Skribit Housecleaning: Virtual SSTO w/ Drop Tanks, Biamese Vehicles, and Sea Dragon

I want to clean out some of the Skribit suggestions that I’ve been neglecting for a while. Some of these may end up somewhat brief, but I wanted to at least try to be responsive.

Drop Tanks to Enable SSTO?
I don’t like drop tanks. While they do definitely make the rest of the SSTO easier, they come with several significant drawbacks:

  1. Large expendable hardware on an otherwise RLV somewhat misses the point.  While rocket tanks are relative cheap compared to the rest of the stage, they’re still pretty expensive compared to the propellant cost, or even the likely maintenance cost of the rest of the vehicle.
  2. Dropping stuff on people is usually considered somewhat anti-social.  That means you’re stuck launching over water like the ELVs (there might be one or two over land paths where the drop zone would be in a safely unpopulated area that isn’t a national park, but it’s unlikely).  Launching overwater means aborts have a much higher chance of costing you the launch vehicle.  It also likely limits you to existing ranges, and ties you in with their practices that may not be very conducive to RLV operations.
  3. You still get some of the drawbacks of TSTOs.  You still have to design and analyze two vehicles instead of one.  You still have to design and analyze a separation system.  You still have to integrate two stages together at the pad instead of dealing with just one.  You still have to figure out how you handle aborts (if possible) while the drop tank is on.
  4. Drop tanks reduce the “fluffiness” of the orbiter stage, which may complicate reentry TPS considerations (though maybe not enough to matter).
  5. The dry weight to orbit savings might not be as much as you think.  Propellant tanks are pretty lightweight for pump-fed vehicles.  Mechanical connections, separation systems, plumbing, pressurization systems, quick disconnects all add quite a bit of mass.   How much mass do you really save compared to just building slightly bigger tanks on the SSTO side?
  6. Drop tanks probably complicate the aerodynamics.  If you have one drop tank, you now have something like a Biamese vehicle (I’ll get to those in a minute), which has much more complicated aerodynamics, harder abort environments, etc.  If you go with a lot of smaller tanks, the aerodynamics becomes easier, but your scar weight to structures, plumbing, etc becomes a lot heavier.

That’s not saying they don’t have some advantages.  Any reduction in dry mass can go a long way towards making an SSTO shift from marginally infeasible to marginally feasible.  Expendable tanks can be designed to lower safety factors since they don’t need to consider fatigue issues like RLV tanks do.  Less dry mass means a lighter landing weight, which decreases the amount of landing propellants required and the weight of landing gear (or the wing and landing gear weight if you’re doing an HTHL design).

It’s not that they don’t have advantages, but the disadvantages are enough that I’m not convinced it pays for itself.

Biamese or Parallel Staged TSTOs
Even though my boss Dave is fan of Biamese approaches, I’ve never been.  He gives me crap about far-out stuff like FLOC, I give him crap about Biamese.

While once again there are enough benefits for Biamese vehicles to make them sound interesting, I think the drawbacks once again win out:

  1. The engines have to be able to operate from launch altitude all the way to vacuum.  While altitude compensation can often help TSTO RLV designs, and while there are some techniques like TAN and Flow Separation Control that might make it easier to have an engine operating over that wide of a regime, you’re still making some pretty big compromises to both stages (or you’re making compromises to the biamese concept and losing some of the benefits).  The upper stage now has to have far more thrust than it would’ve wanted anyway.  That means more weight that the upper stage didn’t need.  It also needs some way of operating its space-rated engines at low altitudes.  This means altitude compensation (which tends to be heavy and complicated), high pressures (which makes the engine harder to develop and more complicated, or something like TAN or Flow Separation Control, which it might not have otherwise needed.  If the booster and upper stage engines are the same (as they would be in a purist Biamese design), the booster engines now have to have a much higher expansion ratio than they would have otherwise.  Once again this leads to higher pressures or other complications that they might not have needed otherwise.
  2. A nearly 50/50 mass split is not very optimal for staging.  Especially when you figure in that the first stage wants to get back to the launch site.  It isn’t heinously bad, but it does mean you end up having moderately high delta-V requirements on both stages (since the first stage is likely going to be far enough downrange that you’ll need to do some sort of boostback).
  3. TPS requirements for the two stages are vastly different.  For a purist Biamese vehicle this means the booster is lugging around a lot of weight and complexity it really doesn’t need.
  4. If you don’t do a purist Biamese design (where the two stages are really identical and interchangeable), you’re back to designing two separate stages, but now with all sorts of unneccessary constraints.  The more divergences you make to simplify things (like going with a lower expansion ratio on the booster stage, or going with a lighter TPS on the booster stage), the more the two designs become different, and the more and more you start losing any real benefit from the process.
  5. Parallel staged vehicles have uglier aerodynamics.  Aerodynamic design and analysis for supersonic vehicles can be very complicated and expensive.  I’ve never seen a TSTO Biamese design that didn’t look like it would be a bear to analyze and design the control system for.
  6. Biamese RLVs tend to lead to compromised structural design.  Rocket vehicles are most weight efficient (and easiest to design and fabricate) when they are bodies of revolution.  In order to get good mechanical connections, most Biamese vehicles I’ve seen end up being lifting bodies, which starts driving either really weird propellant tank shapes (with added weight and fabrication complexity) or really inefficient structures (where the propellant tanks fit inside a more complicated shell.
  7. Because of the different operating modes of the stages, you’re really stuck still designing and analyzing three different vehicles (the two together, the first stage independently and the upper stage), not just one stage.
  8. You do potentially reduce the number of engines you need to make, and may allow you to design some subsystems only once, but now they’re being designed to meet more constraints.  Many times it’s easier to design two slightly different subsystems with 10 constraints each than one with 15.  You can still reuse a lot of the design and analysis work if you do things right, but each of the two designs are easier.

