Thrust Limiter Limitations

[SpacedCowboy]

How is it, in the VAB or SPH, that you can adjust the thrust on both the rocket engines and SRB's but, on the fly you cannot adjust the thrust limiter on a SRB? I understand that a SRB is an off or on thing but, I just don't get the logic! Either it is what it is or, it is what it isn't!? If it's adjustable in the VAB, then it should be adjustable in flight! Right? Comments welcome.

Edited March 5, 2014 by SpacedCowboy

[Vanamonde]

There isn't a control mechanism on an SRB engine. The solid propellant is placed in the booster so that the thrust is determined by how much surface area is exposed to combustion from one moment to the next. In other words, the throttle is set when it's made, not while it's operated.

[AlexinTokyo]

My thoughts:

Adjusting the SRB thrust in the VAB is done by opening up the engine (popping off panels, etc.) and tinkering with the settings on valves; it's not possible to do this in flight. (Maybe you could do this on an EVA, for enhanced reality)

Adjusting the liquid engine thrust is done with the throttle handle, and you can limit the maximum throttle either by physically limiting the travel of the handle or by tweaking the fly-by-wire control code, both of which could be done in flight.

[SpacedCowboy]

Ok, I get it, but I don't like it. Just depends on how much gun powder you lay down and lighting the fuse! Only wish there was some way to control the fuse.

[Claw]

Vanamonde has the idea. When SRBs are manufactured, the burn rate is set. The fuel is solid and burns straight on through once started, there is no throttle or valves.

If you go look up the SRBs for the space shuttle, they were made with two patterns inside. The forward portion consisted of an 11 point star pattern. The rear portion consisted of a (more or less) tube. The booster ignited from the top down. The top portion burned faster to provide more initial thrust. The aft section burned slower to provide lower thrust for a longer period of time.

[Red Iron Crown]

I think it might be possible to make an SRB that changes thrust as it burns, through careful shaping of the propellant. I don't think it could be manually adjusted on the fly, though.

[MarvinKitFox]

There isn't a control mechanism on an SRB engine. The solid propellant is placed in the booster so that the thrust is determined by how much surface area is exposed to combustion from one moment to the next. In other words, the throttle is set when it's made, not while it's operated.

Incorrect. Or at least, horridly incomplete.

You are skipping the third and most important factor, being chamber pressure. Solid rocket fuel burns faster as the pressure increases.

Quote from a very helpful into document: (link in next post)

"Burn rate is profoundly affected by chamber pressure. The usual representation of the pressure dependence on burn rate is the Saint-Robert's Law,

where r is the burn rate, a is the burn rate coefficient, n is the pressure exponent, and Pc is the combustion chamber pressure.

r = a Pc^n"

One can *easily* fiddle with the burn rate, and thus the thrust, of a Solid rocket by fiddling with the nozzle aperture.

Make it larger, and you get less pressure, less burn rate. Unfortunately, less ISP too.

Make it smaller, and pressure increases. Make it too small and the propellant will MAKE a bigger exit, either by ablating the nozzle to a suitable size, or more usually by rupturing its casing.

If mass was not a factor, one could throttle a SRB by using some symmetrical aperture-adjusting nozzle, something like a camera shutter. But one capable of withstanding SRB heat, pressure and abrasion would also serve as a good anchor to keep this SRB on the ground.

Edited March 5, 2014 by MarvinKitFox

[MarvinKitFox]

I think it might be possible to make an SRB that changes thrust as it burns, through careful shaping of the propellant. I don't think it could be manually adjusted on the fly, though.

YepYep.

Shaping the inner burning surfaces of SRB is a fine art, a good example is the SRB that was used on the Shuttle launches. These were tuned to deliver maxx thrust at launch, then taper down by a THIRD at about t=50s, so as to reduce thrust during Max-Q, then increase again after t=70s.

There's some good-ish basic info about it to be found here. http://www.braeunig.us/space/propuls.htm#solid

[Kasuha]

The point is that devs chose to implement it this way for the sake of limited realism in the game. In real SRBs, you can do miracles when constructing them but once you ignite them there's no stopping or limiting them, too.

Whether or not will we get more than constant thrust over the burn period is a question but not really related to the topic.

[Red Iron Crown]

YepYep.

Shaping the inner burning surfaces of SRB is a fine art, a good example is the SRB that was used on the Shuttle launches. These were tuned to deliver maxx thrust at launch, then taper down by a THIRD at about t=50s, so as to reduce thrust during Max-Q, then increase again after t=70s.

There's some good-ish basic info about it to be found here. http://www.braeunig.us/space/propuls.htm#solid

That's a great link, and not just for the solid rocket info. Thanks.

Looks like it's trickier to make a constant thrust SRB than a variable thrust one.