Ok, I’ve been toying with another orbital access methodology, but I wasn’t sure whether to file it under Random Thoughts (which tend to be my more half-baked, far-out ideas) or with the rest of the Orbital Access Methodologies series (which I’ve tried to keep a lot more professional/high-brow). This idea is actually an offshoot of two ideas I’ve posted about previously (air-launched glideforward TSTO and Fleet Launched Orbital Craft), along with the Boom Rendezvous idea I just wrote about.
Carrier Craft By-the-Slice
Anyhow, the key thing that led to this concept was the realization that for air-launched vehicles, you really want to be able to buy the carrier aircraft “by-the-slice”, instead of having to own it outright. Ie, you want to be able to call up Virgin Galactic and say “what’s your schedule for this week…ok, can I buy the noon-5pm slice on your WK2 next Thursday? Usual ammenities. We’ll meet you on the flight line at 12 o’clock sharp. Thanks!”
Especially for early-generation airlaunched RLVs, not having to pay the full burdened cost of an aircraft in addition to the rocket stages is really important. It’s even more important to avoid having to pay the development cost of a custom carrier craft. If you only have to rent the carrier craft when you need it, you are much more able to handle ups and downs in demand, which will likely be fairly volatile for the first few years. It also means that you can use the carrier plane during development without having to have that big of a capital investment early on. IIRC, this was one of the keys to making Zero-G’s parabolic services possible–the same aircraft can be converted to cargo-hauling overnight. I don’t know if they still use this capability, but not having to pay for the full aircraft just out of space revenues makes it easier to charge an attractive price.
The problem is that there aren’t any airlaunch carriers that you can “buy by the slice” that are big enough for an manned orbital RLV. That’s where exo-atmospheric refueling could come in.
Exo-Atmospheric Suborbital Refueling
Many years ago, people in the military realized that being able to refuel aircraft in-flight could greatly expand their operational capabilities. Of course, at the time that the idea had first been raised, mid-air refueling sounded about as crazy as exo-atmospheric refueling sounds today. The first mid-air refueling involved linking two biplanes, and having a guy walk from one aircraft to the other carrying a 5lb gas can! Of course, over time, much safer procedures and techniques were invented, and nowadays its easy to forget how insane this idea must have looked before it had been tried.
That said, while it might be possible in some cases to eliminate the need for mid-air refueling by using a bigger vehicle with larger propellant tanks, there are some missions that would be flat-out impossible without the capability. The typical response I get when I suggest something crazy is, “why don’t you just build a bigger rocket”. While there are some cases where you’d be better off just “building a bigger rocket”, there are some times where additional operational complexity more than pay for themselves. And I think the added complexity in this case is worth it if it buys you the ability to use an off-the-shelf carrier plane, bought by-the-slice, and keeping your rocket stage sizes small, and the rocket engine sizes in the low 10s of klb range, while still being able to feasibly deliver people to orbit.
So, what do I mean by exoatmospheric suborbital refueling?
Basically, it means that at some time after the vehicle leaves the main sensible atmosphere, it hooks up with another vehicle on the same trajectory, propellant is transferred from one vehicle to the other, and then the first vehicle continues on to orbit (while the mostly-empty tanker vehicle reenters and lands). [Note: Gary Hudson reminded me that Mitchell Burnside-Clapp of Pioneer Rocketplane investigated just such an enhancement to their system, during their work on the RASCAL project.]
The configuration that I’ve used in my analysis is a pair of TSTO vehicles launched off of a WK2. The first stage in both stacks is identical, and are roughly 25klb wet, 5klb dry. The orbiter and tanker stages are about 10klb wet each, with the orbiter upper stage launching a little more than half full, and the tanker stage having smaller tanks fully-loaded on takeoff. The tanker and orbital stage would be built to more aggressive mass fraction targets than the first stages. [Note: you can find a copy of the spreadsheet I used, in case it's helpful, here --Jon]
The operational procedure would be something like this:
- Both WK2’s head uprange to the appropriate drop zone, spaced as close as is reasonably possible–maybe 1km apart?
- The two TSTO stacks drop and light at the same time.
- During the first stage burn, the two stages slowly close the relative distance between each other, so that at staging they’re maybe 50-100m apart.
- As soon as the dynamic pressure is low enough (possibly even before staging), booms are extended between the two orbital stages.
- The first stages separate from the upper stages, and glide forward from the staging point to the launch field for landing and reuse.
- The upper stage and tanker stage would start their engines, while finalizing the boom connection. During this phase, the upper stage would be following its own trajectory, and the tanker stage trying to match.
- The booms have built in propellant transfer hoses, and a quick disconnect possibly like the one ULA proposed for cryo prop transfer based on their slip-joint duct design. The QD would engage, and as soon as it is sufficiently engaged, propellant would start flowing between the vehicles, either pump-fed or using differential pressure.