I guess to me it boils down to the fact that jacks of all trades really tend to be compromised kludges by the time they make it into operations.  In a Biamese system, both stages are carrying stuff they don’t need, and are being designed to more constraints than were necessary.  I really don’t see how that will lead to a cheaper system than one that has the two stages scaled the way that performance and operations want them to scale, and that can be more custom-suited for the task they’re being asked to perform.  Hybrids tend to give you the worst of both worlds.

Sea Dragon
For those of you not familiar with this concept, the Sea Dragon was an old Aerojet design by Bob Truax for putting 1,000,000lb of payload into orbit on a single TSTO launch vehicle (whose first stage might be recoverable). The design was a Big Dumb Booster, with pressure fed tanks made of maraging steels, built more like a submarine than a rocket vehicle. The first stage engine would’ve been something like 70x higher thrust than the F-1 engine on the Saturn-V. You can get more details here.

I’m a tiny bit more torn on this one than the others, but I still think it makes sense in today’s world.  It might make sense at some future date, but not right now.

Here’s my big concerns:

  1. Where’s the demand?  I don’t think we currently as a species launch a million lb or payload into orbit in a year.  Until other systems like RLVs get the cost down and the flight rate up there’s never going to be enough demand for more than one or maybe two of these per year.  While the marginal cost of one of these would be pretty low, the fixed costs and development costs aren’t going to be trivial, and they have to be amortized over those flights, cutting into any cost advantage the design might have.  Now, if RLVs do get the cost down to the point where you start having enough demand where Sea Dragon could make sense, you run into a different problem–the Sea Dragon is no longer competing against expensive existing ELVs, it would be competing against RLVs.  Sea Dragon may get stuck only launching payloads where the integration costs of launching them separately and putting them together in orbit outweigh the cost diffrence between the two.  Now, we live in a world where even though most stuff gets shipped in tiny intermodal containers, there are still Super Guppies and Belugas that get used occasionally.  In an RLV centric world, there may still be situations where a Super Heavy Launch Vehicle might be useful enough frequent enough to justify its existence.  But we’re nowhere near that point in time.
  2. What’s it going to be like developing and testing a 70 Mlbf rocket engine?  Pintles are a pretty cool, pretty scalable combustion system, but will they really scale up to something 70x bigger than has ever been built before?  We have no idea what unknowns lurk between here and there.  Maybe pintles will turn out to work fine without any problems, but we’re pushing far past what has ever been done in the past.   But pintles tend to get worse c* as they scale up, will they still have adequate performance at those scales?  Nobody knows, and nobody will know until they start.  That’s scary.
  3. Testing an engine this big is going to be mindbogglingly expensive as well.  Every second the engine would be going through about 5 Falcon-1’s worth of propellant.  That’s only $110k/s of propellant (the upper stage uses much more expensive propellants, so even though it’s only 8Mlbf, it’s still likely going to cost a lot), but that’s not counting anything else.  You’re talking about $25M per full-duration burn test.  With how expensive the payloads would likely be for a vehicle this big (see below), you’re likely going to need to do a lot of tests.  Just the injector testing alone for something like this would likely run you into the $1B+ range.  If you did even a fraction of the number of runs typically done in a rocket engine project, you’d be talking about billions of dollars up front.  And where are you going to test a monster that big?  You’d pretty much have to do it out at sea a long ways.  How are you going to vacuum test an engine the size of the upper stage engine?  I guess you can get away with not doing the full nozzle extension tests, but that’s still putting a lot of risk into the first few flights.
  4. Development costs would be insane.   Between testing the huge engines, and doing at least one or two flight tests, you’re likely talking several billion dollars to develop–if it’s done on a commercial basis!  The marginal cost of one of these things in 2009 dollars is likely going to be in the $1B range, so that starts adding up fast.  It’ll be a long time before there’s enough demand to justify putting up that kind of money.
  5. Payload costs per launch would likely be very high.  While I full-heartedly agree that relaxing mass constraints can reduce the cost of space payloads, it’s only one part of the cost involved.  Being able to go with welded steel and an FOS of 3 may reduce some design and fabrication costs a lot, you’ve still got the fact that unless you’re launching bulk commodities, you’re designing hardware that has to operate in a very harsh environment, that still needs to be fairly complex, and which very few will be built.  People talk about stuff like being able to launch an ISS in a single launch.  While avoiding all of the EVAs and integration stuff would take a lot of work out of ISS, you’d still be talking about a several billion dollar payload.  How would you get launch insurance?  How often can you afford to fly a vehicle that costs $500M-1B to launch and has payloads that will tend to cost several times more?

