SRBs throttling down with time

[Gaarst]

As the title says, would it be possible to have a mod that adds this feature: SRBs not having constant thrust over their burn time, at least as a tweakable option.

The most obvious example of why this would be useful is making space shuttles. If you balance the thrusts of the SRBs and shuttle engines at launch and do not want to put 20+ Vernier engines, then as the SRBs lose mass, their TWR increases and you will flip, or at least lose control.

I know that SRBs cannot be shut off or that their thurst may not be tweakable at will, but take a look at the Space Shuttle's SRBs: their thrust was anything but constant over the duration of the flight.

This might have been requested and discussed over and over again, but I didn't find any recent post about it. So if anyone knows a mod which includes this, I'd be glad to know it too; or if anyone would make such a mod, please do so !

[Yemo]

You can use HybridRocketBoosters.

This is the thread by LordAurelius:

http://forum.kerbalspaceprogram.com/threads/104236

With his permission, they were included in SETIctt as well. If you install ProceduralParts + SETIctt, there is a procedural HRB available @basicRocketry.

[Gaarst]

You can use HybridRocketBoosters.

This is the thread by LordAurelius:

http://forum.kerbalspaceprogram.com/threads/104236

With his permission, they were included in SETIctt as well. If you install ProceduralParts + SETIctt, there is a procedural HRB available @basicRocketry.

Thank you but I read in the config that the hybrid booster has 200 kN thrust. The thing is, the ones I'm currently using have 5,000 kN thrust (from SpaceY) ^^

Also, the problem with freely throttleable boosters is that their throttle changes along the other liquid engines, so for a space shuttle, you will be unable to balance boosters thrust and shuttle engines thrust, only to change overall thrust... :/

[asmi]

RealFuels supports setting up thrust curves for SRBs for a long time. However there is no GUI for that atm - you've gotta do it via config-editing.

[Gaarst]

RealFuels supports setting up thrust curves for SRBs for a long time. However there is no GUI for that atm - you've gotta do it via config-editing.

Yeah, I thought something like that could be made with config files (with RF or not), but considering I have approximately no skill in editing configs I was hoping someone else could do it

[Cheesecake]

There is ETCwith a config which slow down the Thrust. I don´t have a link. But it was in the SpaceShuttle Pack.

[Zuthal]

I would really like a mod where you can for any SRB just make a thrust curve in a GUI. You just set a bunch of points (with the axes being "Fuel percentage burned" and "Percentage of max thrust" or something like that) and it automagically generates a smooth thrust curve from that.

[Moach]

as far as I reckon, SRB thrust curves are determined largely by the geometry of the "fire-goo stuffed inside the go-tube" (yes, I chose the most Kerbal-ish words available there)

the mechanics of SRB thrust envelopes are determined by the surface area of the fuel (which has a putty-like mushy consistency, I gather) that is exposed to combustion gases at any given time through the length of it's burn...

there are only so many possible configurations, but they do cover a different range of applications and allow some limited flexibility to suit your (planned, unlikely actual) ascent profile

it so then appears, the SRB's used by KSP are all solid-packed constant-area models... there are no specific cross section "hollows" to allow any other kind of burn profiles

which is a valid option, no doubt... but then again - it does favour "creative omni-stage decoupling" a bit more frequently than crews would prefer....

the tradeoff here, is that a constant-area solid really does have the best volume-to-fuel ratio you can get - since it has no cavities in the fuel-putty inside... so to have SRB's with a tailored burn curve it is inevitable you'd have to remove at least some fuel - and given the indifferent bulk of the empty casing, this would detract from your final mass fractions a bit (less bang for your buck)

this is a mod begging to be made, if anyone's looking for ideas (if not, I might go and make it myself one of these days) -- using modulemanager, this could be applied onto all solid boosters in general too (which is something that wasn't available last time I had a mod running that offered these features (a good while ago))

my (dead and soon-to-be) Kerbals largely endorse this idea!

Edited September 9, 2015 by Moach

[BudgetHedgehog]

There is ETCwith a config which slow down the Thrust. I don´t have a link. But it was in the SpaceShuttle Pack.

EngineThrustControl by HoneyFox. From memory, no GUI, it's all config based but it does taper off thrust with time.

[Skalou]

right, perfect for you!

I have reresented the stock EngineThrustControl curve:

you can do custom one's by tuning the points in the .cfg and add a config for other mod's SRB with a module manager patch.

you can select a time option(instead of fuel left) but never tried myself, someone has tried the differents options? (useTimeCurve = false ; canAdjustAtAnytime = false ; showItemInList = false)

Edited September 9, 2015 by Skalou

[Gaarst]

Thank you all for your replies, I'll try the EngineThrustControl thing tomorrow

[Armchair Rocket Scientist]

it so then appears, the SRB's used by KSP are all solid-packed constant-area models... there are no specific cross section "hollows" to allow any other kind of burn profiles

I rather doubt it: "end-burning" solid rocket motors have two major drawbacks.

1. They burn a lot slower than other profiles. Even in smaller amateur rockets you can end up not having enough thrust for a safe flight. Even a sepatron-sized motor might burn for 30 seconds depending on the fuel composition.

2. In an end-burning motor the lower portion of the casing is exposed to exhaust gases, while in a core-burning motor it's insulated by the unburned fuel. Basically this means that larger endburning motors would need a very bulky and heavy ablative lining to avoid overheating and exploding.

[GDJ]

^^ All true.

Also most hobby sized Solid Rocket engines have a thrust curve that goes from nil to full thrust in a few tenths of a second, then slowly taper down to zero over their burn time.

There are a few that maintain their thrust over their burn time (some CTI, Aerotech H and baby I motors for example) so I would love a actual thrust curve attached to these solid rocket boosters.

[Moach]

I rather doubt it: "end-burning" solid rocket motors have two major drawbacks.

1. They burn a lot slower than other profiles. Even in smaller amateur rockets you can end up not having enough thrust for a safe flight. Even a sepatron-sized motor might burn for 30 seconds depending on the fuel composition.

2. In an end-burning motor the lower portion of the casing is exposed to exhaust gases, while in a core-burning motor it's insulated by the unburned fuel. Basically this means that larger endburning motors would need a very bulky and heavy ablative lining to avoid overheating and exploding.

aye, true enough!

so we have determined that KSP solids are not tail-burning, tho it is still apparent they have a constant-area profile to them in some other way... this is visible as your SRB's become lighter and your G loads start to become more and more favourable to "juicing" the poor brave kerman strapped to them...

still, your usage of the term "safe" does imply of a very unkerbal choice of booster model

either way -- doesn't solve the problem much.... must look into this engine control thing - that sounds promising

Edited September 10, 2015 by Moach

[NathanKell]

Given the casing mass, I'm not sure you can safely assume the casing was not reinforced for end-burning.