Comment by goku12
6 months ago
As the other commenter already pointed out, it's the mass that's the limiting factor here, not the cost. The key idea here is that rockets and jets need two things - a reaction mass and energy. Scramjets and other air breathing engines don't just take oxygen from the atmosphere. They derive much of the reaction mass also from it. Even the inert nitrogen absorbs heat from combustion and acts as reaction mass. The primary purpose of the fuel onboard is to provide the energy. It's contribution as reaction mass is only secondary (note that this is for air breathing engines). This is very evident in the case of turbofan engines, where much of the thrust is contributed by the uncombusted air from the fan.
A scramjet stage will be very light compared to an equivalent rocket stage, since it carries only the energy source (fuel) and not the full reaction mass. If this scramjet stage is able to impart a velocity close to the orbital velocity by the time it reaches the upper atmosphere, the subsequent rocket stage will have much less work to do to get it into orbit. And that translates to much less propellants (including oxidizer) and much less mass in the upper stage. It's not necessary to collect oxygen from the atmosphere to see an advantage.
Obviously, the raising of the perigee at apogee is going to need this rocket engine again. There are no launcher concepts that depend purely on scramjets.
What is the point of saving mass here? LOX is cheap, so it isn't the cost of the LOX. Does saving LOX make the vehicle cheaper? No... it increases the quantity of fuel needed, which (particularly if it's LH2) makes the empty vehicle much larger and more massive. This is doubly bad, since every last gram of that empty mass is taken to orbit, unlike the mass of LOX.
Minimizing fueled mass of the vehicle is a stupid thing to do. It's optimizing the wrong metric.
Scramjets also suffer from bad thrust/mass and thrust/$ ratios compared to rocket engines.
Overall scramjet launch vehicles are an example of pyrrhic engineering: even if one could make such a vehicle "work", no one would want it.
The point here is how it alleviates the problem of the tyranny of the rocket equation. But you seem to have made up your mind.
I'm looking at the issue deeply, not using the superficial reasoning you seem to be engaging in. This reminds me of "mark logic" where if you are promised pie in the sky you commit yourself to believing it even if it's a scam.
Scramjet launchers (and air breathing launchers in general) have been looked at extensively, for decades. They don't make any sense. The math just doesn't work. Here's an insightful post from the Old Usenet that illustrates the difficulty:
https://web.archive.org/web/20220131115058/http://www.island...
When reading that, remember that LOX is 20x the density of LH2 (or 17x the density of "slush hydrogen").
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