The Bigelow stuff was very promising and showed that it could work. The larger units on extruded spokes was a viable path to a .5G space station. This would be doable with three (possibly 4) Starship launches[1].

[1] Caveat Starship has to reach its goal of transporting 100 tonnes to LEO

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zackmorris 4 hours ago | parent [-]

Ya I really like their airship to orbit concept. I asked AI about lift to drag (L/D) ratios in plasma at 10,000-17,5000 mph (5-8 km/s) and it suggested that lifting bodies achieve between about a 1:1 and 3:1 L/D ratio. If we assume the generous 3:1 L/D ratio, that would seem to make a single-stage to orbit space plane possible.

A bit off-topic, but an aerospike engine is half of a rocket nozzle, with a virtual half created by the supersonic shockwave. So we could envision a retractable nozzle half that moves through subsonic, transonic and supersonic modes to power the airship.

Also the SABRE engine uses (according to AI) 16,800 thin-walled tubes filled with liquid hydrogen to cool ambient air to -238 F (-150 C or 123 K) in 10 milliseconds so that it can be compressed up to 140 atmospheres and fed into a combined-cycle engine. That would allow it to be air-breathing up to mach 5.4 (3,600 mph or 1.6 km/s) and transition to liquid oxygen after leaving the atmosphere.

I also asked it about using something like titanium to withstand the heat of exiting the atmosphere (since the titanium SR-71 reached mach 3+) but it said that it can't withstand a high enough temperature. So an ablative coating might need to be applied between launches. Quite a bit of research was done for that through about the 1970s before NASA chose the space shuttle with its reusable tiles.

It seems like most of the hard work has already been done to achieve this. So I don't really understand why so many billions of dollars get devoted to other high-risk ventures like SpaceX. When for a comparatively smaller amount of money, a prototype spaceplane could be built. I'm guessing that the risk/reward value just wasn't proven yet. But really shouldn't VC money chase the biggest bet?

This is the kind of stuff that I went down rabbit holes for when I dreamed of winning the internet lottery. Now that AI is here, I can feel the opportunity for that slipping away. A more likely future is the democratization of problem solving, where everyone knows everything, but has little or no money and doesn't want to pay for anything. So really not much different from today. So maybe it's better to let these half-baked ideas go so that someone else can manifest them.

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pixl97 4 hours ago | parent | next [-]

Pretty sure it's Scott Manley that did an episode on the aerospike and just how hard they are to get to work right.

Needless to say, getting anything to go to space is hard.

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CableNinja an hour ago | parent [-]

Aerospikes are hard because you cant control the external pressure. At altitude A you have X atmospheric pressure, but at altitude B, Y pressure, that pressure is what keeps the exhaust against the surfave and exerting force, you can only design an aerospike for a certain effecient operational altitude and outside of that its just not great.

A nozzle engine doesnt have to account for this as much because the nozzle is keeping the pressure of the exhaust

	▲	dabluecaboose 21 minutes ago \| parent [-]
		Nozzle engines absolutely have to account for the external pressure. The optimal pressure as the exhaust leaves the bell should be as close as possible to ambient for full thrust. If the pressure at exhaust is higher than ambient, the exhaust pushes outward against the ambient pressure and you get huge exhaust plumes, and lost efficiency. Conversely, if the pressure at exhaust is lower, the ambient pressure pushes the exhaust inward into shock diamonds[1] and you, again, lose efficiency. Engine bells specifically yield their max efficiency at one external pressure/altitude. The reason you see shock diamonds is most often from ground-level testing (or takeoff) of engines that perform best at altitude. [1] https://en.wikipedia.org/wiki/Shock_diamond

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nkoren 2 hours ago | parent | prev [-]

Airship To Orbit is JP Aerospace, not Bigelow. It seems like an utterly bonkers and fairly implausible concept and I'm definitely not equipped to analyze its merits. But the JP team have some legitimate accomplishments in the rockoon world, and appear to be honest, hardworking people. Definitely not grifters. I've been following their work on ATO since they first announced it at a Space Access conference in ... 2003, I think? Still can't figure out whether it's real or not.