Orbital Access Methodologies Part I: Air Launched SSTO

As I mentioned last month, I would like to briefly discuss in a series of blog posts some of the more promising potential approaches for reusable orbital transportation. There is often a tendency among engineers to completely dismiss any idea other than ones own preferred approach as being unrealistic, naive, flawed, impossible, inefficient, etc. However, the more I’ve studied the problem, the more I’ve come to the conclusion that there are probably several technical approaches that can be made to work for providing reliable, low-cost access to orbit. Each of them has its own set of strengths, challenges, unresolved questions, and operating characteristics. By their nature, this means that different approaches may lend themselves better to different potential market niches and different development paths.

The first such approach I would like to introduce for discussion is epitomized by a proposed design (illustrated below, credit: Teledyne Brown) that was brought to my attention about a year ago. This proposed design, termed “Spaceplane” was developed at Teledyne Brown by Dan DeLong (who later became one of the founders XCOR Aerospace and is currently their Vice President and Chief Engineer, and who also currently owns all the rights to the Spaceplane design). Dan’s proposed concept was a winged, “assisted” single-stage to orbit (SSTO) design that was launched off of the back of a converted 747. The LOX/LH2 stage, powered by 1x SSME and 6xRL-10s would theoretically be capable of delivering ~14klb of unmanned cargo to a 400km circular orbit. The vehicle would be reusable, using an Inconel-foil over fiberglass insulation concept for its reentry TPS, and using a runway landing for its recovery method.


While the specifics of Dan’s proposed design are now a bit dated (the concept was proposed back in the late 80’s), the general approach still merits investigation.

To Stage or Not to Stage: That Is The Question

Now, before I go into the specifics of this approach, I know at least a few of you are probably already thinking things along the line of “SSTO? He can’t be serious. Everyone knows that SSTOs are totally unrealistic!” While to be honest, I’m mostly a TSTO guy myself (as is Dan DeLong these days), but I think there’s a real danger in how quickly and without contemplation people tend to buy into new conventional wisdoms.

The fundamental reason why anyone would even want to stage a rocket vehicle has to do with the physics of the rocket-powered flight. The rocket equation, says that the change in velocity due to a rocket in flight is linearly proportional to the specific impulse of the propulsion system and proportional to the natural logarithm of the vehicle’s mass ratio (the ratio of the mass at ignition to the mass at shutdown of the engines).

DV = Isp * g * ln (MR)

Another way of looking at this equation is that the required mass ratio of a vehicle is exponentially proportional to the required velocity change divided by the vehicle’s specific impulse:

MR = e^(DV/(Isp * g))

The inverse of the mass ratio is the dry fraction of the vehicle, ie. the percentage of the vehicle’s gross takeoff weight that can be allocated to structures, propulsion, payload, recovery systems, controls, power, life-support, etc, etc. The rest is fuel. Rewriting it in terms of dry fraction (df), we get:

df = e^-(DV/(Isp * g))

Now this is a fairly simplistic way of viewing things (ie. the Isp actually varies quite a bit with time based on the altitude at a given time, the engine throttle level, if you’re using thrust augmentation, etc, etc.), but shows the crux of the problem. The total delta-V needed to attain a low earth orbit can range anywhere from ~8-10+ km/s, while you’d be lucky to get a mission-averaged Isp much higher than ~400-440s even using the highest Isp propellants in service, LOX and LH2. Now there are all sorts of subtle nuances that we could go into. Things like how dense propellants typically require lower overall delta-V because they end up having less gravity and drag losses, or that depending on what latitude you’re launching from you can get a small “boost” due to the earth’s rotation. But the crux of the matter is that for a single-stage system, you’re dealing with a dry fraction of less than 10% (and typically quite a bit less than 10%).

That 10% has to cover all those categories mentioned above while still providing a high enough payload fraction that your system doesn’t have to get too gargantuan to deliver a sufficiently sized payload. And it has to be robust enough to be reused many times. And your system needs to be maintainable. And it needs to have graceful failure modes, and safe abort modes throughout the flight path. And it needs to be buildable on a realistic budget and timeframe.

All of those issues make the concept of staging very desireable. By staging you get to drop off some of your dry mass along the way, instead of having to lug it all up to orbit. This tends to relax the required mass ratios substantially, which makes it a lot easier to do all those things that make a reusable vehicle truly reusable (as opposed to recoverable, refurbishable, or scavengeable).

But that staging comes at a price. Staging creates a lot of complexity, and introduces some potential failure modes that can be hard to actually check-out on the ground. Staging is one of the single highest risks of failure for existing launch vehicles. Additionally, with a TSTO, now you’re really designing three vehicles, not just one. A first stage, an upper stage, and a combined entity. You now have to come up with abort modes for all the different configurations.

Probably one of the biggest headaches for TSTOs is how to recover and reuse the first stage. Getting to orbit is only a little bit about going up, and mostly about hurtling yourself sideways fast enough to “throw yourself at the ground and continually miss”. Doing so entails gathering quite a bit of horizontal velocity with a first stage, which means that the first stage gets quite a bit of horizontal distance between it and the launch site by the time it releases the upper stage. Most of the TSTO approaches I’ll discuss later revolve around how to get that first stage back. This is a real challenge for TSTO vehicles, though as Dan put it about SSTOs, they have their own challenges with getting the stage back (mostly due to trying to pack a robust heat shield and a robust structure into such a limited available mass budget).

So, in spite of the real challenges of developing SSTOs, there is a reason why some sane and rational people still look at them from time to time. There are real drawbacks to all approaches, and if an SSTO can be technically feasible, it might actually be desirable economically.

With that in mind, I’d like to get back to the topic of this post: air-launched “assisted” SSTOs.

The Benefits of Air Launching

One of the lessons I’ve learned as an engineer is that many times the best way to solve a really nasty and intractable-looking problem is to find a way to not actually solve that problem, but to replace it with an easier problem, and solve that one instead. In the case of an SSTO, trying to make a ground launched, horizontal takeoff and landing SSTO is a horrible challenge. You have very little dry mass to start with, and ground launching requires landing gear rated for the fully loaded weight of your vehicle, wings that have to be able to produce sufficient lift at very low speeds for takeoff, engines that can operate near sea level while still being efficient in vacuum (which entails either really high pressure designs, altitude compensations, or carrying around different engines with some optimized for high thrust at low altitudes, and some optimized for high efficiency in vacuum), and several other challenges. According to Dr Livingston, a Boeing engineer several years ago suggested that such a system was just not technologically feasible with modern materials and propulsion systems. While there have been some improvements on both fronts since he made that comment back in the mid-90s, I wouldn’t be surprised if a ground takeoff HTHL SSTO is still unrealistic.

So the real engineer finds a way to cheat.

And a good way to relax all of those constraints is to not try taking off from the ground, but to start at a reasonable altitude, by using a subsonic airbreathing carrier aircraft. Starting, as SpaceShipOne did, at a reasonable altitude gives several distinct advantages over ground launch (the following list comes from Dan DeLong, with some thoughts from me [in brackets]):

  1. The airplane carrier contributes to the overall altitude and velocity. These advantages are small. [Total savings are probably on the order of 100-200m/s. While this is a small fraction of the overall delta-V, the exponential nature of the problem means that even a small decrease in required delta-V makes a big difference.]

  2. Meteorological uncertainties are mostly below launch altitude. Propellant reserves can thus be less. [Or this means that you can fly on a more dependable schedule, and that you can have more robust propellant reserves without paying as much of a penalty for such.]

  3. Total integrated aerodynamic drag losses are less, as the launch is above much of the atmosphere. [This provides a bigger benefit to low density propellant combinations such as LOX/Methane or LOX/LH2, but overall could be worth several hundred m/s of delta-V, particularly for smaller vehicles]

  4. Max Q is less, which reduces structural mass, and may allow lower density thermal insulation. [You may also be able to “split the difference” on the structural mass somewhat–allowing for a higher FOS on the structure, which allows much less maintenance/inspection, while still pocketing at least some of the mass savings.]