Unlike the other two ideas, I’m not convinced that super heavy launch vehicles will never have a place in the rocket world.  I’m just not convinced we’re even within visual range of such a time where they make sense.  We’re still back not too far past the Wright Flyer stage of launch vehicle design.  We’re nowhere near the point where a Beluga or Super Guppy makes sense.

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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

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 Commercial Space, Launch Vehicles, Space Transportation, Thrust Augmented Nozzles. Bookmark the permalink.

38 Responses to Skribit Housecleaning: Virtual SSTO w/ Drop Tanks, Biamese Vehicles, and Sea Dragon

  1. Martijn Meijering says:

    Smaller versions of the Sea Dragon, such as the ones Beal and especially Microcosm were considering may make more sense. They would be made from composites, not steel to keep weight down. You could even have the advantage of nontoxic, dense and storable propellants like kerosene or methanol and peroxide. I’m especially intrigued by the possibility of combining this with ramjets (not scramjets). With three stages this could dramatically reduce the delta-v requirements for a reusable crew vehicle mounted on top or off the side, giving more generous mass fractions and easier reuse.

  2. Martijn Meijering says:

    Drop tanks reduce the “fluffiness” of the orbiter stage, which may complicate reentry TPS considerations (though maybe not enough to matter).

    I don’t think this is true. Fluffiness is limited by delta-v/Isp not propellant density. You can make the crew/cargo compartment fluffy too. In fact that would be a good idea anyway. And drop tanks or multiple stages improve your delta-v/Isp. It looks like a net win to me.

    The dry weight to orbit savings might not be as much as you think. Propellant tanks are pretty lightweight for pump-fed vehicles.

    For a cheap pressure-fed composite launcher however, the difference would be substantial.

  3. Tim says:

    With regards to the biamese concept, I wonder if you could use a basic airframe/engine combination with a few bolt on components to allow you to optimise each unit for its particular job on a particular mission, but allow you the flexibility to use the basic vehicle for different roles in other missions. For instance, you might be able to attach an extended skirt to the orbiter stage but not the booster. Similarly, you might be able to remove the TPS from the booster stage if it is not needed (or of its an ablative TPS, simply not apply it to the booster stage).
    Would you actually have to test three vehicles? Surely for a pure biamese vehicle the first stage operating independently and the upper stage are the same vehicle in different flight regimes?

  4. I’ve really never understood what people liked about the Bimese concept. Langley tasked us to look at it back when I was in grad school at Georgia Tech and it was one of those things that seemed good on the surface but as soon as you peeled back the first layer it was a total mess.

  5. Kelly Starks says:

    I’m rather surprized by your reaction to the biamese idea? By making both stages identicle, you pretty much halve you design costs, cut servicing servicing costs a lot (easier to maintain a fleet of identicle craft), and frankly its pretty close to what we do now with the shuttles.

    A 50/50 weight split would have the stage seperation about where the shuttles drop their SRBs, both the US and Russian shuttles engines burned from pad to near orbit. etc.

    So say if you weanted to take the current orbiters, steach the hulls adn wings for LOx/Kero storage, put no RD-180s rather then SSMEs, adn attach 2 belly to belly with fuel cross feed, your set.

