Upper stage SRBs (PAMs etc.)

[monophonic]

20 minutes ago, StarStreak2109 said:

How do you shut down an SRB? Modding idea?

Here: http://everything2.com/title/thrust+termination

Bummer, links won't work when posting from my phone.

Edited February 26, 2016 by monophonic

[samstarman5]

1 hour ago, DerekL1963 said:

In the interest of full disclosure - that SRB also showed one of the downsides of solid motors.   Without a shutdown system (which do exist), they provide a fixed amount of d/v, and that amount varies somewhat from motor to motor even with the same batch.   New Horizon's Star 48B third stage burned 'hot' and imparted (IIRC) around 5 m/s more velocity than planned.

And no doubt a lot was learned through the procedure for improvement in further missions. NASA and the companies that design the engines don't employ them as helter skelter as we tend to do in KSP.

---------

For KSP, the ideal way to remove an SRB's thrust through abort or planned termination is to eject it. Naturally, just popping it loose during thrust is likely to just screw everything else up. Sepratrons would be your best friend here, and with some practice you can figure to aim them just so to make sure the SRB does not come back upon the craft. One might even be able to use one sepratron to an SRB thrust engine alone if angled just right, in order to save on weight.

It should be noted that in actual usage, once an SRB is extinguished through either depletion or extinguishing, it can not be lit again, for various explosive reasons alone. So using ejection as a tactic for stock play fits quite well.

[GoSlash27]

35 minutes ago, el_coyoto said:

I can't find the exact source, but an American nuclear missile used SRB all the way to space, and had to be stopped at a precise time for their trajectory to take them to the target.
To solve the problem, engineers used "venting ports" at the base of the missile, which covers would be blown to redirect thrust to the "sides" of the booster, nullifying forward thrust at that moment. In fact, it worked so well that they were able to snuff the SRB in space like a candle...
I'd be very happy if someone knew which missile it was, my google fu is weak this morning.

Sounds like the Minuteman III, although the MIRV bus used a liquid fuel system for the bus.

https://youtu.be/HMcauXSUAcU?t=225

[el_coyoto]

1 minute ago, GoSlash27 said:

Sounds like the Minuteman III, although the MIRV bus used a liquid fuel system for the bus.

https://youtu.be/HMcauXSUAcU?t=225

Yup, that's it, I found the sentence I had in mind in the wiki : "A series of ports were added inside the rocket nozzle that were opened when the guidance systems called for engine cut-off. The reduction in pressure was so abrupt that the last burning fuel ejected itself and the flame was snuffed out."

Thanks for the video link @GoSlash27, I'll check it out once I'm home.

[djnattyd]

SRBs for the upper stage only? Why limit yourself when you can build the entire ship out of SRBs and go to Laythe and back?

 

Video Linky

[regex]

4 hours ago, djnattyd said:

SRBs for the upper stage only? Why limit yourself when you can build the entire ship out of SRBs and go to Laythe and back?

That's pretty cool but do you have a more modern example?  You simply cannot slam into atmospheres anymore using default heating and drag options; there's a lot more precision involved in KSP 1.0.5.

It's certainly not impossible, though.

[John FX]

you can `shut down` your SRB by having them fixed to the larger radial connectors and staging them when you don`t need them any more. A slight angle would stop them impacting the stack.

[DerekL1963]

7 hours ago, el_coyoto said:

8 hours ago, StarStreak2109 said:

How do you shut down an SRB? Modding idea?

I can't find the exact source, but an American nuclear missile used SRB all the way to space, and had to be stopped at a precise time for their trajectory to take them to the target.
To solve the problem, engineers used "venting ports" at the base of the missile, which covers would be blown to redirect thrust to the "sides" of the booster, nullifying forward thrust at that moment. In fact, it worked so well that they were able to snuff the SRB in space like a candle...
I'd be very happy if someone knew which missile it was, my google fu is weak this morning.

Some missiles use ports in the base dome, other missiles use ports in the forward dome, other missiles blow off the nozzle, other missiles use a combination of two or more of these.

Anyhow, the idea isn't to nullify forward thrust (though that can be, and often is, done), it's to reduce pressure in the case.   When the pressure drops below a certain critical value (which varies) the burn cannot sustain itself and the motor 'shuts down'.  AIUI (from a reliable source) only some of the burn occurs in the solid phase.  Mostly the solid sublimes, the resulting gases burn, and heat from those gases sublimes the solid phase.   If you vent the gas phase, the burn rate drops essentially to zero.

