Clark Lindsey has already commented on this paper, but I figured it was worth a little further discussion here, since I’ve already written a few articles about air-launched RLV ideas. Air launch is definitely an idea I am warming up to.
The paper covers a lot of ground we’ve discussed here in previous posts. Of course, that probably shouldn’t be surprising, since Gary and some of the others have given me help in writing those posts. They discuss a lot about the benefits of air launching, which we’ve been over previously, namely:
- higher efficiency propulsion due to lower atmospheric back pressure
- better orbital phasing allowing easier first-orbit rendezvous opportunities (often two per day from a given launch site)
- more control over launch point, allowing you to avoid populated areas
- less need for launch escape systems, and thus more benign aborts
- less gravity and drag losses (although they came to a different number than other sources I’ve read)
They also spend a decent part of the paper discussing various air-launch methodologies, and discussing the strengths and weaknesses of each. Some of the interesting takeaways from that discussion were:
- Tow Launch requires a large glider attached to the rocket in order to generate enough lift, and could have issues with takeoff aborts (which do happen), though they do reduce modifications required to the tow-plane compared to other approaches.
- Overslung launches tend to have more drag on the way up, thus decreasing maximum vehicle size. They also require either large wings or potentially risky maneuvers.
- Underslung launchers came out the best, but with the main limitation being ground clearance.
- The most capable option was a Scaled Composite conceptual design for a White Knight derivative vehicle capable of carrying 680,000lb (!!!) to an altitude of 50,000ft and at a speed of Mach 0.7. Unfortunately, they came to the same conclusion I came to in my previous posts–if they had to be the anchor tenant for a vehicle that big, the development cost would be too high. If there were other users though….
- Stilts or sled launch of modified airliners were both discussed. Both had issues either with landing loads in the case of the stilts, or with the incapability of aborted takeoffs with the sled.
- There was a trick described that would allow them to modify an existing 747 enough to give adequate ground clearance for a smaller diameter, three-core vehicle.
- The expected cost for a used and modified 747-200 was a lot lower than I had expected: $6-11M for the actual airplane, and an estimated $15-25M extra for the modifications. This with a payload capacity of roughly 240klb. While a custom-built WK3 might have more capacity, it would only make financial sense if there were multiple customers and you could wet-lease the aircraft.
There was also some interesting discussion of the Trapeze/Lanyard Air-Drop technique. This was the staging technique t/Space flight tested back during the initial CE&R studies back in 2005, before Griffin came in and decided to give us all The Shaft. Basically, a trapeze and lanyard keep the rocket from recontacting the aircraft, and cause it to drop, pivoting upwards. A few seconds later a drogue slows the rotation, the engine is lit, and then the rocket ascends, behind the flightpath of the carrier aircraft.
All in all, it seems like a pretty innovative idea. It’s too bad it wasn’t able to get the funding needed to take it into practice.
That said, for an RLV system there are some things it might make sense to do differently. First off, for an RLV, since you’re going to be reusing it multiple times, it might make more sense to go with a slightly higher performance feed system. Some form of expander cycle (turbo, piston, or pistonless) pump preferably. By reducing the pressure in the tanks (while also improving the density by quite a bit), you could get a much higher performance system that would allow them to get down to a single-launch for an ISS mission (instead of their current two-launch plus rendezvous approach). Sometimes a little extra performance is actually worthwhile, and this seems like one of those cases. Obviously this design would work best with a glide-forward TSTO style concept. It also might be worth going to a RL-10 powered LOX/LH2 reusable upper stage. Sure, the LH2 is a pain, but you would then have enough mass to spare to have a good Airborne Cryogenic Conditioning system that could keep the tanks topped off before launch. Also, these performance improvements might allow you to pull things back to a single-stick configuration while still keeping the required ground clearance.
Anyhow, that’s enough speculation. All told it’s quite an interesting idea, and if Scaled never gets around to building a SS3, it seems like a very interesting option worth reinvestigating.