    Yeah, the lower stage carrys around TPS it doesn’t need as a booster (assuming the aero-thermal isn’t to bad before sep), but so what? Assuming you don’t use a dumb brittle TPS ilke shuttle did, its not a big cost driver. Capital costs to dev craft are, their nearly all the launch costs. Needing to develop 2 sep craft – even if fairly close in design, is close to twice as expensive as designing 1 that can do both. Servicnig 2 differnt optimized craft is allso a lot more expensive. So a biamese looks like a big cost saver – possibliy offering most of the cost savnigs of a SSTO versus a TSTO.

  6. Kelly Starks says:

    SeaDragon

    I thought both stages weer recoverable?

    As to 1,000,000 lb a year. I think so far Humanity has only launched about 12million(?) ni 50+ years.

    Like all CATS concepts, the tech not hard – but without a huge increaes in market demand, its not going to work.

  7. Pete says:

    Not that I am advocating Biamese, but I think it is more favored with air launch – avoids needing different engines and reduces the aerodynamic constraints.

    With regard to drop tanks, it might be worth leaving them in LEO. If they can be reused there, then the effective payload just got greatly increased. If tank cost per kilogram is less than payload launch cost per kilogram, this may be worth doing. Recently I heard some things about slightly lower strength but much lower cost carbon fiber (like less than $10/kg). This might also better enable a sea dragon…

    I have a suspicion that drop tanks can simplify vehicle design and reduce structural weight. If the tanks are side mounted ending at the engines then the load paths can be very clean and separate. This perhaps allows a very uncompromised, simple and light weight vehicle (orbiter) design. Integrated design has its appeal, but it can increase complexity, which is not necessarily desirable for an initial prototype vehicle which is all development effort. At small scale, I am not sure the added fluffiness of bringing the tanks back is worth the extra mass and complexity.

  8. Martijn,
    I was talking specifically about Sea Dragon, not about minimum cost designed vehicles in general. Sea Dragon isn’t the name of a class of vehicles, but of a specific design. Other, smaller MCD vehicles like Beal and Microcosm could reasonable work, and possibly be competitive in the current launch market (because they’re small enough that there are actual markets for their services). I don’t think any of them would be revolutionary cost-wise though. I could see a 2-3x drop in price compared to currently flying boosters, but no the 10x drop you’d need to really see a big change in the market.

    And ramjets/scramjets? Blech.

    ~Jon

  9. Bob Steinke says:

    As far as SSTO with drop tanks I tend to think of it more as TSTO with an expendable first stage that is made as cheap as possible (no engines, no electronics, no RCS, no TPS, etc.). My primary interest in this idea is that there are instances of expendable components in non-aerospace industries like disposable soda cans. I’m curious to see a detailed cost model that compares a fully reusable TSTO to a drop tank TSTO. Now, the major cost driver for reusable soda cans is recovering them from millions of individual consumers, which wouldn’t be an issue for a launch vehicle. So I don’t think drop tanks would actually be cheaper. I’m just curious to see how close it would be. I guess no one has the kind of cost models needed to get a credible answer to that for a new space style development program.

    As far as operational issues of where you drop the drop tanks I think it’s not much worse than a reusable RTLS first stage, but slightly more constrained. You have to have a trajectory where the first stage passively returns to the launch site so you are pretty much limited to pop-up or air-launch glide-forward (fall forward for drop tanks), but those are viable options.

    Kelly Starks,

    I think Jon’s point is that biamese doesn’t halve your development costs. For example, you only have to develop one TPS instead of two, but the one you have to develop is the hard one. The upper stage TPS might cost ten times as much to develop as a first stage TPS. So just using upper stage TPS on both vehicles will not cut your development cost in half. Likewise, developing one engine to operate well in both atmosphere and vacuum is going to cost more than half of two engines that are each simpler because they are optimized for only one environment. Or if you take Tim’s suggestion of different bolt on components for the two stages you are back to developing two engines.

  10. Paul Breed says:

    I’m personally very enamored with a miniature sea dragon as simple as possible, large for the payload delivered. Probably composite structures rather than steel, but otherwise holding to sea dragon MCD ideals.

  11. Eric Collins says:

    What if we were to take a slightly hybrid approach to this. Take a little bit of biamese, perhaps a little bit of drop tank, and maybe a little bit of flyback booster too.

    Imagine something like two or three small shuttle like craft, maybe about the size of the the Dream Chaser. These craft are side-mounted to a fuel tank about the size of the STS external fuel tank. At launch all of the engines are firing so that sufficient thrust is generated to get the whole thing off the ground. All of the craft are drawing fuel and oxidizer from the main tank. After the fuel load has been sufficiently depleted, one or more of the attached vehicles can separate and do a RTLS maneuver or land at some other convenient down range location. The one or more vehicles that continue can stage as many times as necessary such that at least one of them has sufficient fuel to make it all of the way to orbit.