  5. Engine average Isp is increased because the atmospheric back-pressure effect affects a smaller fraction of the trajectory. [This means that your mission averaged Isp is going to be much closer to your vacuum Isp than is typical for a booster engine.]

  6. Engine expansion ratio (non-variable geometry assumed) can be greater because overexpansion is less problematical. [For instance, IIRC, you can light an RL-10 at 30,000ft without risk of unsteady flow-separation caused by overexpansion. This can make a huge difference, as it means you can use an engine with a much higher vacuum Isp. Possibly a benefit of as much as 5-10%, with greater improvements seen by lower pressure systems that often have higher reliability than the ultra-high pressure staged combustion engines preferred for booster applications these days. When combined with benefit #5 above, this can have a large impact on the required propellant fraction due to the exponential nature of the rocket equation.]

  7. Wing area can be smaller because the wings do not need to lift the gross weight at low subsonic speed. Air launch Q is greater than runway rotation Q.

  8. Wing airfoil shape need not be designed to work well at high gross weight and low subsonic speeds.

  9. Wing bending structure need not be designed for gross weight takeoffs or gust loads. Wings can reasonably be stressed for 0.7 g working plus margin. This is a large weight advantage made possible by the carrier aircraft flying a lofted trajectory and releasing the orbiter at an initial angle of at least 15 degrees. (25 degrees is much better but not crucial, more than 60 degrees has no value) This initial angle decays in the first 10 seconds of flight but picks up again as propellant is burned and the constant wing stress trajectory yields a better lift/weight ratio. The thing to keep in mind is that the wings are sized and stressed for landing, and that insofar as they exist, are used to augment launch performance. [A comment I heard from a professor of mine back at BYU was that many people try to use composites as “black aluminum”, i.e. they don’t try to understand the nuances of the material, and thus miss out on most of the benefits. I think that that may often be the case with wings on rocket vehicles–if you design a vehicle to take the maximum advantage of your wings, you can negate some or all of the supposed “penalty” for carrying them in the first place. And that’s coming from a VTVL guy!]

  10. Thrust/weight ratio can be smaller because the low initial trajectory angle does not have large gravity losses. This allows a smaller engine, propellant feed, and thrust structure mass fraction. I found 1.25 at release to be about optimum. This is a bigger advantage in air launching because total integrated aerodynamic drag losses are less and the trajectory need not get the orbiter out of the thick stuff as fast. [Lower gravity losses due to the flight angle reduces the required delta-V somewhat, and is probably a bigger benefit once again for high performance, low-density propellants, which typically suffer from higher gravity losses. Lower required thrust-to-weight is also big because your propulsion system is often a large part of the dry mass of an SSTO, so being able to get away with a lower required T/W ratio for the vehicle can make a large difference.]

  11. The lower mass/(total planform area) yields lower entry temperatures. I assumed inconel foil stretched over fibrous blanket insulation for much of the vehicle undersurface. Titanium over blankets, or no insulation worked on the top surface. Payload bay doors peaked at 185 F. [Having a better ballistic coefficient (the relationship of mass to planform area) means that your vehicle starts decelerating at a higher altitude where the atmospheric density is lower. Basically, drag force is proportional to area, while since F=ma, the acceleration is inversely proportional to mass.

    In other words, “Fluffy” is good for reentry vehicles, which means that by necessity, a fixed geometry SSTO is probably going to have gentler reentry heating loads than a fixed-geometry TSTO. This is increased by the fact that many of the benefits/constraints of air-launching push vehicles towards lower density propellant combinations like LOX/Methane or LOX/LH2. This is a good thing, because an SSTO has a lot less mass to cram that TPS system into. This is also good, because lower temperatures and more robust TPS systems mean lower maintenance, lower costs, and higher “availability”.]

  12. Mission flexibility is greater. For example, the carrier airplane can fly uprange before release to allow a wider return-to-launch-site abort window. Good ferry capability, etc. [The other major benefit for missions to specific orbital destinations, like say a Bigelow station, is that the carrier airplane can move the launch point around. By being able to place the launch point at just the right position relative to the station, you can provide for first-orbit rendezvous opportunities even if your launch site isn’t directly underneath the given station. The ability to move the launch point also potentially opens up longer launch windows. Lastly, being able to move the launch point allows options like operating out of an airport closer to “civilization” while still launching out of an area with low population density, like say over an ocean or a desert.]

  13. [Update: A commenter noticed that Dan and I both forgot to include an important additional benefit of this approach–landing gear for an air-launched SSTO can be designed based on landing weight instead of takeoff weight. This is a big deal for SSTO designs. Boeing had another proposed design, RAS-V that used a trolley for takeoff, but would probably be pretty dicey for an abort. Dan also mentioned the point I forgot to bring up that the RL10s on his design could be used to establish a subsonic cruise of a respectable distance, so you wouldn’t actually dump propellants, you’d burn them off in your smaller engines. All in all this ability helps Mass Ratio substantially since the landing gear for a ground takeoff HTHL SSTO is typically a large chunk of the dry weight of the vehicle.]

As can be seen from this list, by “cheating” a little bit on the boundary conditions, assisted SSTO approaches can avoid many of the typically largest drawbacks of ground-launched SSTOs. What was a probably intractable problem before (ground-launched HTHL SSTO) becomes a lot more feasible by adding the air-launch “assist”. Now, technically you could say that the carrier airplane in an air-launched “assisted” SSTO is really a stage, and therefore the idea isn’t really SSTO–and you would be technically correct. But, I do think there is a fundamental difference between an airbreathing carrier plane and a true first stage, such as: no worries about TPS for the carrier plane, no need for RCS systems, no need for rocket propulsion (probably), no need for high propellant fractions, etc.

So all in all, there’s a fairly compelling case that if you’re interested in developing a SSTO vehicle, and a winged one at that, that air-launching is a big win over ground launching.

The Constraints, Challenges, and Drawbacks of Air-Launching

But as with everything in engineering, air-launching is not without its constraints, challenges, and drawbacks. While I’m sure that someone like Dan DeLong, or Antonio Elias of OSC could probably do better justice to this section than I could, I’ll try to touch on some of the high-points:

  1. There are a limited number of existing aircraft designs that can be used for air launching. What this means is that the design space for gross takeoff weight vs. carrier price is not a smooth continuous function. If you are near the upper limits of a given carrier craft, even a small increase in takeoff weight might end up forcing you to use a much larger carrier craft.
  2. Most existing aircraft aren’t that great for air-launching large vehicles. If you drop the vehicle from beneath most commercial aircraft, you’re very limited on maximum volume beneath the wings or the hull. If you launch off of the back of an aircraft, you now need to have a higher L/D wing (or light the engines before separating) so as to not collide with the carrier after separation. Also, if you use a top-launched configuration, now you have to mount the stage on top of your carrier, which requires a substantial amount of ground handling equipment (compared to a bottom-dropper).
  3. Due to needing to fit on an existing carrier aircraft, air-launched SSTOs are a lot more Gross Take-Off Weight (GTOW) limited than ground-launched SSTOs (which can grow to arbitrarily big sizes).
  4. Related to point #3, there are certain systems on a launch vehicle that don’t scale down very linearly. There are also minimum gage issues. These two realities mean that as an SSTO gets smaller, the maximum achievable mass ratio for the system gets worse and worse. Below some minimum size, it’s no longer possible to reach orbit with any appreciable payload at all. I’m not positive where that exact point is (and it probably depends on a *lot* of details, but it is probably in the ~50klb range.
  5. This is still an SSTO, and even if you cheat by air-launching, you still have a very demanding mass ratio to meet while still making the system robust enough for reuse.
  6. Air-Launching a cryogenic propellant stage requires either very good insulation, or some sort of propellant storage capabilities on the carrier craft, or at least some sort of propellant conditioning equipment (ie something to pull heat out of the propellants and prevent them from boiling off). Or possibly all of the above.
  7. Due to upper limits on the size of available carrier craft, this concept is unlikely to be scalable to payloads much bigger than 20-25klb.

Now, none of these are necessarily deal-killers, but its important to know a design choice’s drawbacks.

