Do SRBs throttle down in reality?

[Bobe]

I was just about to post a thread in the suggestions forum about making SRBs throttle down before separation (essentially just by adjusting the thrust output asymptotically towards 0 fuel), but then I thought maybe I am misunderstanding how solid boosters operate.

Obviously in Kerbal SRBs burn at a constant rate, varied only by the thrust limiter, until empty, at which point the vehicle experiences an abrupt change in acceleration, depending on the mass. When I watch a real launch, solid boosters appear to throttle down and burn at a low rate before separation while still burning so as to maintain constant acceleration. Is this the case, or do we just get the illusion of the boosters burning at lower thrust because the lower air density doesn't produce the same exhaust condensation as near the ground?

[*Aqua*]

According to this

Pic shows left and right SRBs of a Space Shuttle.

they loose thrust. But it's not possible to manually adjust thrust. When a SRB is ignigted it will burn until all fuel is gone. If you somehow limit the exhaust the pressure inside will rise until it explodes.

Edit:

I quickly made this. Just in case you didn't know how a SRB works.

Edited November 26, 2014 by *Aqua*

[Bobe]

But it's not possible to manually adjust thrust. When a SRB is ignigted it will burn until all fuel is gone.

Do you mean in game or in real life?

[sal_vager]

You can't throttle them in the game during flight, and they don't lose power.

But in real life they do change their output depending on the design (shape) of the fuel inside, as they burn that shape changes and surface area changes, affecting thrust.

Plus it is possible to reduce the thrust further by blowing off panels, if you put a hole in the combustion chamber you lose pressure, and lose thrust, it's not reversible though

[Bobe]

You can't throttle them in the game during flight, and they don't lose power.

Right, like I said.

But in real life they do change their output depending on the design (shape) of the fuel inside, as they burn that shape changes and surface area changes, affecting thrust.

Plus it is possible to reduce the thrust further by blowing off panels, if you put a hole in the combustion chamber you lose pressure, and lose thrust, it's not reversible though

So would you think it's reasonable to have SRBs in the game gradually lose thrust as the fuel approaches zero?

Not sure if something like this is achievable with a mod.

[sal_vager]

It probably is, but they are coded to have fixed throttles, that'd have to be overridden.

Also, they may lose thrust due to changes in fuel grain, but they gain efficiency with reduced atmospheric pressure just like any other rocket engine

[battlefieldaces]

Also, solid boosters work on a chemical reaction that uses some atmospheric oxygen so SRBs lose thrust as they run low on it.

[kerbiloid]

Solid rocket throttle changing is pre-defined by geometric profile of solid fuel charge. May as decrease, as increase, as stay constant while fuel burns out.

Also there is an option (not in KSP) to shut down it by fire exhaust.

[*Aqua*]

Also, solid boosters work on a chemical reaction that uses some atmospheric oxygen so SRBs lose thrust as they run low on it.

I never heard of that. Someone explained to me the fuel will burn no matter of the atmosphere. A SRB should work in space, too.

[magnemoe]

So would you think it's reasonable to have SRBs in the game gradually lose thrust as the fuel approaches zero?

Not sure if something like this is achievable with a mod.

Yes, normal SRB do, the hole in the center is usually star shaped to give higher surface area and trust in the start, the falling trust in the end is because the SRB is burning the rest of the fuel.

[Streetwind]

Also, solid boosters work on a chemical reaction that uses some atmospheric oxygen so SRBs lose thrust as they run low on it.

I never heard of that. Someone explained to me the fuel will burn no matter of the atmosphere. A SRB should work in space, too.

The oxidizer is baked into the fuel, it does not need atmospheric oxygen to burn.

[lajoswinkler]

SRBs are just like any other solid fuel motors and I think they use the star design of the fuel assembly. There's obviously not gonna be ON and OFF in thrust like in KSP, no matter which design you use.