    In a way, it’s sort of like a fly-back booster approach, except that the boosters are being fed by the external fuel tank, and they are fundamentally identical to the vehicle that actually makes it into orbit. So, not only will you likely have a fairly large fleet of the vehicles produced (amortizing their development costs), each and everyone of them should be designed to be able to perform a nominal (i.e. non-catastrophic) abort from anywhere in the launch trajectory. If sufficient performance is available, it may even be possible to carry the whole external fuel tank to orbit along with the last booster vehicle, thus making it possible to reuse or repurpose the entire stack.

    So, any thoughts on this kind of launch configuration?

  12. Martijn Meijering says:

    I don’t think any of them would be revolutionary cost-wise though. I could see a 2-3x drop in price compared to currently flying boosters, but no the 10x drop you’d need to really see a big change in the market.

    I’m surprised to hear that. Wouldn’t such a booster be little more than a glorified drop tank? No turbopumps, no igniters, no regenerative cooling, no gimbals, no throttling capability, no RCS, no pressurisation system, no avionics, no cryogenics, no insulation. Would the body itself be expensive to produce? No friction stir welding, no paper-thin walls. I imagine the composites wouldn’t be cheap compared to steel, but would composite tanks be more expensive than high tech Al-Li tanks?

  13. Kelly Starks says:

    > Bob Steinke
    >
    > As far as SSTO with drop tanks I tend to think of it more
    > as TSTO with an expendable first stage that is made as cheap
    > as possible==

    Nit – but it (and a biamese – and the shuttles) would be a 1+STO, not a TSTO, given the main craft would boost from take off to orbit (or near orbit in the case of the shuttle).

    Oh, this also impacts the argument about droping tanks on people. Drop them after your at high speed and near orbital speed. As long as they burn up – it doesn’t mater.

    >== I’m curious to see a detailed cost model that compares a
    > fully reusable TSTO to a drop tank TSTO.

    Tanks can be pretty cheap. excluding liquid hydrogen tanks (which are extreamly heavy and bulky) LOx or Kerosene tanks could be light say2% of the weight of fuel, and given the cost I’ve heard for shuttle tanks — under $100 per pound of cargo to orbit should be pretty easy to do. Maybe under $10 per pound of cargo if the tanks are smaller and mass produced. Given the margin costs for shuttle is about $1,200 ish per pound to orbit, thats not to bad. Results of the DC-X program suggest a well designed RLV could have only a couple dollars labor cost per cargo pound. Fuel/LOx costs should be under $10 per cargo pound.

    > RE: Kelly Starks,
    >
    > I think Jon’s point is that biamese doesn’t halve your
    > development costs. ==

    From what I’ve seen and heard about development costs it really should.

    >===For example, you only have to develop one TPS instead
    > of two, ==
    Eiather way you only need to design the one heavy duty TPS. Once you design the TPS – puting it on both craft doesn’t up your development costs at all.

    😉

    Using the same design eliminates all the expense for the design for the other crafts TPS, and modern TPS designs arn’t a big constructions or operations cost.

    > Likewise, developing one engine to operate well in both
    > atmosphere and vacuum is going to cost more than half of
    > two engines that are each simpler because they are optimized
    > for only one environment. ==

    Actually they needed cost much more at all. Look at how easily engine makers have addapted space worthy engines to surface take off, or vica versa. Certain types of rocket engines are hard to adapt – but aparently not ones in common use. It appears its actually much harder to design inflight start ability to a engine. Like the SSMEs that could not be adapted to work as a second stage engine. As apposed to the RL-10s which can work to both.

  14. Kelly Starks says:

    > Jonathan Goff
    >
    > == And ramjets/scramjets? Blech.

    ??
    What you don’t like light weight high ISP? NASA estimated RBCC engines would double the average ISP from take-off to orbit. For LOx Kerosene thats a ISP of 700, and a mass ratio of 3.2. Making even runway take of capable SSTOs pretty easy.

    If I was doing a start up – or developing a new engine, I’ld put adding at least some airbreather wraper to it. If you can’t do a ramjet around your rocket engine – you probably shouldn’t be trying to build the rocket engine.

  15. Paul Breed says:

    Can’t really test a ramjet bolted to a concrete block on the ground. So all testing has to be in flight at high speed, something like that is going to have associated costs….

    Now if you have a regularly flying RLV then adding a ramject experiment as part of the payload might make sense….