Potential Enabling Technologies

There are a couple of recent technologies that could make a vehicle like this a lot more realistic than back when Dan DeLong first developed the concept. Specifically, cryogenic composite tank materials, some advanced cryogenic insulation techniques that are under development, the White Knight series of carrier aircraft, thrust augmented nozzles, and orbital tugs.

First off, cryogenic composite tank materials (such as XCOR’s “NonBurnite” flouropolymer matrix composites) allow for somewhat lighter tank masses, allow for cryogenic “wet wings” if desired, and allow for insulation and the tank to be integrated into the vehicle structure.

The advanced cryogenic insulation technique I mentioned would help a lot with reducing/eliminating boiloff issues for cryogenic propellants (particularly LH2 if you go that way). I can’t really go into the specifics on this approach quite yet. I had written an SBIR proposal for pursuing this technology (along with some teaming partners in industry), but we barely lost out, so it may take a lot longer before the idea is proven out. Suffice it to say that it could cut down on boiloff substantially in gravity, and even moreso in microgravity. Keep your fingers crossed.

The benefit of the White Knight series of carrier aircraft should be obvious. Having a large carrier aircraft with a high undercarriage that is purpose-built for carrying large rocket powered vehicles is immense. I don’t have exact specs for WK2 (I figured it would be really bad form to try and pump my friends on the Scaled Propulsion team for such info), but my guess is that its at least 40klb, and possibly as much as 60-80klb. Depending on the exact numbers it might be just barely big enough for a fully orbital SSTO, though I’m not sure how much payload you could get with a vehicle that small. I really don’t have a great feel for how the scaling performance for the SSTO works. There have also been several rumors (from all sorts of sources) about the possibility of a White Knight 3 down the road. T/Space showed such a vehicle in their original presentations. That would likely be capable of carrying a booster in the 300-500klb range, which is about the weight of Dan’s original “Space Plane” proposal. The benefit of using a White Knight 2 or 3 for your carrier plane (above and beyond being able to buy an airplane that is purpose built for air-launching) is that the SSTO wouldn’t be the only customer for the carrier aircraft. Which means the SSTO would only have to pay a fraction of the amortization costs of the WK2/3 development. More importantly, if you can get away with something like WK2, there may very well be several of these built for Virgin Galactic (and other customers), which means that the unit price of the airplane will be lower, parts will be more available, there will be a larger operational/maintenance experience base for it, and depending on the required flight-rate, it might even be possible to just rent a WK2/3 from a SS2 operator instead of having to own one outright.

Ok, I’m sure I’m starting to sound like I have a bit of a hobbyhorse thing going, but I think that thrust augmented nozzles would be a very good match for an air-launched SSTO. Especially if they were running in a “tripropellant” configuration (ie with the fuel in the thrust augmentation section being a denser fuel like kerosene, methane, or subcooled propane). The first big advantage is that it would allow an engine with a much higher thrust to weight ratio compared to a more traditional engine. This would allow for a much lighter engine to be used, which directly translates into more mass for the rest of the vehicle (and the payload). Another benefit is that depending on the fuel used (and the construction technique for the wings), a “wet-wing” tank could be used for the TAN fuel, which would allow a lot more fuel to be carried at almost no extra dry-weight. Combine this with the fact that the LOX tank would be bigger, and the LH2 tank smaller, and it ends up giving you a much higher achievable Mass Ratio for a given construction technology. Using TAN, you can also get away with a larger expansion ratio on the nozzle, giving better Isp after the TAN propellants burn out. Also, if the TAN injectors are broken up into quadrants with separate valves, they could possibly be used for Liquid Injection Thrust Vector Control. This would eliminate the need for the gimbal, and possibly allow for the now much bigger rocket engine to package better into the rocket vehicle. Lastly, if the thrust augmentation is light enough, it might allow for the possibility of keeping some “go-around” propellant for increased landing reliability. While adding the denser TAN propellant doesn’t give quite the same drag and gravity loss benefits as it would for a vertical ground launched vehicle, it would still likely increase the payload fraction for the vehicle at a slight increase in GTOW. Aerojet was estimating, IIRC, a 3x increase in payload for a less than 50% increase in GTOW.

Lastly, space tugs (possibly based on the Orbital Express design, or possibly based on the Loral/Constellation Services tug designs) could greatly help such a system if it turns out to have lower performance than hoped for. Instead of taking the payloads all the way to their destination, a tug could possibly allow the SSTO to place payloads into a much lower temporary orbit (which would increase payload mass). Having a tug would also reduce the mass and complexity of the SSTO as it would no longer need its own rendezvous and docking hardware. Also, having a tug means that the cargo (or propellant) could be stored in generic containers, which would simplify ground handling and payload installation. A pressurized tug would be necessary if you wanted to fly people on the spaceplane, but that isn’t too unreasonable.

All of these new technologies, most of them which have only come out in the past 5 years or so, make a system like this a lot more feasible today than back in 1986.

Preferred Instantiation

[Update: I also forgot to include this section in the original post]

While I think Dan’s original design provides a lot of useful ideas, I think that my preferred instantiation of an air-launched “assisted” SSTO would be a lot smaller. After Space Plane, Dan also went the direction of a smaller vehicle–one he called “Frequent Flyer”. I don’t recall the exact specs for that design, but they were around 40-50klb GTOW, and required a solid strapon “0th stage” to provide enough thrust. I’d go instead with a wet-wing tripropellant design using kerosene in the wings burned in a single RL10 modified for LOX/Kero thrust augmentation. The gross weight would go up a bit, probably to up around 70-75klb (which is hopefully below the upper limit of what White Knight 2 can carry–I don’t know for sure), but you’d get a better mission averaged Isp, would have a fully reusable system, and would probably increase the payload a bit over the Frequent Flyer. But Dan would be in a much better position to say. My goal with this instantiation would be basically either two people, or 1-2klb worth of cargo to LEO. If a vehicle this size works, and if it can fit on WK2, it would be possible to do a larger follow-on using something like a WK3 down the road.

That’s just my opinion though.

Remaining Unknowns and Some Potential Paths Forward

So the question becomes, where are we at now with regards to this concept? What unknowns are that we currently know about? Where do we go from here?

The key “known unknowns” I can think of include:

  1. TPS design and reentry aerodynamics–is it feasible to make a reusable TPS system that will work for this vehicle that is robust enough, and what moldline/airfoil design will provide the best balance of needed subsonic performance, and workable hypersonic aerodynamics?
  2. Cryogenic propellant tanks and insulation–can tanks be designed that are both light enough and robust enough for the application? Can long-lifetime cryogenic composite tanks be built that work at LH2 tempeatures? Can an insulation technique be found that is adequate enough to prevent boiloff during the ferry to the launch site? Do we need to use some form of subcooling, propellant conditioning, or “top-off tanks” on the carrier plane?
  3. Thrust Augmentation–can thrust augmentation actually deliver enough of an advantage to justify its use in this application? Can an existing engine (such as an RL10 variant) be readily modified for use with thrust augmentation? What is the optimal augmentation level? Does the better payload fraction provided allow you to use a smaller vehicle? What TAN fuel is best? Kerosene? Subcooled Propane? LH2? Can the thrust augmentation be combined with an LITVC system? Does that gain you anything? Can you adequately control the CG shift during flight with the TAN fuel in a “wet wing”? Could an RL10 type engine operating on “vapors” in turbine bypass mode provide enough of a core flow to ignite the thrust augmentation for a go-around burn at landing? Or would you need separate go-around thrusters?
  4. Vehicle Sizing–what’s the smallest vehicle size that can reasonably deliver (with margin) the payload in question? What are the actual carrying capacities of WK2 or 3? Would such a minimal vehicle be small enough to fit under WK2, or would WK3 be necessary?
  5. Mass Ratio–what mass ratio would be required for the vehicle? Based on existing technologies, how feasible is that mass ratio to attain? Is the required mass ratio more doable using denser propellants, and if so, can a denser propellant vehicle still keep a low enough GTOW to fit on potential carrier planes?

To me, the most critical questions that are also the most unknown, are the ones regarding the TPS and reentry aerodynamics. Most of the other questions, while important, are much more straightforward to answer.

As for the path forward, I think there are multiple prongs that can be taken.

