Ok, before I go into this latest blog post, I want to put a disclaimer up front:
- This idea is crazy.
- I’m not posting this because I think it’s the greatest idea since sliced bread.
- I don’t think that this the one and true way to get to space.
- Don’t try this at home.
- I don’t necessarily thing this is better than
- Oh, and did I mention I thought this idea was a bit crazy?
But even if it’s crazy, it is interesting. And it was developed by a guy with the same last name as me, Allen Goff of Novatia Labs (Sacramento, CA), so while it’s a bit crazy, it’s definitely clever.
The idea is what Allen called Fleet Launched Orbital Craft, or which another author calls “Separated Ascent Stage Launch Vehicles” (I’ll call it by Allen’s term “FLOC” from here on out). The idea is somewhat related to bimese and trimese launch vehicles. In a bimese launch vehicle, you have a TSTO vehicle where both stages are identical to each other. Both stages ignite for a vertical takeoff, and the ideas typically use propellant crossfeed to guarantee that one stage is still full when the other one’s propellants are almost depleted. At that point the empty stage separates and returns home while the other one proceeds on to orbit. Trimese approaches are similar, but use three identical stages. The theory being that by designing just one stage and using it several times, you can get a lower development cost, even if both stages are somewhat suboptimal.
FLOC is just the logical extension (or maybe reductio ad absurdum) of the Bimese/Trimese concept. You could technically call it an n-mese concept, where instead of only 2 or 3 similar stages you instead have “n” similar stages. John Carmack’s looking at one version of the n-mese concept (like OTRAG did in the past), but this one is different from John’s modular concept. The big difference between FLOC and a more traditional n-mese approach like John’s is that for FLOC, not all of the stages are attached when the vehicles are on the ground. An illustration from Chris Taylor’s AIAA article on the economics of the approach (AIAA 2006-4783) might clarify things a bit:
Basically, you have 2^n stages at takeoff, each of them paired together into a bimese configuration. They all takeoff together, all from right near each other (ie they probably all launch within 1-2 seconds of each other, and within 1-2km of each other). They fly as close together as is physically safe. They use propellant crossfeed to guarantee that one stage on each of the bimese pairs is still full when the other one runs dry. When those stages run dry, each bimese pair stages. The “empty” stages all return to the launch site (possibly using an airbreathing engine once back down to subsonic speed to cruise back). The full stages then perform a…wait for it…exoatmospheric rendezvous with each other, mechanically hook-up so they can operate as a new bimese pair, reestablish propellant crossfeed, and then continue on their way. You then lather, rinse, and repeat until your final stage ends up in orbit.
I mean, what could possibly go wrong?
Seriously though, as Chris points out in his AIAA paper (linked above), while the idea is totally crazy, it does have some interesting ramifications. Chris points out that using a fleet of 8-32 launch vehicles, you can place extremely large payloads into orbit using stages that have a propellant fraction similar to a 747 without having to use cryogenic propellants. Also, you can “tune” your payload to orbit (or your suborbital performance) by adding more or less bimese pairs. Using a 747 sized launch vehicle, they were predicting up to 200 metric tonnes (!!) of payload to orbit in a single launch campaign using 32 vehicles. That’s about twice as much as a Saturn V, with a stage design that’s so low performance it’s more like a commercial airplane than a rocketship. As I said previously, smaller payloads could be done as well using smaller numbers of stages.
How hard can doing a dozen exoatmospheric rendezvous be? I mean, you have about a 2 minute window for each rendezvous operation. That’s plenty of time….
Needless to say, there would need to be a lot of operations development and practice before such a system could become practical. The rendezvous happen outside of the atmosphere, which definitely helps a lot. And the launch vehicles can all launch from the same area at the same time, which also helps a lot. It’s a lot easier to do a rendezvous when you’re already flying along an almost identical trajectory at the same time. You’d probably need some sort of automated hold-down mechanism for the bimese pairs, or some way of starting up all the rockets in idle and automatically checking to make sure things are working on each vehicle before you commit to launch. You’d probably also want to do things like building in more thrust than the vehicles actually need (so that underperformance on one engine, or a premature engine shut down doesn’t risk the mission). Having a redundant bimese pair or two (depending on the number of stages you’re launching already) could also help. Another point that should be made is that in the 2 minute rendezvous window, you only have to get the vehicle pairs mated back together–you can actually hook up the propellant crossfeed after the engines have lit, so long as it happens before you burn about say 1/3 of the propellant of the combined vehicles. This allows you to take the process in two steps instead of having to do both all at once.
More importantly, I’d also like to see the rapid rendezvous and mating demonstrated a lot of times on a subscale basis before trying this for real. You could demonstrate at least some of the basics using cold-gas thrusters on armed robots on something like a Zero-G flight. That would at least allow you to get some of the basics of grappling and mating down in an environment where you could easily get 50-100+ attempts for only a few tens of thousands of dollars. The next step would be demonstrating with two subscale suborbital vehicles that you could consistently do the full rendezvous and mating operation exoatmospherically. With suborbital vehicles, you could start out with a more relaxed rendezvous window of say 5 minutes, and work your way down as you get the bugs worked out.
The good news is that a single one of these stages (or a single bimese pair) should have enough performance to perform a suborbital flight with a decent sized payload. You can then slowly work your way up from there. With a 4 stage configuration, you should be able to at least get to orbit with some payload (something light, probably in the 1-2 tonnes range) once you’ve demonstrated and debugged doing a single exoatmospheric rendezvous mission. After that, it’s mostly operations from there, working your way up to the point where you have enough reliability to reliably pull off larger missions.
Anyhow, for more details, read the paper. I just thought that while this idea was crazy, it was a very fun and interesting form of crazy, and does actually get you thinking.
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