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Considering that there is plenty of oxygen in the air at the altitudes that the SRBs flew to (<50 KM), why did the STS SRBs use solid propellant. Specifically, why use a system that must carry its own oxidizer as opposed to a jet engine?

My guess is that solid fuel was used due to the amazing thrust those things provided (12,000 kN). However, a hypothetical jet engine would be able to throttle or even shut down, increasing crew safety. Unlike the first stages of other launch systems, the SRBs even had an exposed surface in the direction of flight, providing a convenient air intake.

I suppose also that the salt-water recovery might also be an issue with jet engines.

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    $\begingroup$ Jet engines are good to maybe 25 km altitude, air pressure is too low to operate them much above that. $\endgroup$ – Hobbes Dec 8 '14 at 18:09
  • $\begingroup$ On the way down, Baikal is an idea by the Russian rocket company Khrunichev to use jet engines on winged versions of the rocket boosters on their new Angara family of rockets, in order to reuse them by landing them horizontally like ordinary aircrafts on airports. They proposed the concept already for use with Energia in the 1980's. $\endgroup$ – LocalFluff Dec 8 '14 at 18:59
  • $\begingroup$ @LocalFluff Many things were proposed. Few delivered. Buran's test model had jet engines for practicing landings. (Actually built). $\endgroup$ – geoffc Dec 8 '14 at 21:50
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    $\begingroup$ Well, it works in KSP. $\endgroup$ – Thomas Dec 9 '14 at 15:54
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Rockets and jet engines are vaguely similar in that they both burn fuel and oxidizer and use the exhaust out the back end to generate thrust.

Turns out rockets are just way better at it.

Also, 2.8 million lbs of thrust per SRB is so mind-numbingly large an amount of thrust it is hard to visualize.

The largest liquid fueled engines ever really produced and used are the US F-1 (Saturn V had 5 of them) at 1.5-1.8 million lbs of thrust and the Russian RD-170 (Ukranian Zenit used one, cut in half it is the RD-180 (Atlas V), in quarters it is the RD-190 (South Korean booster, and as RD-193 for export in Antares 201) at about 1.8 million lbs of thrust.

The Boeing 777 engines are in the 100 Klbs thrust level and they are some of the largest jet engines built (by thrust).

So 100,000 vs 2,800,000 which is at least one order of magnitude, and then a half again. That is a HUGE difference. The size of the 777 engine is vaguely the diameter of a 737's main body. So imagine if it scaled linearly in terms of diameter.

The SRB segments are narrow enough that they can travel by train. So no more than 10-15 feet. How wide would a 2.8 million lb thrust jet engine be? Can you imagine the spinning blades issues? Are there actually materials strong enough to spin at the size and speed required that would not self destruct?

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    $\begingroup$ More engines = more points of failure. You'd need 24 engines each twice as big as the GE90. $\endgroup$ – Russell Borogove Dec 8 '14 at 16:07
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    $\begingroup$ Correct answer, of course, but it does leave open why the 777 doesn't use two small rockets if they're so efficient. (A: Rockets aren't efficient ISP-wise.) $\endgroup$ – MSalters Dec 9 '14 at 1:38
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    $\begingroup$ One problem with this argument is that turbofan engines are designed for efficiency foremost, not thrust. These engines are large not because they need to produce thrust, but because it is both quieter and more fuel efficient to make them that way. If you were designing a "jet engine" for a rocket, you'd probably start with a turbojet (which has precedent for use in cruise missiles).¶ The larger problems here are complexity and reciprocating mass. $\endgroup$ – imallett Dec 9 '14 at 7:11
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    $\begingroup$ Yes, I would accept that - the reaction force propels the aircraft, not the exhaust. In a jet engine, the reaction force occurs at the impeller, not at the rear of the engine. The reaction force occurs at the point where the gases are accelerated, not the point where they are expelled. I agree it's a minor technicality but misunderstanding of it leads to things like "the blast deflector gives the plane something to push on" which I've heard more than once. And it avoids the error of "rockets are used because they don't need to push on anything" as if jets do. $\endgroup$ – Jasmine Dec 9 '14 at 18:33
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    $\begingroup$ @LorenPechtel I concede that point of course. That is why they are called high-bypass turbofans. But we are not talking about 50% more power, we are talking about 28 times more power, or 2800% more. It does not seem like Jet technology would scale that high. $\endgroup$ – geoffc Aug 7 '15 at 0:43
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One factor that would likely disqualify a jet engine for consideration is that they can't operate over such a large speed range. The SRBs propel the orbiter to about 3,500 mph (Mach 5.3). Ramjets and scramjets can operate at this speed, but can not be operated at a zero airspeed when the vehicle needs an enormous amount of thrust to leave the pad.

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