Why does the Shuttle have Solid Rocket Boosters?

[Ian Curtis]

Alrighty, I've been searching all over the internet over the past few days, and can't find a satisfactory answer to this question.

Why does the Shuttle have Solid Rocket Boosters? And why do they provide more thrust than the External Tank? I was always under the impression that there was more thrust to be had in liquid rocket propellants as compared to solid ones.

I looked it up, and one potential reason I found was that the spent SRBs are simple (relative to liquid boosters, that is) and so can survive the fall into the ocean better. A downside to solid propellants is you can't turn them off once you start them, so if something starts going wrong, you've got to get out quick.

If the Solid Rocket Boosters can provide so much thrust, how come there aren't simply three of them to send the Shuttle skyward? And why do most (all?) launch vehicles use liquid stages when clearly so much thrust can come from solid propellants?

[Joe G.]

I believe that the liquid fuel engines are also used for maneuvering purposes while in orbit, and breaking orbit for reentry. Since, as you say, solid motors cannot be turned off, they would be impractical for this.

As someone with some experience in hobby rocketry, big ditto on solid motors being simpler, and more reliable if used right. Liquid fuel motors are considered a big no-no at most launches I have attended, even those that allow and encourage custom made solid fuel motors.

Cost may also be a factor.

[DonRotolo]

The SRBs are compact, simple in operation and stable in handling.

Liquid boosters need complex engines and support systems (pumps, storage bottles, valves, etc.) and have to be filled with relatively volatile stuff shortly before launch. While you can get more thrust from a liquid booster, it is physically larger.

So the advantages of a SRB include reliability and simplicity of use. Disadvantage as you mentioned is lack of throttle control.

There are proposals to switch to liquid boosters, but highly doubtful this generation of shuttles will ever consider them.

Don

[Leav]

you should read more about the design stage of the shuttle.

There was a big fight over the design. If I recall correctly almost no one at NASA wanted the SRBs, but they were required because the CIA or NSA or some other intelligence agency wanted the shuttle to be able to haul huge surveillance satellites into orbit.

This is mainly discussed in relation to the Challenger accident, because the SRB O-Rings failed.

I remember at least 2 Discovery shows talking about the original alternatives to the current shuttle design and how much the engineers were opposed to the final outcome.

-Leav

[Ian Curtis]

The SRBs are compact, simple in operation and stable in handling.

Liquid boosters need complex engines and support systems (pumps, storage bottles, valves, etc.) and have to be filled with relatively volatile stuff shortly before launch. While you can get more thrust from a liquid booster, it is physically larger.

So the advantages of a SRB include reliability and simplicity of use. Disadvantage as you mentioned is lack of throttle control.

There are proposals to switch to liquid boosters, but highly doubtful this generation of shuttles will ever consider them.

If solid propellants are so reliable and simple, why aren't they used exclusively for the shuttle? They generate more thrust than the external tank, and from this discussion seem relatively safe too. I understand the reasoning about being able to turn them on and off, but the external tank falls away too, and the shuttle uses the Orbiter engines to change orbits, doesn't it?

I think that's why I find this so confusing is it seems very at odds with itself. Why would they use liquid fuels for launch (from the earth to LEO at any rate) if the SRBs work so well?

[EricH]

I understand the reasoning about being able to turn them on and off, but the external tank falls away too, and the shuttle uses the Orbiter engines to change orbits, doesn't it?

I think that's why I find this so confusing is it seems very at odds with itself. Why would they use liquid fuels for launch (from the earth to LEO at any rate) if the SRBs work so well?The main engines are ONLY used during launch. After launch, they shut down for the duration.

However, there are those two pods right above the three main engines. Those have smaller, liquid-fueled engines and are used for larger orbital maneuvers. There's a technical name for them, but I can't remember it offhand. There are also the various steering engines at various places around the shuttle.

[Gdeaver]

Solid fuel motors can not be turned off. Once they are lit there is no turning back. The thrust curve from a solid can be preprogrammed by the grain geometry and chemical composition but they can not be modulated dynamically. Also, efforts to thrust vector solids have not been very successful. The liquid motors provide the throttling and thrust vectoring for stability and course corrections.The shuttle design restricted the acceleration to 3 g's. The solids provide the bulk of the thrust at low altitudes while the liquid is throttled back. As the solids burn out the liquid is throttled up allowing the accel to stay under 3 g's. There is no ideal geometry for a classic nozzle from sea level to a vacuum. The solid motor nozzle is optimized for low atmosphere and the liquid is optimized for low atmospheric pressure to orbit. Note that Rutan used a hybrid motor for thier effort. Hybrids are beginning to catch up. Hybrids are solid fuel and liquid oxidizer.

[David Brinza]

The main engines are ONLY used during launch. After launch, they shut down for the duration.

However, there are those two pods right above the three main engines. Those have smaller, liquid-fueled engines and are used for larger orbital maneuvers. There's a technical name for them, but I can't remember it offhand. There are also the various steering engines at various places around the shuttle.
Eric's referring to the Orbital Maneuvering System (OMS) engines, which are used to adjust the Space Shuttle's orbit after launch and to perform the re-entry burn.

The replacement launch vehicle for the Shuttle being developed for the Constellation program, Ares-1 has a first stage that is only a solid rocket. Originally, the Ares-1 second stage was going to use the Space Shuttle Main Engine (SSME), but concerns on lighting this engine in the air rather than on the ground drove NASA to use the J-2X engine. The J-2X is derived from the design of the J-2 engine that was used on Saturn V rockets that launched the Apollo missions to the moon.

[DonRotolo]

Solid fuel motors can not be turned off. Once they are lit there is no turning back. The thrust curve from a solid can be preprogrammed by the grain geometry and chemical composition but they can not be modulated dynamically.
Exactly. At the max-Q point (where the aerodynamic stresses on the airframe peak) the SSMEs are throttled way down, and once the Max-Q has passed (the atmosphere is thinning fast at that altitude) they can throttle back up. Wide throttle control is necessary, but not easy with SRBs, and the timing (of thrust variations) has to be set during SRB manufacture, limiting flexibility with launch weight (for example).