First off, for the carrier plane, WK2 is mostly built and will probably be flying this year. More exact information about its maximum carrying capacity can probably be had in the relatively near future. Trying to find a way to make a vehicle that closes using WK2 would be the most preferable option.

Second off, the TPS/Reentry Aerodynamics. Some of this can be worked on the “traditional way” using CFD and special wind tunnels (at places like NASA Ames). However at some point, it would probably be worthwhile to move on to subscale models launched from suborbital vehicles. Basically, a suborbital vehicle with a “nanosat launcher” upper stage could probably put up a small, instrumented reentry model to nearly orbital speeds. A lot of care would be necessary in designing the experiment and analyzing the data to get the actual data you want, because there are all sorts of scaling laws going on a the same time. Things like different reynolds numbers, the fact that the standoff distance of the bow shock is going to be proportional to the linear dimensions of the vehicle, so a subscale model is likely going to see more intense heating, etc. It should be possible to design a series of low-cost experiments though that can at least retire some of the risk in advance before trying to build an operational version.

As for overall vehicle integration and Mass Ratio control issues, an HTHL vehicle like Xerus actually provides a useful starting point for working ones way up to an SSTO. Now, the XCOR people aren’t SSTO fans. And they’re especially not LH2 fueled SSTO fans. But, the best approach for trying this would probably be to hire someone like XCOR to try and build a lower-performance iterative prototype first to test out some of the key functionality, and then work your way up to the performance needed for SSTO. The first prototype might use just a traditional LOX/Methane engine with as high of a mass ratio as possible. Make sure that the handling and basic aerodynamics work out right. Test out the cryo insulation, and air-launch cryo-propellant handling procedures. Make sure that the TPS functions as expected in the suborbital (though relatively high velocity) environment that such a vehicle could provide. Upgrade the engine to a TAN system and get some experience operating that and making sure that the LITVC scheme works. Test out RCS functionality. Test abort modes.

Do a second iteration that has LH2 as well as the TAN propellant. Develop and test out a TAN-modified RL10. Get experience using such an engine. Get in-flight performance data. Make sure the cryo insulation still works. Make sure the tank can handle the cold cycles. See how close you can get to the mass ratio. Instrument the crap out of your vehicle and figure out where you can shave weight, and how robust/reusable the TPS is in an almost orbital situation. Figure out if you need to scale up the vehicle, or what other changes will be needed to reach a sufficient payload target. Start expanding the envelope to orbit.

Now some of these steps might be skippable depending on how previous steps go. Some of them might be doable as part of other programs (for instance figuring out the cryo composite tanks for LH2 or the special insulation system might benefit other projects, developing flightweight propellant conditioning hardware that can fit on a WK2 or 3 might also be useful for other projects). But these are just some thoughts on what has to be done from a technical standpoint to get “there” from “here”.

Conclusions

In spite of the bad reputation that SSTOs have earned during the last decade, there are at least some versions, like the air-launched SSTO that aren’t entirely crazy. They still might not make sense, but if any SSTO RLV design ever makes it, my guess is it would likely be something like this.

The following two tabs change content below.
Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
Jonathan Goff

Latest posts by Jonathan Goff (see all)

Jonathan Goff

About Jonathan Goff

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
This entry was posted in Launch Vehicles, Orbital Access Methodologies, Space Transportation, Technology. Bookmark the permalink.

74 Responses to Orbital Access Methodologies Part I: Air Launched SSTO

  1. Antonio Elias says:

    Hi, Jon! Nice piece, and thanks for the heads up it was coming! Your summary of the “rocket equation” consequences of staging, and of the energy efficiency benefits of air launch are right on the spot. I’d like to make a few comments, though, if I may:

    I don’t fully agree that there is “a tendency among engineers to completely dismiss any idea other than ones own preferred approach”. A true engineer takes maximum advantage of the improvements that his/her predecessors have conceived – the advantages of using other people’s ideas overcome the advantages of pooh-pooing them. If you see somebody who does, he/she is NOT a true engineer. Usurpation of other people’s ideas without proper recognition is a far, far more common sin among engineers than NIH (other professions, however, gloat in NIH).

    As a consequence, do not dismiss criticism of new ideas as simple NIH without digging further into it: first, new ideas are not always new ideas; second, the value of a certain idea or concept is usually VERY contents-dependent. Let me try to illustrate:

    You seems to like the concept of air-launch. Well, I have a bit of experience in air-launch to orbit, having done it 38 times. I did not invent the concept air-launch (in spite of having a patent associated with it) but I was the first one (and, I’m afraid, so far the ONLY one) to, as they say, “reduce it to practice”. In other words, I met and fought all the devils in the details that pop up after you advance beyond the concept design stage. So you would expect me to be a staunch advocate of air-launch, right?

    Unfortunately, the answer to the question “is air-launch a good thing” is: “it depends”. It is a good thing for a certain SIZE of launcher, if you can use solid propulsion, and a bunch of other ifs. Can it be done with larger rockets and liquid propulsion? Yes, it can, but all of sudden a large number of cans of worms open up (moans and guffaws from the audience). Is air-launch a good thing for large rockets? From a cost standpoint, NO. The goodness of the idea of air-launch depends on the size of the rocket.

    Similarly, are fewer stages better than a large number of stages? In a very general and abstract way yes, but it depends on so many things that you cannot extrapolate the idea to ONE stage, and it only applies to two stages is some very, very specific cases (I can think of only two: Atlas and R-7, coincidentally two of the very earliest rockets).

    Launch vehicle design depends on two fundamental technology parameters: specific impulse and mass fraction. Unfortunately (or fortunately, depending on your point of view) we reached the limits of the laws of physics on both parameters in the 1965-1975 time frame (for specific impulse) and the 1955-1965 time frame (for mass fraction).

    It is therefore not surprising that the fundamental architecture of launch vehicles (reusable vs. expendable, multi-stage vs. fewer stages, etc.) has not changed considerably since then. In my view, the lack of major changes in the architecture of launch vehicles is not due to NIH or laziness, but by laws of physics. Note that this statement is made by the engineer that implemented the only significant change in launch vehicle architecture since the Shuttle that actually flies.

    Now, give me a significant change in specific impulse or mass fraction, and all bets are off. Nanotubes, anyone?

    – Antonio Elias

  2. Jon Goff says:

    Antonio,
    Thanks for taking the time to read through and comment on my paper. As I said, I’m not normally a “wings” guy, so a lot of this is just trying to extrapolate a bit from what others (such as yourself) have said.

    Regarding your comment about usurpation of others ideas vs. NIH, I totally agree that NIH is not an attribute of a good engineer. Alas, at least among some of the engineers I know it isn’t entirely uncommon.

    So you would expect me to be a staunch advocate of air-launch, right? Unfortunately, the answer to the question “is air-launch a good thing” is: “it depends”.

    Exactly. Me, I’m personally a VTVL guy. But that doesn’t prevent me from seeing other approaches that also seem to make sense. I intend on pointing out a few of the others that I think are promising over the next several weeks.

    Regarding the benefits of airlaunch as a function of size, I can see your point. Air Launch definitely is not going to scale up to say EELV sized payloads. It’s great for smaller payloads. And I agree that doing a liquid fueled air-launch vehicle can be tricky, especially if you’re talking cryogenic liquids. Not impossible, but as I brought up in the paper, there are lots of challenges.

    Launch vehicle design depends on two fundamental technology parameters: specific impulse and mass fraction.

    I think that at least for RLVs, the non-technical (ie business, finance, and economics) parameters can drive the design just as much as the physics of the situation. I think you’ve said as much on a couple of occasions there on the NASASpaceFlight Forum.

    That said, there are at least a few technologies that have come out lately that could change the equation so to speak a little. I don’t think you’ve had a chance to read my article about Thrust Augmented Nozzles (though if you’re working with Aerojet on Taurus II, I’d be surprised if they haven’t tried to sell you on the idea). Not a panacea, but at least in this question it could effectively raise both the achievable Isp for an engine that has to operate at lower altitudes, while also increasing the MR by providing an engine with drastically better T/W ratios than is typical.