For example, here's a large amateur rocket on solid fuel.

[Kryten]

Throttlable solid rockers have been tested, using movable nozzle plugs, but not used on any actual vehicle yet.

[softweir]

Also, solid boosters work on a chemical reaction that uses some atmospheric oxygen so SRBs lose thrust as they run low on it.

No. No atmospheric oxygen gets into the SRB, so it can't affect the chemical reaction. Differences in external pressure can slightly affect outflow from the nozzle and this in turn can affect combustion pressure, which in turn affects combustion rate and therefore thrust; but these are small effects.

[battlefieldaces]

I never heard of that. Someone explained to me the fuel will burn no matter of the atmosphere. A SRB should work in space, too.

Yes, it will burn, but slower and rather inefficient.

[cicatrix]

Solid fuel it's a mix that carries its own oxidized within. It doesn't need any external oxidizer supply.

[SciMan]

Actually, there are some "solid fueled rocket ramjets" out there that DO use the atmosphere. There's nothing that prevents them from working in space, but they're designed for use in an atmosphere. They're better than solid rockets in an atmosphere, but worse when outside of it.

They are less efficient if they don't have atmosphere, but it's not because it runs out of oxygen. The solid fuel's burn rate and efficiency is unaffected by the presence or absence of an atmosphere. A bottle rocket would work just as well in space as it does on Earth. (probably tumble instead of fly straight, no air drag to stabilize it)

The reason a solid fueled rocket ramjet would work less efficiently in space is because it's using the heat of the solid rocket exhaust to heat up the atmosphere and accelerate both the atmosphere and the solid fuel exhaust out the back.

The way I understand it is you basically take a regular solid fuel rocket motor, put the open end of it partway down an air intake, and put a rocket nozzle after that.

The solid rocket provides enough thrust to start the whole thing moving forward, which pushes outside air down the throat of the air intake, the exhaust heats up the air, and the hot air and rocket exhaust both expand against a rocket nozzle to produce more thrust for the same amount of solid fuel than just a standard solid rocket can provide.

Better atmospheric specific impulse than a standard SRB, but less of the mass of the thing is fuel than a comparable SRB so it won't do as well in vacuum (more dead weight).

Here's a Wikipedia page for more info.

Of course, the same principle can work with a liquid fueled rocket, too. It's only worth it if the extra duct-work ends up weighing less than the fuel you save, but duct-work is generally lightweight so it works great in atmosphere.

Obviously it won't work on an interplanetary transfer stage, but it's great for launchers and pure rocket SSTO's.

There have been several shorter range (low hundreds of km) cruise and anti-ship missiles that use this as the propulsion system.

Better than a standard solid rocket, without the storage and maintenance overhead that a missile powered by a small turbojet (like the Tomahawk and Harpoon) has.

Of course, no rocket that goes to orbit has ever used this type of solid rocket, because they try to get out of thick atmosphere as early in the ascent as possible, and (comparatively) thick atmosphere is exactly what makes these solid rocket ramjets work so much better than a regular solid motor.

Bottom line, you're both right! (Even if neither of you knew it)

Edited November 28, 2014 by SciMan

[battlefieldaces]

Solid fuel it's a mix that carries its own oxidized within. It doesn't need any external oxidizer supply.

Solid fuel is a fairly unstable compound and once it reaches high enough temp. it will spilt into the actual fuel and the part that acts as oxidizer. Unfortunately the oxidizer released when a molecule splits is not enough to burn the fuel molecule. Atmospheric oxygen helps counter this, but there's no oxygen in space so lots of fuel molecules are wasted due to lack of oxidizer.

