[Editor’s Note: It’s been too long since I wrote the first article in this series, and I wanted to write some more on this topic.Â My tendency to try to cram everything into one ginormous ominbus post has been almost completely preventing me from publishing anything original about space lately, so I’m going to try breaking this topic down further than I originally intended.]
The key points I previously touched on were why high flight rates are important for RLVs, and that attaining a high flight-rate would require both technology and market development.Â While there are many potential markets for launch vehicles that have been discussed over the years, there are three markets–people, propellants, and “provisions”–that I think are particularly suited to early commercial RLV efforts.Â
In this article I want to begin making my case for why I think that flying people is one of the most important RLV markets, by sharing an amusing analogy.
The Black Aluminum Analogy
One of the “fun” classes I took as a grad student was on analysis of composite structures. It was interesting, even though most of the class involved lots of matrix math as we worked our way through from first principles till we could understand how multi-layer composite structures behaved. One of the lectures near the end of the course discussed some more qualitative concepts now that we had a mathematical foundation. One of the ideas Professor Eastman drove home during that lecture was that composites weren’t just “black aluminum”. While it is often possible to make a composite that was roughly isotropic (say using chopped fibers in resin) and then use it as a drop-in replacement for an aluminum part, you’d be wasting a lot of the composite’s potential. To truly unlock the potential of composites, you have to understand and take advantage of their anisotropic nature.
For instance, composites allow you to put strength in the areas and directions you need it, while minimizing the strength in directions it isn’t needed. The upshot being that a properly designed composite part for a given application may, and probably should, look drastically different from an aluminum part for the same job. Another upshot is that there are some applications where what you need really doesn’t line-up well with the advantages of composites, and where an aluminum part might have been a much better choice (X33 LH2 tanks anyone?).
Black Aluminum and RLV Markets
I think this analogy is relevant to the discussion of how to use RLVs. It’s possible to use RLVs as though they were ELVs that just happen to be cheaper, or that just happen to come home from work at the end of the day. Most experienced ELV people I know who look at RLVs treat them that way. They talk about how “there aren’t enough payloads to justify developing an RLV today”, usually followed by a comment that “maybe in 20-30 years there might be”. By payloads, they tend to be thinking of satellites–because that’s one of the only things ELVs are any good for. And for a satellite, especially the way they’re done today, reusability is just a nuisance, unless it happens to make the flight cheaper. It’s just that much less payload available.
Now, this isn’t to say that the only thing RLVs have to offer is the potential of lower launch prices. Eventually, I think you’ll see satellites that take advantage of intact abort capabilities, the ability to do on-orbit checkout before release, etc. I’m just saying that for satellites most of the reusability stuff is of only secondary importance.
People though are different.
People more often then not will be flying round trips. For them, the recovery system isn’t some extraneous feature that is only useful if it makes things cheaper–it’s a fundamental part of the service. Being able to make it back home in one piece even when something goes wrong also tends to be more highly valued by breathing cargo.Â The interesting thing is that the needs of the personnel transport market actually turn some of the main “drawbacks” of RLVs into strong benefits.Â That recovery system is no longer “parasitic mass” that can’t be used for payload–Now it’s services already provided by the launch vehicle that don’t have to be deducted from the payload.Â Â Of course, it is possible for a manned RLV to carry its crew in a separate capsule just like an ELV, in which case you’d lose a lot of these benefits, but it’s never been obvious to me why that that approach makes any sense.
One corollary of this is that a manned RLV doesn’t need to be able to carry anywhere near as much nominal cargo capacity to carry people as an ELV would.Â Â Depending on the details, instead of needing 10-20klb worth of payload capacity for a 4-8 person capsule on an ELV, you might be able to fly a 2-3 person compliment with an RLV that has only 1000-3000lb worth of cargo capability.Â Â In fact, you could consider the Falcon 9/Dragon stack to actually be a ~6klb to orbit 3STO RLV, just as readily as a 20klb to orbit TSTO with a capsule on top. Of the systems you need for a manned spacecraft, most of them already need to exist for an RLV stage–TPS, landing systems, avionics, RCS, power systems and radiators, abort recovery systems, and possibly even some basic life support hardware (if you’re shipping pressurized/biological cargoes like some of the stuff Dragon will be shipping to ISS).
While it’s outside of the scope of this blog post, before I go on, it is worth mentioning that there are two technologies/techniques that accentuate the advantages of RLVs even further–tugs and fast rendezvous techniques. But that’s a discussion for a different day. It’s also worth mentioning that the “black aluminum” treatment of RLVs extends not just to how people think about using them, but also in how people think about developing them. But that is also a discussion for another day.
In my next post in this series, I’m going to discuss a counter-intuitive result that this line of thinking led me to.
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