    But as I said, most of those technical details only matter for an RLV if you can build a strong enough non-technical case to even get a chance to work no the technical side of the problem. I think that without big changes on the demand side of space launch (ie new markets that require higher flight rates than existing ones), that even carbon nanotubes might not be enough to change the situation enough.

    But I think you probably know that better than most.

    Thanks for the feedback!

    ~Jon

  3. andy_janes says:

    Great summary Jon.

    My 2c is that Scaled are probably thinking along these lines too, and once WK2/SS2 is flying will be building and testing a SS2.5 to answer these questions and see how close to orbit they can get before work on SS3 starts (indeed, they may be able to keep cutting holes in the structure and just get to orbit with just a pilot and broom handle onboard!)

    Andy

  4. Anonymous says:

    This comment is completely off topic, but I couldn’t help but be blown away by how articulate and well crafted your writing is. You should consider writing books for the interested layperson.

  5. Anonymous says:

    Great post Jon.

    Just wonder how you classify the Launch Assist Platform (LAP) approach that was, literally, the basis of the original Kistler concept? Though it didn’t involve an aircraft, it certainly couldn’t be regarded as a conventional first stage as it gave no horizontal velocity component and was more akin to a high altitude launch pad or even an elevator!

    Given the type of vehicles you’re focused on at Masten, I’m sure you’ve had more that a passing look at the LAP’s +/- points and so wonder if you’ll cover these in a future instalment.

    By the way, the work done for the HOTOL/Antonov study in the early 1990’s indicated that the overall performance advantage of air-launch was to effectively reduce the delta-V to orbit by almost 1km/s, which has a massive impact on any SSTO concept. Moreover, if you believe that 4t is all you need to service the current commercial markets (I guess you know what I’m thinking of here πŸ™‚ the approach looks even more attractive from a business point of view.

    Dave Salt

  6. Anonymous says:

    Jon,
    Nice blog. I haven’t had a chance to fully digest everything,so I might have missed it, but I think you missed another big benefic of air launching. That is a reduction in the mass fraction of the landing gear. Typical aerospace weight estimating equations that I have seen seem to peg landing gear fraction around 2% of gross weight. For an air launch you should be able to size the landing gear based upon an aborted launch weight which could be significantly lower than gross if an adequate propellant dumb system is employed.

    -Raymond

  7. Paul Breed says:

    DCX had composite LH2 tanks.
    I believe that in roughly cylindrical
    shapes they are not the show stoppers one might expect. Getting both cyro and O2 compatibility is harder πŸ˜‰

    I also agree that a TSTO with stage one being a straight up straight down VTVL is an interesting trade space.

    This is even more interesting if the trip up is slow so as to have a very low max Q. Big dumb clunky first stage VTVL followed by a higher tech 2nd stage.

  8. Gary C Hudson says:

    Jon,

    Great post! Of course, I was a VTOL guy too for many, many years, and still see the usefulness of that approach for heavy payloads (>25Klbs) but Dan DeLong convinced me of the value of his original Spaceplane concept decades ago. But I didn’t care for the SSME; I think a pair of J-2s and a pair of RL-10s would be a better choice. At least the J-2s are reusable without rebuilding them, and until we started wasting them in foolish J-2X demonstrations we had a number in inventory.

    Operationally air-launching has many advantages as you have enumerated, but for me the ability to conduct first orbit rendezvous is the killer application.

    The real problem is of course the carrier aircraft; Dan wanted to use the 747-400, but that really drives the cost out of the range of NewSpace firms. Until that problem is solved, we can’t make much progress.

  9. mboeller says:

    Hi;

    As a layman I appreciated your article about airlaunched SSTOs.

    For me the best, sort of airlaunched concept is the LLNL Spacejet concept from the 1970s. This concept used a small airbreathing booster and a large orbiter.

    I like this concept because normaly at take-off the booster of a of airbrething TSTO has by far the highest weight, whereas the orbiter still has to cover the largest deltaV. With the Spacejet concept from LLNL this is different. The orbiter weighted ~80% during TO.

    Today, maybe a booster with 3-4 PW119 would enable a TSTO with a staging velocity of M2.0-2.5 without needing the devlopment of a large airbreathing booster vehicle.

  10. mboeller says:

    damn..

    I made a mistake. LLNL is wrong, I should have wrote Langley.

  11. Anonymous says:

    A very thorough and well written piece Jon!

    I really look forward to reading your thoughts on the “Big first stage VTVL followed by a higher tech 2nd stage”. Since it seems to be more or less the route for both you at Masten and AA.

    It will probably be the next “new” concept that will be reduced to practice, that has clear potential getting us to the orbital ball park.

    – K.L.

  12. Anonymous says:

    this is probably nuts but here goes anyway: for launching off the back of the airplane, couldnt you use relatively small “ejection” SRBs that would lift your SSTO straight up ?

  13. Jon Goff says:

    First off, before I post anymore replies, I want to thank all of you for your excellent comments! One of these days if I get enough of this to put together a small booklet, I’ll have to roll a lot of the comments into the main body.

    ~Jon

  14. Jon Goff says:

    Andy,
    My 2c is that Scaled are probably thinking along these lines too, and once WK2/SS2 is flying will be building and testing a SS2.5 to answer these questions and see how close to orbit they can get before work on SS3 starts.

    Possibly. Scaled has a very talented company. But they have no experience with the kind of propulsion or TPS or other things that would be necessary to make this work. I could see them building a WK3, and maybe the airframe for such an SSTO stage, but I think that it would most likely be another company doing the actual stage design. It really has very little in common with anything they’ve done to date. That’s not dissing on them by any stretch, but I just think that all things considered, they’d be more likely to be a contractor than the lead developer for something that far beyond their current state-of-the-art.

    ~Jon

  15. Jon Goff says:

    Anonymous,
    This comment is completely off topic, but I couldn’t help but be blown away by how articulate and well crafted your writing is. You should consider writing books for the interested layperson.

    I think I may actually try turning these into a short introductory book if I can flesh things out enough. We’ll see. Thanks for the kind words!

    ~Jon

  16. Jon Goff says:

    Dave,
    Just wonder how you classify the Launch Assist Platform (LAP) approach that was, literally, the basis of the original Kistler concept? Though it didn’t involve an aircraft, it certainly couldn’t be regarded as a conventional first stage as it gave no horizontal velocity component and was more akin to a high altitude launch pad or even an elevator!

    “Pop-Up” VTVLs are my next topic in this series. And you are right that there are many similarities between this approach and a pop-up VTVL.

    Given the type of vehicles you’re focused on at Masten, I’m sure you’ve had more that a passing look at the LAP’s +/- points and so wonder if you’ll cover these in a future installment.

    Yeah, I’ll go into both the pop-up approach, as well as the boost-back approach. Both of them have pluses and minuses that I’ll try to go into.

    By the way, the work done for the HOTOL/Antonov study in the early 1990’s indicated that the overall performance advantage of air-launch was to effectively reduce the delta-V to orbit by almost 1km/s, which has a massive impact on any SSTO concept.

    Yeah, especially when combined with the fact that an airlaunched SSTO vehicle will get a better mission-averaged Isp (due to starting at a higher altitude), it really takes the idea from crazy to just very challenging.

    Moreover, if you believe that 4t is all you need to service the current commercial markets (I guess you know what I’m thinking of here πŸ™‚ the approach looks even more attractive from a business point of view.

    I’ll go into some more thoughts on the economics of RLVs after I’ve discussed some of the technical approaches. I actually lean towards payloads even smaller than 4t. I’m looking more at the 1-2.5t range. But I’ll go into the whys and wherefores later. That said, this idea should be able to handle up to about 10-12t before it runs into what are probably insurmountable scaling issues (ie requiring a carrier bigger than a 747), so it should work for your idea.

    ~Jon

  17. Jon Goff says:

    Raymond,

    Nice blog. I haven’t had a chance to fully digest everything,so I might have missed it, but I think you missed another big benefit of air launching. That is a reduction in the mass fraction of the landing gear. Typical aerospace weight estimating equations that I have seen seem to peg landing gear fraction around 2% of gross weight. For an air launch you should be able to size the landing gear based upon an aborted launch weight which could be significantly lower than gross if an adequate propellant dump system is employed.