  16. Tom D says:

    I think we may be underestimating how much energy really needs to be imparted to a vehicle to launch all the way to orbit. Enough energy must be imparted that even Big Dumb Boosters need fairly sophisticated (i.e. expensive) structures and components. I went to part of the Cheap Access to Space Conference a few years ago and saw presentations for both suborbital and full-orbital rocket proposals. Both were for single stage vehicles. The difference in design sophistication needed couldn’t have been more stark.

    Pressure fed engines like what Microcosm proposed make structural design that much more difficult since the tanks themselves must be highly pressurized. The trick to Microcosm’s design is that their motor and tank design could (they believed) be mass produced relatively cheaply. That was probably the case. The hard part appears to be getting enough demand to justify mass production.

    If there is enough demand for mass production, then why not mass produce something that is a bit more expensive but much more capable like the Atlas V? The tragedy of NASA’s Ares launch vehicles is that they are neither very efficient when compared with likely alternatives, nor will they be mass produced.

  17. Pete says:

    At small scale, (say ~500kg payload or less), inflatable tanks can start to become favored due to minimum gauge constraints. Inflatable tanks probably want to be external so that one does not have to waste mass on building structure and heat shielding around them. So at small scale, I think drop tank equivalent layouts become increasingly attractive. Atlas type balloon tanks are I suspect also easier if external and structurally independent.

    In theory, small tanks can be proportionately lighter because the pressure head under acceleration reduces. Small (short) scale enables very light low pressure tanks, perhaps less than 1% of propellant mass. Tank mass is one of those areas where great weight reductions are possible. Also, as tank materials get stronger and lighter, new, different, potentially disruptive RLV designs are enabled.

  18. Kelly Starks says:

    > Paul Breed
    >
    > Can’t really test a ramjet bolted to a concrete block on the
    > ground. So all testing has to be in flight at high speed, ==

    Can’t test anything for flight abilities on the ground. Or you need real good sims.

    Though on the other hand, you can test if your rocket/ramjet hybride works bolted to the ground.

  19. Martijn Meijering says:

    Pressure fed engines like what Microcosm proposed make structural design that much more difficult since the tanks themselves must be highly pressurized.

    I got the impression it was the other way round: the constraints put on your structural design by the high internal pressure are much stronger than those imposed by structural loads, dynamic pressure etc. Pressure vessels are much stronger and stiffer than they would have to be to withstand launch loads. Especially with dense propellants. The price you pay for this is that your tanks are very heavy. Composites reduce the mass but are likely more expensive so you would have to trade back some of the cost benefits.

  20. Bob Steinke says:

    > Kelly Starks
    >
    > Tanks can be pretty cheap…

    Ok, I’m going to put some numbers on this. A 120 gallon, 600 psi burst pressure wellmate tank costs $949.

    http://ohiopurewater.com/shop/customer/product.php?productid=698&cat=353&page=1

    The current national average price for gasoline is $2.64/gal

    http://www.fuelgaugereport.com/

    If used in blowdown mode to hold 60 gallons of gasoline ($158.40) then the tank costs six times the cost of the fuel it holds. Probably not viable in the long term if launch costs get down to 3-4x fuel costs like airlines today. But maybe viable in the interim with launch costs around 100x fuel costs.

  21. I’m the one who put the SSTO/droptank suggestion in skribit. I really appreciate the feedback and the points made.

    I made the proposal based on the assumptions that is was a long-term ‘interim’ RLV system that was viable.

    I also made another important assumption:
    The drop tank would be used in conjunction with an orbiter with near SSTO capability. One vexing thing I’ve noticed about SSTO proposals and actual projects (like X-33) is that they were “all or nothing” propositions, which were subsequently dropped because they couldn’t meet the ‘purity test’ of all-out SSTO. Incorporating a droptank provision into the design from the inception ought to be a relatively simple way to solve the excess structural weight problem that ends up preventing pure SSTO. This means that the drop tank would only be needed in the boost phase, and could be dropped and reused in a recovery process simpler than the one for the Shuttle SRBs. The orbiter would still carry substantial on-board propellant, but less than half the total launch requirement.

    I am aware that, as a basic solid geometry problem, adding a drop tank to the design adds some additional initial strucxtural weight. Nevertheless, the tradeoff is a lighter, smaller orbiter.

  22. My alternate suggestion — which didn’t make it to skribit — to a droptank, is what I call “short-range, high accelleration boost.”

    The idea is, again, to wed the concept to an orbiter with near SSTO capability. The concept amounts to a “virtual rocket sled launch rail” system. Like the launch rail ideas, a high rate of speed is achieved in a short distance, with a trade-off between accelleration (G-forces), and speed. Typically, the up-the-side-of-a-mountain proposals call for Mach 2 at the end of a 2.5 mile rail and 6 Gs.

