Why do SRBs burn out so quick?

[GalaxyGryphon]

I was watching some NASA footage and SRBs last like, 2 minutes...

is it because kerbin is smaller so they tried to balance them out? i think longer lasing SRBs would lead to people making ships with a lot less staging, and really help with heavy lifters. it might even help the people with weak computers. idk.

[lazarus1024]

I believe you answered your question. Everything is smaller scale.

[MR4Y]

Woth some clever designing choices, you can make it to orbit with as few as 2 or 3 stages.

[legoheli]

You can make it to orbit in one stage with no problem at all using throttling and simple design.

The SRBs in KSP are only "really" useable for showing off. Using liquid boosters is much more effecient, and the real world limitation of liquid boosters being expensive is something we are completely able to ignore. Of course it's a lot easier to throw SRBs on than LFBs.

[llamatoes]

I disagree, IMHO, they are really rather useful, as they have a massive ISP, and they, have an enormous thrust, and they can get you past the first 200 metres really easily.

[Anglave]

I disagree, IMHO, they are really rather useful, as they have a massive ISP, and they, have an enormous thrust, and they can get you past the first 200 metres really easily.

You think 230-250 ISP is "massive"?

They're sometimes useful as first-stage boosters (as they're intended to be). Where high thrust and low ISP are best. But carrying any low efficiency engine longer into your flight or mission is a bad idea. Boosters are no exception.

[llamatoes]

They still have a great TWR, and that's good enough for me.

[GalaxyGryphon]

yeah i guess that makes sense, real life liquid fuel and engines are hella expensive

[AmpsterMan]

I find two stage to orbit with stage zero boosters to be sufficient for most missions. If you think SRBs are underpowered now, you should have seen them pre-0.17; they were horrible in every sense of the word

[Frederf]

Does anyone know the scale height of the Earth v Kerbin atmosphere and the typical burn out altitude for the STS SRBs v Kerbal SRBs? Maybe it's in good scale.

[Guest]

OK, for KSP SRB's to have a comparable burn time to STS SRB's, they would have to burn for ~76 seconds. Has anyone clocked stock SRB burn time? No, I haven't compared ISP data. It's late and I'm knackered.

[Fel]

28s / 46s

But remember that kerbin is 10.6x smaller... so scale should be a bit similar.

[Brofessional]

I still use them on my first stage. The fact that they always burn for the same length of time makes it easier to time separations with them. With LFB's you have to do more work to make sure your mainsails don't burn through fuel faster than the boosters.

[biggles1]

Usually I build a 3-stage liquid boosted rocket, then slap some big SRB's on for lolz and giggles. Plus it looks hella impressive.

[Guest]

28s / 46s

But remember that kerbin is 10.6x smaller... so scale should be a bit similar.

I was working from the standpoint of the real world SRBs burning for 120 seconds, then took 64% of that for the relative differential, and came up with ~76 seconds. Feel free to skewer my maths, as I'm not that attached to them anyway (meaning, I'll gladly accept informed correction since I was purely spitballing).

[Deadweasel]

SRBs represent "free" boost out of lower atmosphere, in that the containers and engines are considered disposable (at least in KSP). I think the initial thinking in adding the boosters was along the same lines, but players quickly figured out you can toss out whole liquid fuel engines just as easily anyway and get better lower atmosphere performance in the process.

Wait until Career Mode makes an appearance. You won't be so quick to pounce on those orange tank/mainsail combo boosters after that!

[John FX]

SRBs represent "free" boost out of lower atmosphere, in that the containers and engines are considered disposable (at least in KSP). I think the initial thinking in adding the boosters was along the same lines, but players quickly figured out you can toss out whole liquid fuel engines just as easily anyway and get better lower atmosphere performance in the process.

Wait until Career Mode makes an appearance. You won't be so quick to pounce on those orange tank/mainsail combo boosters after that!

I think a lot of designs will have to be rethought if price/performance ratio is an issue. A SRB is really cheap for what it does. eight of them is like having two extra main engines for the hardest bit of the launch. The extra performance of the liquid engines probably comes with a high cost premium.

I think the CORE engine expansion has SRBs that last about 2 minutes. I use them and then stack a stock SRB to the outside of that to get a more realistic looking craft (which will fly better when they introduce more realistic aerodynamics)

Edited May 25, 2013 by John FX

[karolus10]

Burn time of SRB depend from 2 variables: diameter and fuel grain burn rate.

SRB burn from center shaft to booster walls, so thicker layer of fuel (this green stuff on picture) = longer burn time, so both 1.25m boosters should had nearly identical burn time if they used same fuel inside.

Increasing only length of the booster (or stacking more fuel segments if we had modular one) can't increase burn time - it's increasing chamber pressure (more fuel burning at once) causing increase of thrust (longer booster = more thrust).

Second variable changing burn time is fuel grain size (other structural and chemical composition, pretty much different variants of solid fuel), different fuel is burning at different pace.

For example we can use faster burning fuel to increase SRB trust in expense of burn time (and risk of too large pressure/heat inside => explosion) or slower burning fuel to get longer (but weaker) burning boosters or to avoid blowing up modular SRB with too many segments (every next fuel segment add-up pressure and trust to SRB) stacked up.

Edited May 25, 2013 by karolus10