[Iskierka]

*snip*

While yes, this rocket design does technically exist, you got some factors slightly wrong. The solid fuel gas-generator section uses a fuel that's actually extremely oxygen-deprived, practically only containing enough to physically burn itself. The remaining solid fuel, now gasified then gets mixed with the air, to provide far more heat for far less mass in the ramjet section, creating a much more efficient ramjet than the design you described. A pure solid booster with air-augmentation has been tested but not used much, as it's minimal Isp improvement for a much more complex design, while the proper design, known as a throttleable ducted rocket, gets much higher Isp, only somewhat less than a liquid fuel ramjet, with the storage advantages of solid fuels.

[KSK]

Solid fuel is a fairly unstable compound and once it reaches high enough temp. it will spilt into the actual fuel and the part that acts as oxidizer. Unfortunately the oxidizer released when a molecule splits is not enough to burn the fuel molecule. Atmospheric oxygen helps counter this, but there's no oxygen in space so lots of fuel molecules are wasted due to lack of oxidizer.

I'm not saying you're wrong for all solid fuels, but the aluminium /ammonium perchlorate mix used by the Shuttle SRBs seems to be about balanced, or at best, very slightly fuel rich. From Wikipedia:

The propellant mixture in each SRB motor consists of ammonium perchlorate (oxidizer, 69.6% by weight), aluminium (fuel, 16%), iron oxide (a catalyst, 0.4%), a polymer (such as PBAN or HTPB, serving as a binder that holds the mixture together and acting as secondary fuel, 12.04%), and an epoxy curing agent (1.96%).[4][5] This propellant is commonly referred to as Ammonium Perchlorate Composite Propellant, or simply APCP. This mixture develops a specific impulse of 242 seconds (2.37 km/s) at sea level or 268 seconds (2.63 km/s) in a vacuum.

The main fuel, aluminum, is used because it has a reasonable specific energy density of about 31.0 MJ/kg, but a high volumetric energy density, and is difficult to ignite accidentally.

Back of an envelope calculation, that's an approximately 1:1 molar ratio of aluminium to perchlorate. Assume that ammonium perchlorate decomposes as follows:

NH4ClO4 -----> NH4Cl + 2O2.

Now for aluminium burning:

Al + 3/2 O2 ----> Al2O3

So, there's enough oxidiser in the mix to completely burn all the aluminium, with 25% left over to burn off the polymer binder. I don't have any data on the binder, so I have no idea if that's quite enough, hence my TL: DR guess of balanced, or maybe slightly fuel rich combustion. I think 'lots of fuel is wasted' would be an exaggeration though.

Incidentally:

The propellant has an 11-point star-shaped perforation in the forward motor segment and a double-truncated-cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to avoid overstressing the vehicle during maximum dynamic pressure (Max Q).[4]

[Torquemadus]

Unfortunately the oxidizer released when a molecule splits is not enough to burn the fuel molecule. Atmospheric oxygen helps counter this, but there's no oxygen in space so lots of fuel molecules are wasted due to lack of oxidizer.

This would suggest that the Shuttle SRB is an air breathing rocket and would therefore need some kind of air intake. The oxygen from the atmosphere would have to somehow get inside the SRB so that it can be used to burn the fuel. Otherwise, the fuel wouldn't encounter the atmosphere until after it had exited the exhaust nozzle.

[sgt_flyer]

There are several existing SRB upper stages designed to work out of atmosphere. They typically have a longer nozzle than those used at ground level (to counteract the lower air pressures)

Like the Payload Assist Module, which was used on various rockets (and even in the space shuttle - it was stored in the Orbiter's cargo bay along with it's payload, then released and fired once in orbit.)

The minotaur V and Vega rockets use solid rockets, whose upper stages operates in near vacuum and even vacuum conditions (not much of an atmosphere up there) (once the first stage has flamed out, those rockets are almost in near vacuum conditions already)

The initial configurations of ariane VI would have used solid rocket booster as upper stages too.