    Wow, you’re right, I forgot to bring that one up. Good catch. Yeah, when you’re dropping from 30kft, you have plenty of time to dump propellants, so your landing gear and wings can be designed for the landing weight, not the takeoff weight. And seeing as how there’s about a factor of 10x difference between those two weights, it’s a big deal.

    ~Jon

  18. Jon Goff says:

    Paul,
    DCX had composite LH2 tanks.

    Well, DC-XA did, DC-X had aluminum tanks. When they did the upgrade to DC-XA they changed the LH2 tank to composite (with internal insulation), they also added lots of composite plumbing and even composite valves! But that’s nitpicking. Sorry.

    I believe that in roughly cylindrical shapes they are not the show stoppers one might expect. Getting both cyro and O2 compatibility is harder πŸ˜‰

    With internal insulation and something like XCOR’s “nonburnite”, I think it would be a lot easier. If DC-XA hadn’t used internal insulation their tank wouldn’t have worked. By putting insulation inside the tank, it kept the tank walls at a much warmer temperature, one that was a lot easier to work with. As I said, the key to good engineering is not trying to solve the hard problems, but avoiding them altogether.

    I also agree that a TSTO with stage one being a straight up straight down VTVL is an interesting trade space.

    It is, and as I said to Dave, it has a lot in common with this approach (both pluses and minuses). I’m hoping to have a post on that out sometime next week.

    This is even more interesting if the trip up is slow so as to have a very low max Q. Big dumb clunky first stage VTVL followed by a higher tech 2nd stage.

    Calling an almost SSTO upper stage “higher tech” is being euphemistic. We shouldn’t kid ourselves into thinking that building such a stage is going to be anything bug an extreme challenge.

    ~Jon

  19. gravity loss says:

    Nice post!
    For me as a more of a rocket person of course the intro was a bit long winded but is probably a good thing. One thing I’ve noticed in your writing is that you use a style quite similar to a certain known alt space guru… πŸ™‚

    Pressure fed lox-kero mixed monopropellant as second stage for the LAP/pop-up/elevator should be the simplest approach. πŸ˜‰

    The analysis of Part II requires some simulation for the ascent part though because of the drag and gravity losses and the various ways they can be approached. Remember to run the sims with big enough vehicles.

  20. Matt Metcalf says:

    Maybe I’m just crazy, but it seems to me that having a carrier automatically makes this kind of solution two-stage, not single-stage. Instead of the initial rocket stage, you have a carrier stage that solves the reusability problem that you usually have with an initial rocket stage, and your second stage has to fire earlier than it would otherwise.

  21. Jon Goff says:

    Gary,
    Great post! Of course, I was a VTOL guy too for many, many years, and still see the usefulness of that approach for heavy payloads (>25Klbs) but Dan DeLong convinced me of the value of his original Spaceplane concept decades ago.

    Yeah, VTVL is probably the most scalable approach for when you want big RLVs…but such huge RLVs aren’t going to happen anytime soon. They just cost too much to develop. Dan mentioned that he had given both you and Burt the “air-launch” pitch a long time ago, and at least from the directions both of you have taken, it appears he was pretty successful at convincing you. πŸ™‚

    But I didn’t care for the SSME; I think a pair of J-2s and a pair of RL-10s would be a better choice. At least the J-2s are reusable without rebuilding them, and until we started wasting them in foolish J-2X demonstrations we had a number in inventory.

    Hmm…I just realized that in my rush to get this out the door, I forgot to give my preferred instantiation.

    As I see it (not having access to the kind of numbers and tools needed to do a thorough design analysis), I’m a fan of a vehicle in the 50-80klb GTOW range, powered by 1-2 RL10s modified with TAN injected LOX/Kero, with the Kero stored in a “wet wing” configuration. Payload would be fairly low, probably on the order of 1-2 people or 1-2klb. But I definitely agree that putting an air-startable SSME on board is a non-starter.

    Operationally air-launching has many advantages as you have enumerated, but for me the ability to conduct first orbit rendezvous is the killer application.

    Yeah, the other advantages just make it feasible. The much easier first orbit rendezvous is what truly sets it apart compared to other approaches.

    The real problem is of course the carrier aircraft; Dan wanted to use the 747-400, but that really drives the cost out of the range of NewSpace firms. Until that problem is solved, we can’t make much progress.

    Depending on the carrying capacity of WK2 (it would be nice if Burt or VG could release that), it might be possible to do the scaled down version I was mentioning using that. And since he’s trying to make several WK2s for the suborbital market, buying or leasing one of those should be affordable. But yeah, if WK2 doesn’t have the capacity, something like WK3 would be a prerequisite for making this concept work.

    ~Jon

  22. Jon Goff says:

    Gravity Loss,
    For me as a more of a rocket person of course the intro was a bit long winded but is probably a good thing.

    Me, long-winded?

    One thing I’ve noticed in your writing is that you use a style quite similar to a certain known alt space guru… πŸ™‚

    Do I want to know who? πŸ™‚

    Pressure fed lox-kero mixed monopropellant as second stage for the LAP/pop-up/elevator should be the simplest approach. πŸ˜‰

    To quote Doug Jones about LOX-Kero mixed monoprops “That scares me, and I’m fearless.”

    ~Jon

  23. Jon Goff says:

    Matt,
    Yeah, whether you consider it a form of TSTO or an “assisted” SSTO is purely a matter of taste (or marketing). As I said in the post though, since the carrier is an off-the-shelf, or nearly off-the-shelf air-breather, which doesn’t need TPS, or RCS, or anything else common in a rocket stage, doesn’t need any exotic materials, etc., there really is at least some fundamental difference between an air-launched, “assisted” SSTO and a more traditional TSTO.

    Now, if the first stage were rocket powered, or supersonic, or custom built, etc…

    ~Jon

  24. Dan DeLong says:

    “Typical aerospace weight estimating equations that I have seen seem to peg landing gear fraction around 2% of gross weight. For an air launch you should be able to size the landing gear based upon an aborted launch weight which could be significantly lower than gross if an adequate propellant dump system is employed.

    Wow, you’re right, I forgot to bring that one up. Good catch. Yeah, when you’re dropping from 30kft, you have plenty of time to dump propellants, so your landing gear and wings can be designed for the landing weight, not the takeoff weight. “

    Geez, I can’t believe I forgot to list that advantage. The Spaceplane assumed the gear was designed for landing, not takeoff weight, and the 4 RL-10s had enough thrust to establish a subsonic cruise and get the vehicle back to the takeoff runway. So propellants were not dumped, they were used to save the vehicle in the event of an SSME no-light. (Jon had an earlier version with 6 RL-10s, but 4 worked well enough.)

    Also, the HTOL SSTO Boeing RAS-V seems not to have been mentioned. It used a trolley for takeoff.

  25. Jon Goff says:

    Dan,
    I added that benefit to the list, and added a section about my thoughts on doing a preferred first generation instantiation of the idea.

    Also, I didn’t go into RAS-V or other SSTO approaches mostly because I wanted to focus on a handful of approaches that I think are most promising. Trying to go over every idea that’s been thought up in the past is far beyond what I can handle with my limited bandwidth.

    ~Jon

  26. Karl Gallagher says:

    The RASV would be another one for the trade space–powered sled/SSTO.

    Some others you might want to look at are the aerial propellant transfer (Pioneer Rocketplane) and towed launch (Kelly Aerospace) options. They get a lot of the benefit of the carrier aircraft without the ops penalties.

  27. Jon Goff says:

    Karl,
    I know there are parts of the tradespace that I won’t be spending much time on, but that’s mostly because of limited bandwidth. Sure, the propellant transfer, or the tow-launch ideas both have some benefits, but at least to me, when you compare the three approaches, the air-launch idea has some real benefits over either of the alternatives.