Missiles known to use this:  Polaris A-1, Polaris A-2, Poseidon C-3, SUBROC, ASROC, Minuteman-I, Minuteman-II, Minuteman-III, Peacekeeper.  (The Titan's for Dyna-Soar and MOL would have used this method thrust termination too...)

If you look at this picture [source] of a Posiedon C-3 you can see oval 'patches' in the skin - those are where stacks leading from the forward dome meet the skin of the missile.   When it's time to shut down, the dome and the skin are blown and simultaneously the 'bus' (the part with the 'do not touch' sign on it) separates.  The motor stays behind (slowed by the venting gasses) while the bus continues on to start releasing the warheads.

Edited February 26, 2016 by DerekL1963

[Renegrade]

There's nothing quite like using a reduced-fuel Hammer to put something on course for the Mun or Minmus... So fast.. much G..  many fun.  very scared.  wow.  /doge

It's not the most practical or efficient thing, but most of my final stages will have liquid engines of some kind, so any small errors in the burn can be easily corrected.  The only thing to watch out for is making sure not to accidentally trigger it...you don't want the engine activation to happen until you're at the node

(sure beats the snot out of slowly burning away at 30 er 18 er 50 er 60 kn fun-wise and time-wise..)

[Stratzenblitz75]

2 hours ago, regex said:

That's pretty cool but do you have a more modern example?  You simply cannot slam into atmospheres anymore using default heating and drag options; there's a lot more precision involved in KSP 1.0.5.

It's certainly not impossible, though.

It is possible to go to Eve and back with SRBs in 1.05, so I imagine a Laythe return trip would be possible as well:

 

[regex]

2 minutes ago, Stratzenblitz75 said:

It is possible to go to Eve and back with SRBs in 1.05, so I imagine a Laythe return trip would be possible as well

Wasn't disputing whether it was possible, was asking whether people had done it without slamming into atmospheres at interplanetary velocities using unrealistic drag and lack of friction to make it possible.

I'm much more impressed by the video you posted.

[Snark]

14 hours ago, StarStreak2109 said:

How do you shut down an SRB? Modding idea?

I thought about that.  My idea would be to have a "scuttle" mod.  It adds a lightweight, "physicsless" radially-attachable part that's a high explosive.  When activated, it blows up whatever part it's attached to.  It would be optionally activated via staging, but the default is "off".  Would have an action group associated with it.

Thus you could, for example, tie it to your "abort" action to blow up the boosters.  I can imagine all sorts of creative uses you could use for something like that. 

[bonyetty]

28 minutes ago, Snark said:

I thought about that.  My idea would be to have a "scuttle" mod.  It adds a lightweight, "physicsless" radially-attachable part that's a high explosive.  When activated, it blows up whatever part it's attached to.  It would be optionally activated via staging, but the default is "off".  Would have an action group associated with it.

Thus you could, for example, tie it to your "abort" action to blow up the boosters.  I can imagine all sorts of creative uses you could use for something like that. 

No mod needed. A  sepratron facing inward works as a good self destruct trigger by staging or action group. 

 

Edited February 26, 2016 by bonyetty

[Snark]

56 minutes ago, bonyetty said:

No mod needed. A  sepratron facing inward works as a good self destruct trigger by staging or action group. 

Yeah, but it's bulky, ugly, draggy, and has potentially unwanted side effects.

In other words, perfectly Kerbal. 

But I'd still like a self-destruct charge.

[John FX]

14 hours ago, Snark said:

Yeah, but it's bulky, ugly, draggy, and has potentially unwanted side effects.

In other words, perfectly Kerbal. 

But I'd still like a self-destruct charge.

You could fit a few seperatrons inside a nosecone if you wanted cleaner lines or even inside the fuel tank itself.

[Amianoob]

It's simple; match your PAM DV to a maneuver, personally I've used the Flea as a PAM many times, if I know I've got 900ms DV I plot a maneuver that takes exactly 900ms, you can even do a burn in a slightly inefficient way if you have excess DV you don't need, but, either way you use the entire SRB and it gets you where you want to go without having to explode your stage, which is just asking for trouble if you ask me.

Edited February 27, 2016 by Amianoob

[Dfthu]

Well I did this thing once where I had a hammer SRB at about 25% thrust and put 4 sepatrons and 1 sepatron pointing away for the craft so when I decoupled the SRB it wont hit the craft once the sepatrons run out of fuel.

