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Not necessarily as much as you think. The issue is orbital velocity is an absolutely bonkers high velocity. Nothing can even come close to that velocity in th

Not necessarily as much as you think. The issue is orbital velocity is an absolutely bonkers high velocity. Nothing can even come close to that velocity in the atmosphere. The SR-71, fastest air-breathing aircraft ever flow topped out at around 1/9th of orbital velocity. Yes scramjets could certainly beat that speed, once they are developed but even if that doubles the speed of a SR-71 (which already was pushing the limits of heating) then you are still only doing 1/4th of the speed you need for orbit. So you have a bit of a catch-22 situation - You can have plenty of oxygen for your engines all around you, or you can have the speed you need for orbit - But not both at the same time. Yes, a scramjet can reduce the amount of oxidizer required (by a lot) but to do this you need the extra weight of wings and everything else you need for proper aerodynamic flight.

aeternum 2y ago
Right but the SR-71 had a pretty significant fuselage cross-section that is basically pure drag. If going for max efficiency, the entire nosecone of the rocket should probably be the air intake. Basically take one of the SR-71's engines put it vertical and mount the payload inside of it under the shock cone. TWR needs to be high enough to eliminate the need for wings. Some of the newer missiles like the BrahMos are designed like this and have a ramjet powered second stage to save on oxidizer. It doesn't reach anywhere near orbital speed, but with this design, the shock cone takes the majority of the heating vs. the SR-71 where there were many exposed parts and materials.
radu_floricica 2y ago
A booster stage solves exactly that. I noticed in the launch yesterday that the booster was a hog - it was bigger, but it also had a lot of engines and a lot of work to do, so it burned through its fuel faster. This particular configuration is set up for the separation to happen pretty high, but one can imagine different configurations as well - a bigger ship and a smaller, fuel only booster that works only as long as there's atmosphere. Or even better - keep the current setup but have a ring of air breathing engines in the booster that work as long as they can. There's a lot of stuff some extra engineering can do, given enough time and resources. For now, SpaceX is going for the biggest bang for the buck, and they have a very healthy aversion of complicated solutions. But in time, adding a few air breathing engines may become simple enough to be worth it.

aeternum 2y ago

Right but the SR-71 had a pretty significant fuselage cross-section that is basically pure drag. If going for max efficiency, the entire nosecone of the rocket should probably be the air intake. Basically take one of the SR-71's engines put it vertical and mount the payload inside of it under the shock cone. TWR needs to be high enough to eliminate the need for wings. Some of the newer missiles like the BrahMos are designed like this and have a ramjet powered second stage to save on oxidizer. It doesn't reach anywhere near orbital speed, but with this design, the shock cone takes the majority of the heating vs. the SR-71 where there were many exposed parts and materials.

radu_floricica 2y ago

A booster stage solves exactly that. I noticed in the launch yesterday that the booster was a hog - it was bigger, but it also had a lot of engines and a lot of work to do, so it burned through its fuel faster. This particular configuration is set up for the separation to happen pretty high, but one can imagine different configurations as well - a bigger ship and a smaller, fuel only booster that works only as long as there's atmosphere. Or even better - keep the current setup but have a ring of air breathing engines in the booster that work as long as they can. There's a lot of stuff some extra engineering can do, given enough time and resources. For now, SpaceX is going for the biggest bang for the buck, and they have a very healthy aversion of complicated solutions. But in time, adding a few air breathing engines may become simple enough to be worth it.