Upper stage SRBs (PAMs etc.)

[StarStreak2109]

Hello folks,

my question is, how do I use upper stage SRBs, e.g. payload assist modules (PAMs)? Since I can't turn them off when I need to, how do I now, they have exactly the right amount of dV to achieve a given task?

Sorry, noob question, but I can't wrap my head around this...

Thanks,

Sebastian

[steuben]

You can't shut off a SRB once started. You can adjust the amount of fuel and thrust in a SRB in the VAB/SPH. But when it comes to last boost to orbit and position you're better of with LFO or mono engines.

Unless you are far better at orbit planning than I am.

[Laie]

6 minutes ago, StarStreak2109 said:

how do I now, they have exactly the right amount of dV to achieve a given task?

You need to know how much delta-V you need, from experience, or from a delta-V chart, or from doing the maths yourself.

Then you need to build a vessel-with-PAM that has exactly that amount of delta-V (difficult in stock as you can adjust the propellant only in 10% increments).

Then you need to reach the requisite transfer orbit to launch the PAM from, with high precision. That's pretty hard to do in KSP  even with information mods.

 

That said, depending on circumstances you can sometimes fake it.
Example 1: If you want to go to the Mun from LKO, you can pack a little more delta-V than strictly necessary (say, 870m/s) and send your payload on a trajectory that's not highly efficient but a) hits the Mun and b) consumes 870m/s.
Example 2: if you want your payload in a stationary orbit, bring just the right amount of propellant, do a best-effort insertion, then compensate for the inevitable error with RCS. That's how they do it in real life, by the way; only that the real-life error tends to be quite a bit smaller than what we can to in KSP.

[regex]

5 minutes ago, Laie said:

You need to know how much delta-V you need, from experience, or from a delta-V chart, or from doing the maths yourself.

This.  Using a PAM requires planning and foreknowledge.  Even on RO/RSS, I'd also have backup engines/RCS on my payload to make corrections as needed as the PAM tends to be slightly inaccurate.

Edited February 25, 2016 by regex

[StarStreak2109]

Mmmh, okay, I'll try that out! Thanks for your input!!!

[Snark]

SRBs are powerful and cheap, but they have absolutely abysmal Isp.  This makes them fantastic for the launchpad, and terrible anywhere else.  In general, you really don't want to be using an SRB anywhere but the pad.

For an upper stage, you'll get more dV for less mass if you use a liquid-fueled solution.

Is there some reason you want to use SRBs on an upper stage?

Edited February 25, 2016 by Snark

[StarStreak2109]

I'm just asking because there are some examples IRL, where SRBs were used.

[Nich]

Are Pam's a real Rockets thing or is that just an SRB for second stage? In the cheap and cheerful payload competition it was shown that SRB second stages cost more then LFO second stages because the the first stage has to be exponentially bigger

[StarStreak2109]

https://en.wikipedia.org/wiki/Payload_Assist_Module

[regex]

PAMs are real rocket things and a great exercise in planning.  Selling yourself short because other things are cheaper, easier, or more efficient in-game is a great way to limit your KSP experience.

[Nich]

 I have done SRBs to orbit it is interesting but I don't know if I would call it fun. SRBs to the mun would be a great challenge  but I don't know if I could put up with the frustration or the part count

[moogoob]

1 hour ago, regex said:

PAMs are real rocket things and a great exercise in planning.  Selling yourself short because other things are cheaper, easier, or more efficient in-game is a great way to limit your KSP experience.

Repetitive things like LKO to Mun transfers can be fun with SRBs to do the transfer with. Not efficient, but fun. Galileo went to Jupiter with the Inertial Upper Stage, a solid orbital booster.

Edited February 25, 2016 by moogoob
Inertial, not Kinetic, and Galileo, not Cassini

[Wjolcz]

I remember doing my RATOL with SRBs. What I wanted to do is to try and land a big plane on thr island runway. What I did was pretty easy. I took the plane for a flight and tried fly as slow as possible without stalling. When I knew how slow I could go I added the SRBs and adjusted their fuel levels to have the same exact (or as close as possible) dV as the slowest speed of my plane.

Basically, fly your mission first. Note how fast you are going when you are at the Apo, add the needed dV by adding appropriate amount of fuel in your SRB and activate it to achieve the orbit. At least that's what I'd do.

Btw: you need KER to figure this stuff out.

[GoSlash27]

7 hours ago, StarStreak2109 said:

Hello folks,

my question is, how do I use upper stage SRBs, e.g. payload assist modules (PAMs)? Since I can't turn them off when I need to, how do I now, they have exactly the right amount of dV to achieve a given task?

Sorry, noob question, but I can't wrap my head around this...

Thanks,

Sebastian

Sebastian,

 Figuring out the DV of a stage is easy. It's the rocket equation. Figuring out how much DV you need to circularize an orbit is also easy. That's the vis- viva equation.

The tricky part is in knowing what orbit you will be in when the PAM stage fires. You *have* to be in the orbit you expected to be in at that moment, or else the circularization will be off. You will have to plan that out and execute it flawlessly.

So for starters, we need to establish 3 things:

1) What is the mass of your payload?

2) What orbit will it be in when the PAM fires?

3) What orbit do you wish it to be in when the PAM burns out?