    What I had in mind was relatively small boosters providing a ‘virtual, vertical (or high angle of attack) sled/rail system, but without either sled or rail. By providing a boost to Mach 2-3 at anywhere from 4.5 to 6 Gs to anywhere from 3 to 6 miles altitude, you have a booster system that can be recovered at relatively short downrange, possibly even onsite of a likely launch range. You also don’t have to build a rail system at every launch site, nor find a suitable mountain, or engage in mega-engineering contruction.

    In other words, it turns the TSTO concept on its head, with the off-the-pad assist boosters being the smaller element, and the orbiter the larger. Another way to look at it is as a “Super JATO” system.

    I think it would be operationally simpler and less costly than TSTO.

    There is again this assumption: it is intended as a means of insuring orbit for a near-SSTO-capable orbiter.

  23. Bob Steinke says:

    > Roderick Reilly
    >
    > The drop tank would be used in conjunction with an orbiter with
    > near SSTO capability.
    >
    > Another way to look at it is as a “Super JATO” system.
    >
    > it is intended as a means of insuring orbit for a near-SSTO-
    > capable orbiter.

    Jon, I think Roderick has an operational advantage that you missed. It eases the evolutionary path from TSTO to SSTO. After you’ve been successfully flying TSTO for many years you go to investors and ask for $100M to develop an SSTO, but they say, “If you miss your mass targets by 3% you won’t make orbit, too risky for me.” Instead you have an SSTO + droptank design where the drop tanks are your performance margin and with evolutionary performance improvements you will grow out of them.

  24. Kelly Starks says:

    >Bob Steinke
    >
    >> Kelly Starks
    >>
    >> Tanks can be pretty cheap…
    >
    > Ok, I’m going to put some numbers on this. A 120 gallon,
    > 600 psi burst pressure wellmate tank costs $949.==

    And why would you use that? Hell the shuttle tanks are (ignoring overhead which is another hornets nest) a couple milion, for 650 ish tons of LOx and 80 of LH. Or something like a dollar per pound of fuel.

  25. Kelly Starks says:

    > Roderick Reilly
    >
    > ==
    > One vexing thing I’ve noticed about SSTO proposals and actual
    > projects (like X-33) is that they were “all or nothing” propositions,
    > which were subsequently dropped because they couldn’t meet
    > the ‘purity test’ of all-out SSTO. ==

    Thats not actually true. Bioth the DC-X and X-33 had fall back designs. If they ran a little heavy for pure SSTO, they could add in small solid boosters to give them a little boost off the pad. McDonnel Douglas all so had evaluated it for occasional super heavy loads.

    DC-X and X-33 were abandoned for political reasons, not really technical.

    > ..By providing a boost to Mach 2-3…
    A all rocket SSTO burns up half its total reaction mass by Mach 6, so carrying it to Mach 3 could save almost 1/3rd the fuel/LOx from takeoff. (Thats why adding ramjets make such a huge difference.)

  26. “”””” Thats not actually true. Bioth the DC-X and X-33 had fall back designs. If they ran a little heavy for pure SSTO, they could add in small solid boosters to give them a little boost off the pad. McDonnel Douglas all so had evaluated it for occasional super heavy loads. “””””

    Thank you, I stand corrected. And I was on the admin staff for DC-X!

    That is essentially a variant of what I was talking about.

    There is one thing, though: part of the “political” reason for the demise of X-33 is because it required additional modification to achieve orbit.

    “””” A all rocket SSTO burns up half its total reaction mass by Mach 6, so carrying it to Mach 3 could save almost 1/3rd the fuel/LOx from takeoff. “””””

    Which was a major rationale for my proposal.

  27. Bob Steinke says:

    >>> Tanks can be pretty cheap…
    >>
    >> Ok, I’m going to put some numbers on this. A 120 gallon,
    >> 600 psi burst pressure wellmate tank costs $949.==
    >
    > And why would you use that? Hell the shuttle tanks are (ignoring > overhead which is another hornets nest) a couple milion, for 650 > ish tons of LOx and 80 of LH. Or something like a dollar per pound > of fuel.

    I was looking for a mass produced tank to get a rough order of magnitude on the ratio between the cost of a tank and the cost of the propellant it holds. 60 gallons of gasoline is about 360 pounds, so the wellmate tank would be $2.63 per pound of fuel. Well within an order of magnitude of your dollar per pound for the shuttle ET. There’s probably some economies of scale as you go to larger tanks. It was just meant to be a rough estimate with at least some evidence behind it.

  28. Martijn Meijering says:

    Can’t really test a ramjet bolted to a concrete block on the ground.

