SRB With Throttle?

[Redrobin]

Could we make something on a SRB that changes the nozzle size, and thus changing the thrust produced while flying?

[Camacha]

I am not sure that nozzle size would change that much, to be honest. The same amount of fuel would still be burned and that needs to go somewhere - you would rather have it shooting out the back than out of another part

I think it would also be pretty hard to make a material that is capable of resisting the full brunt of a SRB, although the vectoring capabilities or the space shuttle boosters suggest that limited control of the burning material is possible.

[toric5]

shouldn't this be in the science labs?

[Vanamonde]

shouldn't this be in the science labs?

Yeah, probably.

[magnemoe]

You can design an SRB to have variable trust during the burn, say high trust at launch then an fast drop in trust as you estimate you get close to terminal speed, and then optimal trust until its almost burned off. If you use the SRB as an an stage and not an booster you typically want trust to tapper off as weight go down.

You can kill trust with blow out panels, basically large ports in the side of the rocket who are opened, this pretty much kill all trust as is used at least on ICBM who must be able to be aimed at targets at various range.

[K^2]

I am not sure that nozzle size would change that much, to be honest. The same amount of fuel would still be burned

That's not true. Rate of burn of fuel in SRB has exponential dependence on pressure. Increasing nozzle size would drop the pressure, reducing the thrust and the burn rate.

Unfortunately, you'll also be losing a lot in terms of I_SP. So while it's a way to throttle SRB, it's not an efficient one.

There is a way to build an SRB whose throttle varies as the fuel burns. It's only going to vary in a predetermined way, but in most situations, that's all you really need. That and an ability to shut SRB down is sufficient to fly a ship powered by solid fuel only all the way to orbit.

[SargeRho]

There are throttleable SRBs, they're called Hybrid Rockets. Solid fuel with liquid oxidizer.

[sgt_flyer]

Advanced SRBs can be throttled with various means (most discussed above) but also by carefully shaping the srb's solid fuel shape, which will vary the thrust as the fuel burns.

(You can find a lot of infos about that here)

http://en.m.wikipedia.org/wiki/Solid-fuel_rocket

[Nuke]

There are throttleable SRBs, they're called Hybrid Rockets. Solid fuel with liquid oxidizer.

i dont think thats entirely accurate. you can turn them off, but i dont think you can get any throttle control. you might be able to control the flow of oxidizer into the engine, but i have a feeling that would greatly diminish its isp.

[shynung]

i dont think thats entirely accurate. you can turn them off, but i dont think you can get any throttle control. you might be able to control the flow of oxidizer into the engine, but i have a feeling that would greatly diminish its isp.

Throttling is definitely possible on hybrid rocket designs. The key here is that the fuel cannot burn itself without presence on the oxidizer, therefore throttling the oxidizer flow would throttle the rate of fuel/oxidizer reaction, hence throttling the thrust.

Problem is, these designs necessitate an additional tank for the oxidizer(which, if we're talking about liquid oxygen, would be HEAVY due to the insulation), ultimately lowering their thrust-to-weight ratio with little increase in I_SP. As such, it would be useful only in very specific situations.

[K^2]

Actually, Nuke is right that this is going to hurt the I_SP. But it's usually not so bad as to make it useless.

[Camacha]

Rate of burn of fuel in SRB has exponential dependence on pressure.

I was not aware of that, I must admit I never delved very deep into SRB physics