    ~Jon

  28. Anonymous says:

    i think i already made this comment, but it seems lost. anyway:
    Air launch is assisting the SSTO from the ground end, while orbiting rotovator could assist the SSTO from the other end, dubbed Single Stage to Tether concept.
    Also, just launching off a high mountain would have some of the air launch benefits, but its own significant drawbacks as well ( support logistics )

  29. Anonymous says:

    I appreciate much of what has been written here. I am an air launch engineer, not because it is my idea or liking, but because in the long run (unless new physics is found), air launch will be the workhorse of ETO in the long term. No other system will have the safety, meet the demanding schedule and have the long-term economics for massive ETO transportation.

    Ok, that is the grandiose future. Lets be practical today. Many ETO concepts will work, SSTO will not. It has to be economical enough. SST (i.e., Concord) was not and was artificially propped up for years as the Europeans were in denial. Why not redesign the rocket to the air launch conditions and see what is the optimum rocket system that comes out rather than jump to the SSTO conclusion before the first numbers are crunched? In all the studies I have seen, or been apart of, the perfect rocket was still 2 stages. Yet it was very different in many ways to VTO design, nevertheless, the 2-stage approach aligned all the advantages of air launching.

    The numbers indicate that air launch has a sweet spot, not in small vehicles, but at today’s ELV levels. That makes it a great reason to hate, and put down technically, and not push forward in real development. It is funny how all the air launch ideas, other than the White Knight, are perfect examples of how not to do air launch!

    Although much is stated well here, there is just enough poison to cast the air launch idea as just another fantasy when in fact it is the practical way to orbit for the many reasons you state and for many more.

  30. andy_janes says:

    Gary, you said “Of course, I was a VTOL guy too for many, many years, and still see the usefulness of that approach for heavy payloads (>25Klbs) but Dan DeLong convinced me of the value of his original Spaceplane concept decades ago.”

    Can I ask when exactly this was? In particular, if it was before the Rotary Rocket era and if so how come you went with that design?

    Andy

  31. Jon Goff says:

    Anonymous,
    i think i already made this comment, but it seems lost. anyway:
    Air launch is assisting the SSTO from the ground end, while orbiting rotovator could assist the SSTO from the other end, dubbed Single Stage to Tether concept.
    Also, just launching off a high mountain would have some of the air launch benefits, but its own significant drawbacks as well ( support logistics )

    Yeah, while there are some advantages to be had by either approach, they both lead to serious bottlenecks. In the case of the single stage to tether concept, it’s the tether. You’re likely going to be limited to only one or two inclinations for the tethers, and the tethers can only support so high of a flight rate. With the high mountains approach, there are only a few areas in the world that really make sense, so once again, you’re limiting yourself needlessly.

    I like the flexibility of air-launch, because it really could turn into something where you could have dozens or hundreds of these things flying every day from different airports around the world.

    ~Jon

  32. Jon Goff says:

    Anonymous,
    I appreciate much of what has been written here. I am an air launch engineer, not because it is my idea or liking, but because in the long run (unless new physics is found), air launch will be the workhorse of ETO in the long term. No other system will have the safety, meet the demanding schedule and have the long-term economics for massive ETO transportation.

    Well, I’m not sure I would go that far. It has its pluses and minuses like all other approaches. It is very interesting though, which is why I started out my series with this post.

    Ok, that is the grandiose future. Lets be practical today. Many ETO concepts will work, SSTO will not. It has to be economical enough. SST (i.e., Concord) was not and was artificially propped up for years as the Europeans were in denial. Why not redesign the rocket to the air launch conditions and see what is the optimum rocket system that comes out rather than jump to the SSTO conclusion before the first numbers are crunched?

    You seem to be misunderstanding why I wrote this article. I wasn’t writing this as a “Air-Launched SSTO is the best design EVER!!!1one!” I wrote it from the standpoint of “of all the RLV options out there, this is one of the few SSTO approaches I really think has a chance of possibly working, and therefore I wanted to dive a bit into the pluses and minuses for the benefit of the readers”. Notice the difference?

    In all the studies I have seen, or been apart of, the perfect rocket was still 2 stages. Yet it was very different in many ways to VTO design, nevertheless, the 2-stage approach aligned all the advantages of air launching.

    Out of curiosity, was this a fully reusable design, a fully expendable design, or a partially reusable design? For a fully expendable design, I would agree that two stages is always going to give you better payload performance, require less stringent mass ratios, and generally be easier to pull off. But I wasn’t talking about expendable launch vehicles.

    For a reusable air-launched vehicle, TSTO might still win out, but it’s a lot closer of a thing. The biggest problem is getting that first stage back. Either you have to provide so little delta-V with that stage that the economics start looking doubtful, or you provide a good delta-V split and now have to recover a first stage that’s far downrange, and coming in fast and hot. You might still come out somewhat ahead. Maybe. But it isn’t anywhere near as closed of a case as you imply.

    The numbers indicate that air launch has a sweet spot, not in small vehicles, but at today’s ELV levels. That makes it a great reason to hate, and put down technically, and not push forward in real development.

    Today’s ELV levels? That’ll be nice and expensive. And that will also require a lot of work on your carrier aircraft. I probably didn’t do a good enough job of pointing this out in this post, but my point was focusing on near-term RLV options. And quite frankly, an EELV sized air-launched TSTO RLV, while probably quite doable down the road, looks way too expensive for the near term. Unless you have lots of connections with “Uncle Sugar”. And even then I’m skeptical.

    It is funny how all the air launch ideas, other than the White Knight, are perfect examples of how not to do air launch!

    Would you care to elaborate? Other than White Knight and Pegasus, nobody else has ever successfully done an air-launched rocket vehicle (well, ignoring missles and such). How do you know, seeing as how if you weren’t on one of those teams, you by definition haven’t actually done it yet, that your approach is the right one, and that everyone else doesn’t know what they’re talking about.

    Although much is stated well here, there is just enough poison to cast the air launch idea as just another fantasy when in fact it is the practical way to orbit for the many reasons you state and for many more.

    I’m sorry you feel that I “poisoned” the idea.

    ~Jon

  33. Rodney says:

    Hello Jon

    How familiar are you with the Andrews-space concept of airlaunch?

    http://www.andrews-space.com/content-main.php?subsection=MTA5

    Taking off horizontally and light and making your LOX out of the air would be advantageous along with the other benefits of air launch.

    Rod Kendrick

  34. Anonymous says:

    While 747-400s are still pricey, there are a lot of old but perhaps still serviceable 747s that have been retired from airline service that could serve as the launch aircraft. Orbital Sciences didn’t buy a new L-1011 for their launch aircraft. They got one that had been retired from airline service.

    Air release from a 747 was demonstrated 5 times back in the late 1970s when they did the Shuttle Enterprise flight tests. One disadvantage was that the 747 had to enter a shallow dive before releasing the Shuttle. That would reduce some of the advantage of air release.

    One idea that came to mind to possibly increase the useful load of the aircraft/spacecraft combination would be to use aerial refueling techniques to top off the spacecraft’s tanks before release. This might be easier to accomplish if you use kerosene for the thrust augmentation nozzle. Refueling using kerosene is a decades-old established practice. I have no idea how difficult (if possible) it would be to deliver cyrogenic propellants this way.

    The idea is that the partially fueled aircraft/spacecraft combination to take off and climb to a reasonable altitude (say 25,000-30,000 feet). Planes like the 747 burn a lot of fuel during takeoff and climb. As the 747 burns off fuel, it’ll have weight margin to accept the extra propellant from the refueling. This idea (if practical) would allow the 747 to take off at gross weight and then to add thousands of additional pounds of propellant to the spacecraft shortly before release.

  35. Jon Goff says:

    Rodney,
    Yeah I’ve looked at their LOX collection idea. I don’t think it’s crazy, just more likely a 3rd or 4th generation performance improvement for once the market is big enough that you need something like that. Making a LOX plant that is flight weight isn’t going to be trivial, and in the near term, you really don’t need it, because the market isn’t there.

    My thoughts:
    1st Gen–if possible small enough to fly under WK2. Very small payload to LEO (2 people or ~2klb).

    2nd Gen–if available, fly under WK3 (if such a beast exists by then), or off the back of a 747 (less preferable, but doable). Will need new engines. Bigger payload. Possibly 3-4 people, or 5-10klb.