    Couldn’t you use a compressor + diffuser/nozzle upstream of your ramjet inlet for ground testing? That might not be cheap either of course.

  29. Kelly Starks says:

    > Roderick Reilly

    >> Thats not actually true. Bioth the DC-X and X-33 had fall
    >> back designs. If they ran a little heavy for pure SSTO, they
    >> could add in small solid boosters to give them a little boost
    >> off the pad. McDonnel Douglas all so had evaluated it for
    >> occasional super heavy loads. “””””
    >
    > Thank you, I stand corrected. And I was on the admin staff for DC-X!

    really? Smal world. A friend of mine from JSC (Rick Jurmain) was on DC-X for a while – I wound up on Spacestation and later Office of space access durring the X-33 mess.

    > That is essentially a variant of what I was talking about.

    Yup – though carrying drop tanks is probably better cheaper safer, then strap on solids.

    😉

    > There is one thing, though: part of the “political” reason for the
    > demise of X-33 is because it required additional modification to
    > achieve orbit.

    Eh, X-33 was never supposed to go to orbit. NASA paid a extra Billion to stop L/M from trying a orbital version. Mainly they got to into games. X-33 was loaded with cool new tech a SSTO wouldn’t really need – and L/M lost interest when they realized their was no possibility of a sale. So when L/M took a cheap route to make the composite tanks (that were heavier then the Aluminum/Lithium alternate tanks), it failed, and was the final excuse to kill the program.

    Really – X-33 is a great case study of why NASA should bein charge of developing space ships or X-craft.

  30. Kelly Starks says:

    >Martijn Meijering
    >
    >> Can’t really test a ramjet bolted to a concrete block on the ground.
    >
    > Couldn’t you use a compressor + diffuser/nozzle upstream of
    > your ramjet inlet for ground testing? That might not be cheap
    > either of course.

    Testing at higher altitudes adn high speeds could be complicated — or you could borrow a wind tunnel.

    😉

    This could really make building a SSTO a snap though.

  31. #29. Kelly Starks:

    “”””” Eh, X-33 was never supposed to go to orbit. “””””

    I stand corrected again. I was confusing X-33 with the intended follow-on: Venturestar, which was intended to be SSTO, as I recall.

  32. This thread may be played out now, but I wanted to ask what you guys think of this ‘ALICE’ (nanoaluminum powder and water ice) propellant concept?

    http://www.nasa.gov/home/hqnews/2009/aug/HQ_09-194_ALICE.html

    There are several variants, including a gel/slurry liquid propellant concept. Supposedly, in theory, the concept could be pushed up to a high performance thrust and ISP rating.

  33. Martijn Meijering says:

    I don’t see why you would want to use ice for an Earth launch vehicle, but there may be a place for metal or carbon loaded gels.

  34. A_M_Swallow says:

    Ice was used rather than liquid water because metals separates out and falls to the bottom of the tank. The school was doing a pretend lunar launch using a mono-propellant and they had heard that there was ice on the Moon.

    On the Moon an alternative to ice is solid or slurry oxygen.

  35. Martijn Meijering says:

    On the moon that makes sense, but on Earth using a fuel in gel form would be more useful since it would give you better performance. It wouldn’t melt either.

  36. Brad says:

    Regarding Sea Dragon

    1,000,000 lbs payload to orbit is no doubt excessive. But a Sea Dragon technique for Saturn V class HLV payloads seems very promising to me, and a cheaper way for a nation to achieve that level of performance. When the development paths of the Saturn V, N-1 and (shudder) Ares V are considered, it seems to me the more sea based a large vehicle is the more successful it is.

  37. Nels Anderson says:

    If anybody’s still reading this thread, I’m wondering about the weight impact of drop tanks from a structural point of view. One of the points that John Whitehead made in his 1996 SSTO paper at the Joint Propulsion Conference was that it’s important to minimize the amount of structure that does not benefit from pressure support. In other words, things like interstages and, when you don’t have commonn bulkheads, inter-tank sections can be quite heavy.

    Let’s say I build a nice, clean single-stage vehicle with a structure consisting mostly of tankage which weighs 1% of its volume in water (Whitehead’s assumption). Then I decide to strap two drop tanks onto the side. Say each has half the capacity of the original tankage. Now I’ve got those two big loads hanging off the side, and the loads on the core vehicle are no longer cylindrically symmetric. About how much weight am I going to have add to the core vehicle to support the drop tanks?

  38. Jonathan Goff Jonathan Goff says:

    Nels,
    Good point. If they’re actual drop tanks, you have to react the whole inertial load of the propellants in those tanks through some sort of structure on the vehicle, which is going to cut even further into the benefit.

    ~Jon

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