- At some point the upper stage gets tanked all the way up. If there’s still propellant to be transferred, the stages may stick together for a short period of time, with the transfer pump operating throttled back in a way so that the upper stage is not using any of its own propellant.
- Before the vehicles separate, the QD is disengaged, and the booms reeled back out a bit.
- Vehicles separate, upper stage continues on to orbit. Lower stage is still within a glide-based RTLS maneuver of the initial starting site, if the propellant transfer operation can be kept quick enough.
The nice thing about such a setup is that if you do things right, most worst-case failures result in an aborted mission, not a loss of vehicle. If one of the TSTO pairs doesn’t ignite when air-dropped, you abort (with the upper stage from that TSTO combo having enough propellant to make it home, and you only have to figure out what to do about the first stage). If you can’t mate up in time, you abort. If the QD doesn’t work, you abort. If you can’t keep the vehicles together exoatmospherically, at worst the boom/hose fails, and you use hydraulic fuses to keep that from becoming a loss of vehicle event. Now, there are many more things that can cause an abort in this scenario, but many of them are things that should get more reliable with practice. The nicest thing is that many of them can be practiced with first-gen suborbital RLVs without even requiring an air-launch.
Performance and other Observations
Here are some observations from my super low-fi simulations:
- Performance-wise, this system behaves like a quasi-3STO, giving you a bit of a benefit over a pure TSTO. For instance, in order to put the same sized upper stage into the same velocity and altitude as the upper stage has at the point the tanker leaves it, you would need something like an 82klb stack instead of two 35klb stacks.
- The quicker you can get propellant flowing, the better. The longer you have to throttle back to stretch out the refueling phase, the further you have to carry the tanker stage, which impacts performance. Ideally you’d like to have propellant transfer done within 90-120s of when the first stages separate.
- You probably want to throttle-back during prop transfer, this makes it so you’re blowing through less propellant during this phase when you still have a lot of dry mass you’re accelerating. This would come at the cost of either more lofting earlier on, or more gravity losses.
- Obviously more exotic propellants for the upper stages tend to provide better payloads, but the quasi-3STO benefits decrease.
- You want as much of the fueling subsystem mass on the tanker side as possible, since it’s the part that doesn’t get hauled all the way to orbit.
- You have to be able to pump enough propellant through the transfer hose over the course of the transfer to overwhelm the amount of propellant being used by the main propulsion system. You are pumping this against the tank backpressure. Both requirements suggest you probably want an actual pump instead of trying to use pressure feed.
At the end of the day, this would give you an orbital capable system, using the existing WK2 that had all stages fully reusable, and could carry at least 2-3 people.
How to Develop Exo-Atmospheric Refueling
There are three major challenge areas for fielding this technology:
- The formation flying GN&C development
- The rapid boom rendezvous system
- The actual propellant transfer interface
The interesting thing is that the second two systems can at least be tested out in the near-term without even reaching space. For instance, using two VTVL hovering vehicles (say Xombie and Xoie, or Xoie and SuperMod), you could fly the two together, demonstrate the boom connection and even swap some propellants operationally. These just require a relatively precise hovering mission, the two vehicles don’t even necessarily need to be actively cooperating. The next level would involve figuring out how to fly the vehicles in close formation at high speeds and altitudes. Finally once you had those, you’d reintroduce the exoatmospheric boom rendezvous step. And if it really looks like it can hook up fast enough, you can demo a little fluid transfer up near apogee, where the air density is low/practically-nonexistent, and the velocities are low. Only once you’ve developed and demonstrated that, do you try and build the full-up system.
The nice thing is that the incremental cost of flight tests for reusable suborbital vehicles should be really cheap compared to building orbital stages and TPS and other things, so once you have those systems available, it only makes sense to try it (especially if you can find someone crazy enough to pay you to try).
There are a few other benefits to trying something crazy like this:
- Once you’ve demonstrated exoatmospheric fuel and LOX transfer, people won’t be able to honestly question if orbital propellant transfer is feasible. This is the acid test.
- The rapid rendezvous techniques end up being very similar to what is needed for a suborbital RLV + MXER tether concept, so by developing this, you open the door for the latter.
- If demand picks up enough eventually to allow you to do a bigger carrier aircraft and bigger system so you don’t need exoatmospheric refueling, having the technique in-hand allows you to launch even bigger payloads in a pinch without having to develop a bigger system.
Complexity should generally be avoided, but sometimes complexity can make things easier, not harder. A comparable ground-launched TSTO system would likely weigh upwards of 300-500klb wet, and would likely cost more to develop. It would be a lot simpler, and a lot operationally easier, but its not clear that it would actually be more cost effective or profitable. Mid-air refueling is still mostly used by the military, but it’s an indispensable part of military operations today, even though it adds complexity. I think the case of exoatmospheric suborbital refueling will likewise be one of those crazy things that we wonder how we ever lived without.