[RocketBlam]

The short answer is, you don't. SRBs are boosters, they are intended to augment liquid engines during the boost phase. This is because a) you can't turn them off, and b) you can't steer their exhaust. 

Now, I'm not saying I've never boosted just with boosters, but... they're not suitable for the final stage of getting into orbit.

[regex]

 

27 minutes ago, RocketBlam said:

The short answer is, you don't. SRBs are boosters, they are intended to augment liquid engines during the boost phase.

Oh really?  The Star 48 would like a word with you.  Hell, I'd like to have a word with you...

27 minutes ago, RocketBlam said:

This is because a) you can't turn them off, and b) you can't steer their exhaust.

Proper planning alleviates point a, a single magical reaction wheel or some RCS negates point b.

27 minutes ago, RocketBlam said:

Now, I'm not saying I've never boosted just with boosters, but... they're not suitable for the final stage of getting into orbit.

They're suitable for whatever task you decide to use them for, provided you understand their limitations and build accordingly.

[Dman979]

@Snark, you might want to check out the TAC Self-destruct mod. Hope that helps!

[RocketBlam]

2 hours ago, regex said:

They're suitable for whatever task you decide to use them for, provided you understand their limitations and build accordingly.

Agreed, and their primary limitation is that you can't turn them off. So unless you're just trying to get into any orbit, no matter how imprecise, they're of no use for the final burn into orbit.

[regex]

10 hours ago, RocketBlam said:

Agreed, and their primary limitation is that you can't turn them off. So unless you're just trying to get into any orbit, no matter how imprecise, they're of no use for the final burn into orbit.

Nonsense.  If they were that useless we wouldn't be using the Minotaur for LEO operations and the U.S.'s first satellite launch would have looked much different.  You just have to plan and fly correctly.

[CobaltWolf]

My solution in BDB is basically just to allow them to be turned off.  ¯\_(ツ)_/¯  SRBs are used as the final kick stage for a lot of IRL rockets. It seemed a shame to just leave them out. Part of the issue is that the stock SRBs are all too big. I have been adding a bunch of smaller sizes of SRB for different applications. They are still not very precise, and they are essentially a fixed amount of DeltaV (in that they can't be extended like a liquid stage) but they're cheap. I figure that allowing them to be shut down is a good middle ground between making them more useable, and making them feel distinct from a liquid stage.

[aeroeng14]

I agree, lots of math basically. You have to figure out the maneuvers you want to perform ahead of time and if they include plane changes, all the better. The idea is to "waste" the extra delta v contained in a standard sized solid fuel motor. An example could include launching into a higher inclination than planned transfer orbit and using the excess PAM delta v to fix the inclination error as well as circularizing at apoapsis with a combined maneuver. This may be hard to accomplish adequately in KSP since you're doing all the flying and it isn't the most precise given the small maneuver times. I'd like to try this in kOS sometime and see how the control goes.

[wumpus]

I should point out the exception to KSP's anti-pam design: the contract "escape Kerbal's sphere of influence" works very well with a last stage as a hammer (or possibly flea).  The problem is that while SRBs have amazing fuel/dry weight (thus excellent delta-v), they don't provide sufficient control: you will probably need to bring control rockets that destroy all your precious dry weight and make the advantage moot.

Kerbal (mad science) suggestion: bring sepratrons.  Bring pairs of them and use them for correcting burns.  Trial and error will be needed to perfect this, but I suspect that you should be able to fix your burns midcourse (note that you might need sepratrons that aren't full). [Side note: I have to try this...]

As far as second stage SRBs, while they are rare they aren't quite the impossible solution typically described.  They will work provided the third stage (and above) is sufficiently massive that the second stage doesn't effect the efficiency that much while the second stage is sufficiently light that the first stage can lift it*.  I'd expect this type of thing to most likely happen once hammers are unlocked (giving you the hammer/thumper/load combo) or NERVAs (using the NERVA for the final circularization burn and not wanting to recover lower stages).  Don't worry about not looking for such unicorns, while they do exist, you can probably find much better solutions using the time it takes to try them on all the other rockets...

* the efficiency of SRBs come down to two things: cheap thrust and (relatively) low mass.  The mass may be a surprise considering how heavy those things are, but as long as the dry mass of the stage is dominated by the upper stages the efficiency will be close to the difference in Isp.  Once the dry mass starts to dominate, expect really bad efficiency.  This is the same reason that they work so well as PAMs (ignoring control issues).