I'm enjoying the conversation too! Your point about the scaling properties making LH2 more favourable for larger vehicles is something I hadn't properly appreciated in those terms, but is certainly correct. As far as T/W goes, I think that you do need to be able to pull 3G off the tower, and moreover do it with one propulsion system (or cluster of identical systems). Again from a systems-engineering point of view, the point where a launch vehicle design jumps the shark is where you're strapping together completely disparate types of rockets to get the desired effect. I don't think that an LH2 engine has ever actually done this, although the the X-33 designs perhaps could have worked. (I always thought that the Venturestar was emblematic of everything that can go wrong with an LH2-driven design, since it basically turned into a (non-)flying kitchen sink exposition. But the DC-Y was elegant enough; I would have loved to see somebody make an honest attempt at that.)
So basically, it sounds like there are three things required to make a cryogenic first stage/RLV viable:
1. A market for large payloads -- large enough to allow the scaling laws to work out in LH2's favour -- with sufficient frequency to pay off the development costs. As such, this doesn't exist yet, but it's certainly conceivable that someday it could.
2. New types of LH2 engine designs, which are much more durable than SSMEs (which I guess would be the current benchmark of LOX/LH2 durability? Or would that be RL-10s?) Presumably with some substantial revolutions in materials sciences. This may be well underway already (as you allude), but is not public info yet.
3. New, cheaper ways of generating hydrogen. As long as the lowest-cost method is steam reformation, then hydrocarbons will, by definition, be cheaper. If that's the case, then you'd end up in a curious situation where LH2 might only be economical in the middle of the spectrum. In an ELV which is primarily concerned with capital costs, hydrocarbons clearly are more more cost-effective, since they're much easier to develop and build. On the other hand, in an RLV which achieves true airline-like operations -- where the cost of fuel begins to become a meaningful concern, which it currently is not -- then hydrocarbons win again. That might only leave a thin band in the middle where LH2-based designs are the most cost-effective. On the other hand, a new low-cost hydrogen-generation technique (eg., bacteria that exhale it) could change the game.
So yes, I guess I'd echo your sentiment that this is something that could be relevant in a decade, if all three of those items develop in the right way. Definitely promising enough to merit some R&D, but not yet assured or inevitable.
Agree on all counts, especially 2. Funny you should mention the [LOX/LH2 Single Stage To Orbit] DC-Y! If you can only have one stage, Hydrogen is also very attractive. I agree with you that someone should make a proper go of it because it's possible and it makes a lot of sense. It was actually this realisation that prompted me to change career and work where I now do. (I am British, work in propulsion research, especially Oxygen (be it stored liquid or from the air...) with Hydrogen, am interested in reusable space vehicles, and I think wings are A Good Thing, and I live in the city where you did your MBA. As a fellow space enthusiast that's hopefully enough for you to figure it out :D ).
Oho! That is indeed a very legitimate and exciting piece of technology which you're working on! Lucky you!
(Just to be clear, I'm not remotely an actual rocket scientist. I'm a transport planner / urban designer who specialises in Personal Rapid Transit systems, and starts companies on the side for fun; I've just spent an inordinate amount of time in pubs with fairly eminent rocket scientists, and have acquired an Opinion or two along the way. Plus, I need to keep one foot planted firmly in that world, so that once I've made my fortune elsewhere, I can move on to colonising the inner solar system without delay...).
Personally, I've always tended to be more of a VTVL guy, but anyone who can make wings work certainly has my support. In the case of of Reaction Engines / Skylon, I have to confess a bit of skepticism -- not from a technical point of view (the credentials and capabilities of the team are superb), but from a business-plan point of view. I've done various offhand models to get it to work, and the financing costs always kill it. It only works if you presuppose A.) an existing launch market of about 250-500 Skylon-class payloads per year, and B.) that SpaceX or Blue Origin haven't succeeded in lowering launch costs with their flyback boosters etc.
In contrast, SpaceX has a much more bootstrap-able evolutionary path, which allows them to largely avoid financing costs, and to theoretically get their customer prices closer to their marginal costs much more quickly. My feeling has always been that Reaction Engines ought to focus more on the possibilities for aviation, since it is a large enough market that it could absorb the R&D / finance costs much more easily. Once you're turning a profit in that market, then use it to bootstrap Skylon.
This gotta be one of the most interesting technical conversations that I have seen on HN!! Since you both seems to have a lot of knowledge about propulsion systems, I wanted to ask you about advice on how to get into the field. I am an aerospace undergrad graduating next year. I am thinking of going to grad school (undecided between masters or phd) and propulsion is one of the areas that I would like to work on.
Well, I'm not at all in the field -- armchair quarterbacking it is just an old hobby for me. But if you're in America, my advice would be: go to Space Access every April, and plan on spending your summers in Mojave. That's how most of the people I know have done it.
So basically, it sounds like there are three things required to make a cryogenic first stage/RLV viable:
1. A market for large payloads -- large enough to allow the scaling laws to work out in LH2's favour -- with sufficient frequency to pay off the development costs. As such, this doesn't exist yet, but it's certainly conceivable that someday it could.
2. New types of LH2 engine designs, which are much more durable than SSMEs (which I guess would be the current benchmark of LOX/LH2 durability? Or would that be RL-10s?) Presumably with some substantial revolutions in materials sciences. This may be well underway already (as you allude), but is not public info yet.
3. New, cheaper ways of generating hydrogen. As long as the lowest-cost method is steam reformation, then hydrocarbons will, by definition, be cheaper. If that's the case, then you'd end up in a curious situation where LH2 might only be economical in the middle of the spectrum. In an ELV which is primarily concerned with capital costs, hydrocarbons clearly are more more cost-effective, since they're much easier to develop and build. On the other hand, in an RLV which achieves true airline-like operations -- where the cost of fuel begins to become a meaningful concern, which it currently is not -- then hydrocarbons win again. That might only leave a thin band in the middle where LH2-based designs are the most cost-effective. On the other hand, a new low-cost hydrogen-generation technique (eg., bacteria that exhale it) could change the game.
So yes, I guess I'd echo your sentiment that this is something that could be relevant in a decade, if all three of those items develop in the right way. Definitely promising enough to merit some R&D, but not yet assured or inevitable.