We're working on a few big "if"s today, but at the end of the day we have a fully reusable SSTO whose components can get hauled into or out of any major airport by a feasible airplane and which can lift 100klb to LEO in a Shuttle-plus payload bay.
So, what are the "if"s?
Let's start with the easy one. Y'all may be familiar with some of the Shuttle follow-on projects. This one is from Langley, a fully reusable TSTO from about 1992. The details aren't all that important, except that we have a size and a dry weight (153klb for the orbiter). I've found that, as far as back-of-the-envelope goes, long-crossrange orbiters scale with about the 1.64 power of length. So this gives us something to start with. We'll presume, with some additional fudge factors, that this holds approximately true.
Here comes the bigger "if".
There is reason to believe that hydrogen may be metastable in metallic form. Long story short, this would be pretty much the craziest chemical rocket fuel ever. Basically just atomic hydrogen combining with itself to form H2 as an exhaust product, with crazy energy density and
the lightest exhaust you can think of. Go watch this video
or read this paper
. A pure mH rocket would get an Isp on the order of 1700s, which is insane and incredible and sci-fi shit. The best we can do in real life is about 450s, and this is an exponential
detail. 900 is not twice as good as 450, but more like e^2 ~ 7.4x as good.
The problem with pure mH is that it's just too energetic. You'd melt your rocket no matter what you built it out of. We need to dilute it, dump a cooling agent into the exhaust. The PDF above suggested liquid hydrogen (Isp ~ 1000s) or water (Isp ~ 500s). Hydrogen is efficient, but it's damn voluminous. Water is nice and dense, but the Isp isn't as exciting. I wondered if we could do better, so I fired up CEQ
to see what we could do with a few other hydrogen-rich compounds: specifically, CH4 (methane) and NH3 (ammonia). It turns out with reasonable temperature limits, we can achieve about 750s with a methane-diluted metallic hydrogen rocket, with a bulk density about half that of water (the physicists suggest a mH density of about 700kg/m3, and liquid methane clocks in at about 420 kg/m3, with a 1:4 mixing ratio). This is basically cheating as far as rocketry goes, and any vaguely
plausible SSTO concept you care to name (VentureStar, Delta Clipper, SERV) would be dead simple with this fuel.
But those projects had some limitations I didn't like -- either they were hard to move the rocket around (compare to the real-life Shuttle atop a 747), or they had too limited a payload weight or volume. So I decided to draw up a "bimese" version of the Langley Shuttle-II, upsize it as necessary (not very much, about a 20% stretch), and fill one side of it with fuel. Both segments would fly all the way to orbit. The booster segment would return to the launch facility rather promptly (it need not be once-around, but there's no reason to leave it up there more than a day or so), and the payload segment would continue on to the mission orbit. Both segments are uncrewed and identical externally. Obviously, only the payload segment has a major bay, although both segments have a small bay containing a docking port (forward of the main bay). If you were flying a Hubble service mission, or whatever, you might fly the payload bay full of mission crap, and then dock a small crewed segment here to perform the mission.
The two segments stack on their 'backs' because the re-entry heating on the backshell is much less severe, and because the aerodynamic interference effects are less crazy. It's annoying that this means we can't access the payload once the system is stacked, but I chose to accept this downside.
There is, necessarily, some propellant cross-feed. Oh well, obviously the Shuttle had this, and it was fine. I envision propellant flowing from the booster segment to the payload segment through the aft structural members, which also would serve as hold-downs on the launch pad.
TPS is sort of handwaved as X-37-like. By 1992, we knew a lot more than we did when we built the real shuttle, and I think my 1.25x safety fudge factor on weight is very conservative and generous. Let me emphasize just how
conservative a design this is: no integral tanks, no complex, multi-lobed internal structures, just cylinders and cones and spheres. "Conservative" and "SSTO" rarely go together, but this fuel is great!
The final details:
Engines: 4x2 RS-25-sized, 2x2 AJ-10-sized OMS
Launch weight: 2.85Mlb
Empty weight: 250klb (per segment) - (342/242)^1.64 * 153klb * 1.25
Payload weight: 100klb to LEO
Payload dimensions: 60x25'
On-orbit delta-V: ~500m/s
A caveat to the payload bay: the OMS propellant kit has to fit in there. Basically, you can have either
40x25'. This is sort of a callback to some early stages of the Shuttle program, where some proposals offered either 60x15' or 30x22.5' on the same design (although it's not clear if you would have been able to easily refit a specific hull from one to the other). Here, it's just where we position our OMS fuel tanks (like the Shuttle EDO kit).
The booster segment could alternately fly alone and carry up to a 200klb payload to orbit on its back. For crew launch, you might envision flying something like a HL-42
up there, with a big abort motor and stuff. Of course, since we're fully reusable, you can fly much smaller payloads than the full capability of the launcher. Just don't fill it all the way up with fuel. On the other hand, you could fly a 400klb payload with two boosters flanking the payload assembly (comfortably more payload than a Saturn V). I suppose in principle you could fly a 600klb
payload with a three-way arrangement, although the wingspan would require the payload strongback to be at least 50' in diameter. I can't imagine ever needing to fly so much payload in one launch, but it's neat that the design would be capable. Again, this fuel is BASICALLY A CHEAT CODE.
One other minor detail: unlike the nice, friendly, harmless rocket exhausts we're used to, this exhaust would have a substantial mass fraction of hot, unreacted molecular hydrogen. The rest of it is hot, unreacted methane. Do not point at face. I'm assuming sufficient engineering effort and water deluge could make a launch pad withstand this, but it has concerned me to the point I've envisioned solid strap-on boosters for launch, and only lighting the mH-CH4 rocket once well airborne. An alternative would be to run regular methane-oxygen at launch, and switch over to methane-mH at altitude. I haven't worked out the math on this. It would be worse, but you could certainly do
it. The rocket might get 10% bigger or something for the same payload. Let's just figure that the pad engineers are very smart dudes.
Anyway, the drawing (WIP):
quite a bit bigger than the real Shuttle, and I figure the airborne transport would end up being something like Stratolaunch's carrier, with fuselages closer together (the wings of the shuttle going underneath the twin fuselages, and the winglets sitting just outside of them). Maybe I'll draw the carrier aircraft later