Vega's final stage use liquid engine, simpky because it's much easier to finetune the final orbit than with an all SRB launch, which would require the payload to make final corrections (or that the final orbit does not require great precision)

[magnemoe]

There are several existing SRB upper stages designed to work out of atmosphere. They typically have a longer nozzle than those used at ground level (to counteract the lower air pressures)

Like the Payload Assist Module, which was used on various rockets (and even in the space shuttle - it was stored in the Orbiter's cargo bay along with it's payload, then released and fired once in orbit.)

The minotaur V and Vega rockets use solid rockets, whose upper stages operates in near vacuum and even vacuum conditions (not much of an atmosphere up there) (once the first stage has flamed out, those rockets are almost in near vacuum conditions already)

The initial configurations of ariane VI would have used solid rocket booster as upper stages too.

Vega's final stage use liquid engine, simpky because it's much easier to finetune the final orbit than with an all SRB launch, which would require the payload to make final corrections (or that the final orbit does not require great precision)

Yes, the only reason why SRB is mostly used at launch is the same as in KSP, they have huge trust and are cheap, downside is lower ISP and no throttle who is an factor for upper stages only. You would anyway adjust the engine bells for the pressure.

[kujuman]

I don't think throttle is really the right word based on what a throttle is, but as has been stated, SRBs can have a preset thrust profile (technically it's a preset burn profile). There's at least one other (aside from AdvSRBs) mod I'm aware of that allows configurable SRB throttling.

Actually...throttling SRBs a bit in stock KSP might be a reasonable compromise for realism/simplicity: set minimum thrust to 80% and maximum to 100% so the player will not be able to shutdown the engine but still give some control of the thrust. Not perfect, not too bad though.

On the OP, there is the question of the throttle down before separation...this is a necessary physical constraint. The entire booster "fuel tank" is also the combustion chamber, so it is many orders of magnitude larger than a LF combustion chamber. The SRB propellant burns and increases pressure inside of the combustion chamber: all else equal, the higher the pressure, the higher the Isp and therefore thrust. Pressure takes some time to build up (not very long) and even if the fuel burn rate instantaneously drops from 100% to 0%, there is still pressure and propellant mass in the chamber. This gas takes some time to equalize pressure with the other side of the nozzle and during this lag there is still residual thrust being produced. This same effect happens in a LF engine, but the small chamber volume requires much less time to equalize pressure. The shuttle SRBs were commanded to separate when the chamber pressure got below some set amount (I think 50 PSI?), not by time or anything else.

[Bobe]

I don't think throttle is really the right word based on what a throttle is, but as has been stated, SRBs can have a preset thrust profile (technically it's a preset burn profile). There's at least one other (aside from AdvSRBs) mod I'm aware of that allows configurable SRB throttling.

Actually...throttling SRBs a bit in stock KSP might be a reasonable compromise for realism/simplicity: set minimum thrust to 80% and maximum to 100% so the player will not be able to shutdown the engine but still give some control of the thrust. Not perfect, not too bad though.

On the OP, there is the question of the throttle down before separation...this is a necessary physical constraint. The entire booster "fuel tank" is also the combustion chamber, so it is many orders of magnitude larger than a LF combustion chamber. The SRB propellant burns and increases pressure inside of the combustion chamber: all else equal, the higher the pressure, the higher the Isp and therefore thrust. Pressure takes some time to build up (not very long) and even if the fuel burn rate instantaneously drops from 100% to 0%, there is still pressure and propellant mass in the chamber. This gas takes some time to equalize pressure with the other side of the nozzle and during this lag there is still residual thrust being produced. This same effect happens in a LF engine, but the small chamber volume requires much less time to equalize pressure. The shuttle SRBs were commanded to separate when the chamber pressure got below some set amount (I think 50 PSI?), not by time or anything else.

Yeah I was just thinking that throttle was a poor choice of word. I know solid boosters can't be throttled, what I was really after was if the boosters' thrust profiles degrade.

I don't want to be able to change the thrust of a solid booster using the in-game throttle, I just want their thrust output to adjust automatically to fit a simple decreasing profile.