Best,
-Slashy

Edited February 26, 2016 by GoSlash27

[regex]

6 minutes ago, moogoob said:

Repetitive things like LKO to Mun transfers can be fun with SRBs to do the transfer with. Not efficient, but fun. Cassini/Huygens went to Saturn with the Kinetic Upper Stage, a solid orbital booster.

And New Horizons used a Star 48B kick motor.  Great stuff!

1 minute ago, Veeltch said:

Btw: you need KER to figure this stuff out.

Nope, just basic math, as Slashy points out.

You can get a half ton tier 5 probe from 72km LKO to Duna intercept with a Flea + TR-18.  You can safely and easily burn so low because the total burn time is less than 9 seconds; zero worries about dipping into the atmosphere, although you might have to reduce fuel by a bit because you'll have @ 100m/s extra.  Carries a thermometer and barometer, uses RCS for course adjustments (sub in an Oscar B and a Spark if you prefer), two OX-STATs and battery backup (just check the "DO NOT USE" on the electric charge to put it into hibernation), use light aerocapture and a braking pass or two to bring the orbit down, or stick with an elliptical orbit.  I'd say you'd have an early, somewhat challenging mission for pretty cheap.

It may not be the most efficient but it'll feel a lot more like actual spaceflight than most other things in KSP.

[Wjolcz]

1 minute ago, regex said:

Nope, just basic math, as Slashy points out.

Eh... You RO guys...

I suggested KER to OP because it's the laziest easiest and fastest way of doing it.

[regex]

2 minutes ago, Veeltch said:

Eh... You RO guys...

Slashy doesn't RO and I'm only not-lazy when I can't open the game to check.

[GoSlash27]

2 hours ago, regex said:

Slashy doesn't RO and I'm only not-lazy when I can't open the game to check.

For real, I'm all about lazy! But KER is severely limited in what it can accomplish, while math is insanely powerful. I only put enough effort into math to make sure I understand it. After that, I write a spreadsheet to do the math for me and revert back to lazy mode...

*edit* Just to be clear, this is NOT a dig against KER. It does what it's supposed to do by all accounts with the info it has. But it can only tell you the capabilities of what you have already built, whereas math can tell you what you need to build in order to do what you want to accomplish.

Best,
-Slashy

Edited February 26, 2016 by GoSlash27

[samstarman5]

2 hours ago, Snark said:

SRBs are powerful and cheap, but they have absolutely abysmal Isp.  This makes them fantastic for the launchpad, and terrible anywhere else.  In general, you really don't want to be using an SRB anywhere but the pad.

For an upper stage, you'll get more dV for less mass if you use a liquid-fueled solution.

Is there some reason you want to use SRBs on an upper stage?

Considering I watched someone perform an intersection burn with just SRB's to Jool, I wouldn't minimize their usage too much(It was Spooty of the Couch Stream, and I would love to share a link but the video was only on Twitch and apparently folks on Twitch either can't keep a big library or just don't want to).  Remember it was an SRB that catapulted New Horizons past the moon and on its way towards Jupiter, faster than any craft had gone before.

[Temstar]

PAMs main advantage is that being a solid rocket it's quite reliable as there are very few moving parts, and being just a can o boom it's safe to carry in a shuttle cargo bay. If the shuttle have to do an abort you just open the cargo bay door, shove the PAM and its payload out, close the door and do whatever abort action you have to do. If instead you are carrying a liquid fuel upper stage like a Centaur you will need specialized equipment to first dump the fuel and oxidizer overboard before you can jettison the stage. Being solid fuel PAMs have lousy Isp just like their bigger SRB cousins. Also since the requirement is for them to be simple they don't have a gimballed nozzle, so payload + PAM have to be spin stabilized.

The advantage of PAM doesn't apply to KSP so there's little reason to use them over liquid upper stages, besides RP of course.

 

[Snark]

If you're interested in upper-stage SRBs for RP reasons, fair 'nuff. 

If you're looking for compact options that will fit in a cargo bay, and you'd like something radial and slender rather than squat and stack-mounted, you might take a look at the SpaceY mod.  That mod is mostly about big parts (5m tanks, 2.5m SRBs, etc.), but it also includes a nice selection of several little 0.625m SRBs-- skinnier but taller than a Flea.

[regex]

After I made the earlier statement I got really curious to see if I what I was talking about would work.

Mind you, the entire thing in orbit could have been done on RCS alone (near 1.3km/s in the tanks) but that just means it's possibly suitable for other destinations like Dres or for changing orbits, that sort of thing.  Reducing the RCS fuel would have made up the 31m/s in the ejection burn that the PAM couldn't complete.

Useless?  Maybe.  It's still pretty fun.

Edited February 26, 2016 by regex

[DerekL1963]

8 hours ago, samstarman5 said:

 Remember it was an SRB that catapulted New Horizons past the moon and on its way towards Jupiter, faster than any craft had gone before.

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.

[StarStreak2109]

First of all, again thanks for all your insights, I will definitely try some of that out, even though I play my game more guessing than calculating - though at some stage I might even try that out, just for the challenge of things!

 

30 minutes 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.

How do you shut down an SRB? Modding idea?

[el_coyoto]

14 minutes 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.