    3rd Gen–add either in-air LOX collection or refueling. Will cost a lot, but hopefully by then there’s a proven market that needs large amounts of payload. This could possibly stretch the payload up to EELV sizes, depending on details.

    I’m just doubtful we can go from where we are now (with the market we have now) to 3rd Generation without first doing Gen 1 and Gen 2 (or their equivalence with other RLV approaches–remember, if anyone decides to run with this idea, they’ll be competing against other approaches as well).

    ~Jon

  36. Jon Goff says:

    Anonymous,
    Used 747s are definitely feasible. Dan’s original plan called for launching off the back of a 747. I’d prefer a vehicle specifically built for air-launch like WK2/3. Having the spacecraft be below the vehicle makes integration and ground handling lots easier, as well as in-flight ops. But if WK2 turns out to be too small, and WK3 doesn’t turn out to be “in the cards”, it’s perfectly reasonable.

    As for in-air refueling…once again, it’s not crazy, but it definitely adds cost and complexity, which means that it probably doesn’t make sense–yet.

    ~Jon

  37. Anonymous says:

    Developing a custom air launcher – especially if the payload is large – can get pretty expensive. Some advantages of using a used airliner such as 747s is that they already exist, there’s a ample supply line of pilots, mechanics, and parts to keep them flying. Offhand, I’d wager that developing a custom airlauncher with the same capacity as a 747 might end up costing quite a bit more than buying or leasing a existing plane.

    You’re definitely correct about the relative ease of mounting, servicing, and launching a payload suspended below an airframe compared to a top mounted payload. However, bottom mounting also imposes limitations and compromises due to issues like ground clearance.

    Aerial refueling has been around since the 1940s. Jet tankers have been around for about 50 years. Several military 747s (e.g. E-4s, Air Force One) have been fitted with aerial refueling equipment so there isn’t any new R&D required if the fuel is something like kerosene. While the idea adds complexity and cost, it could increase the loaded weight of the spacecraft by 50,000 pounds or more.

  38. andy_janes says:

    Hi Jon. How about a C5 for a carrier aircraft. Similar size to a 747 but top wing so could carry the space plane underneath/recessed into the fuselage (which may also help prevent propellant boil-off)

    Andy

  39. Jon Goff says:

    Anonymous,
    As I said, a 747 could make sense, but you have to remember that you can’t just buy an old 747, and slap a rocket on the back. There need to be extensive enough of modifications, that at the end of the day, it might not be that much cheaper than having Scaled build you something specially designed. Going with Scaled makes even more sense if you can “piggyback” off of carrier planes developed for other customers. Ie, WK2 is going to get developed anyway, so if it has enough takeoff capacity, build your plane to fit under that.

    The devil’s in the details, I’m just saying that there’s trades either way. 747s with launching off the top *are* doable, but there are lots of details that might make a custom built carrier cheaper in the long run.

    ~Jon

  40. Jon Goff says:

    Andy,
    I don’t know if you could get enough volume under a C5 wing. Maybe, maybe not. As for if it would reduce boiloff…probably not, IMO. You’re still in the airflow either way.

    ~Jon

  41. Dave Salt says:

    Not sure how much detail you want to capture in your list of +/- points, but it may be worth noting the impact of “flight path angle” at separation and how this drives the design away from “cheap” off-the-shelf solutions.

    Trades-offs show that a sufficiently steep flight path angle results in a significant performance boost but comes at a price because conventional jet engines have little, if any, additional thrust available at the requisite altitude. A dive and pull-up manoeuvre can be of some help (HOTOL/An-225 used this approach) but, if you really want to maximize performance, you’re usually driven to some form of thrust augmentation.

    I remember a Boeing concept from the late 1980’s with an SSME in the tail of a 747 while, more recently, the RASCAL programme considered injecting LOx at the inlet. Also, if memory serves, Dan’s concept envisaged some sort of mild after-burning in the 747’s by-pass ducts.

    So, although it’s not a technical show-stopper, it does tend to drive you to some non-trivial modifications that could prove very expensive, especially if it forces you to re-certify the aircraft!

    Dave Salt

  42. Lampyridae says:

    The Post A Comment form is all in Japanese, so here goes…

    Air launched SSTO is a worthy idea, and the Russians were seriously looking at it in the form of MAKS. You’ll notice that the An-225 has a twin tail specifically for the possibility of air launch, and I think the An-225’s cargo capacity is in the region of 200mT. The MAKS proposal had an external tank and used triprop engines to get into orbit.

    The Skylon spaceplane with its SABRE engine is about the only SSTO proposal that has any merit, and even then it costs something in the region of $10 billion to develop. There’s also a hypersonic transport study, LAPCAT, which would be something of a stepping stone and enable Skylon to see the light of day (fat chance at this point).

    Air-launching could also be used for high L/D hypersonic gliders using conventional rocket engines. That’s a big potential market if the business case could be made to work. This would push the envelope further than suborbital space tourism in terms of reliability and cost.

  43. Anonymous says:

    Great post Jon.

    I would add another significant advantage to this type of architecture compared to a standard VTOL arrangement. The entire aircraft/spacecraft system is assembled in a controlled space, indoors, and when launch time arrives you open the doors and go. The savings in support structures, labor, weather-related issues, etc. would be considerable.

    How much does NASA spend per SS launch on movement of the assembled system from VAB to pad, management of the system while on the pad, and all of the associated hardware? All of that is significantly reduced with this architecture.

  44. Karl Gallagher says:

    Quick comment on Single-Stage-to-Tether:
    The hook-up requires both ship and tether to be at the proper point in their trajectories simultaneously, +/- the reach of the hook or trapeze. That works out to a launch window of a fraction of a second. Not good for reliable ops.

  45. Formerly known as Skeptic says:

    Not applicable to commercial use, but have you ever looked at the feasibility of the B-1 as a carrier? Designed for heavy lift, high undercarriage, lots of thrust for the pop-up (could even potentially add a booster in the tail if you remove the defensive EW suite), and already built-in hardpoints in the form of the current bomb bays. Just a thought which has rattled around for some time.

  46. Anonymous says:

    Karl, it’s not quite that bad. Since there is typically a 30-40 minute coast from engine shutdown to tether rendezvous, you can can the shape of that coast to change your phasing by a few seconds. Doing a prograde trim burn early will make you get there a bit later, doing a +R burn gets you there sooner.

    (I’ve played with Orbiter quite a bit, and by adjusting velocity by a few m/s while observing both the encounter and synchronization MFD’s, you can adjust the encounter distance to zero. Having several minutes to do the adjustment helps a LOT.)

    This stretches the launch window to good ten seconds or so- a whole order of magnitude improvement! πŸ™‚

    -Doug Jones

  47. Anonymous says:

    Oops, I got the earlier/later tweaks backward. Been a while since I did some rendezvous practice.

    -Doug Jones, not entirely-fearless Rocket Plumber

  48. andy_janes says:

    I didn’t mean under the wing Jon, to clarify;

    What I was suggesting was cutting a hole in the botton of the fuselage for a RLV to drop out of. (depends on shape, will also need cuouts for wings) It would be mainly carried inside the cargo hold, which would get it out of the air so propellant boiloff would be minimised

    Andy

  49. Kirk Sorensen says:

    Quick comment on Single-Stage-to-Tether:
    The hook-up requires both ship and tether to be at the proper point in their trajectories simultaneously, +/- the reach of the hook or trapeze. That works out to a launch window of a fraction of a second. Not good for reliable ops.

    It can be done. NASA and Tennessee Tech proved that a few years ago:

    Catch Mechanism Demonstrated

  50. Karl Gallagher says:

    Kirk,
    I have no fears about the mechanical connection part. The issue is how tightly you have to time the launch to get both in the right place at the right time. Doug pointed out that the window gets wider as the DV fraction provided by the tether shrinks, but it’s still damn small. Delay your take-off by 20 seconds and the tether may miss you by so much you won’t even see it (well, you wouldn’t see a rotating one–a radial one probably would stay in sight a while).

Leave a Reply

Your email address will not be published. Required fields are marked *