SRBs vs KSP SRB

[Nori]

So been playing KSP for a long long while and I've always viewed SRBs as a mediocre. That seems odd since SRBs are used quite a bit in real life launches.

Anyway, I was looking up SRBs today and was amazed at the amount of thrust, burn time and size. The two space shuttle SRBs weigh in at 2.8 million pounds and constitute 62.6% of the entire launch weight despite being smaller (combined even) than external tank.

Compare that to the KSP SRBs which all of a sudden appear very large with low thrust.

That makes me wonder if the solid fuel that KSP has setup is just not dense enough and the SRBs produce too little thrust for their size.

Just some thoughts and wondering what other people think.

[Streetwind]

You can't compare the ratio of SRBs to the STS external tank to the ratio of SRBs to liquid fuel tanks in KSP.

The space shuttle had a hydrogen tank. Hydrogen tanks must be made ginormous for any sort of useful content, because liquid hydrogen is ridiculously low density, incredibly cold and absurdly hard to contain. At the same time, the Isp of hydrogen engines is very high, meaning you only need to carry small fuel masses, making the sheer weight of the SRBs seem even bigger by comparison; and the thrust of hydrogen engines is generally low, making the sheer thrust of the SRBs seem even bigger by comparison. Basically everything about the shuttle exaggerates everything about the SRBs by comparison.

Meanwhile, KSP liquid fuel is more analoguous to kerosene (RP-1) fuel. It's much more dense, and thus comes in much smaller tanks. Kerosene engines have less Isp, so the liquid tanks of the rockets must be heavier. And so on and so forth. Thus SRBs automatically seem less massive and less performance-per-size.

Finally: never forget that values in KSP are all fudged for playability in the tiny stock planetary system with its tiny velocities and tiny dV costs and much smaller sized creatures. If you expect things ingame to reflect reality 1:1, I have some bad news for you...

[SAI Peregrinus]

If you expect things ingame to reflect reality 1:1, I have some bad news for you...

And I have a link to the Realism Overhaul thread.

[Nori]

You can't compare the ratio of SRBs to the STS external tank to the ratio of SRBs to liquid fuel tanks in KSP.

The space shuttle had a hydrogen tank. Hydrogen tanks must be made ginormous for any sort of useful content, because liquid hydrogen is ridiculously low density, incredibly cold and absurdly hard to contain. At the same time, the Isp of hydrogen engines is very high, meaning you only need to carry small fuel masses, making the sheer weight of the SRBs seem even bigger by comparison; and the thrust of hydrogen engines is generally low, making the sheer thrust of the SRBs seem even bigger by comparison. Basically everything about the shuttle exaggerates everything about the SRBs by comparison.

Meanwhile, KSP liquid fuel is more analoguous to kerosene (RP-1) fuel. It's much more dense, and thus comes in much smaller tanks. Kerosene engines have less Isp, so the liquid tanks of the rockets must be heavier. And so on and so forth. Thus SRBs automatically seem less massive and less performance-per-size.

Finally: never forget that values in KSP are all fudged for playability in the tiny stock planetary system with its tiny velocities and tiny dV costs and much smaller sized creatures. If you expect things ingame to reflect reality 1:1, I have some bad news for you...

Hmm, yeah I hadn't been thinking about the hydrogen engine part.

I don't expect it to reflect reality. I just like things to make sense via real life ratios and SRBs seemed to be falling short. But my comparison is off since I hadn't thought about the cryo SSME.

[Kuu Lightwing]

KSP SRBs are too weak anyways... Let's look at Atlas V which uses RP-1/LOLX for the first stage an SRBs.

RD-180 engine produces 4,152 kN of thrust and first stage burns for 253 seconds.

AJ-60A SRBs produce 1,270 kN of thrust each and burn for 94 seconds.

Okay, if we are to compare that to KSP engines what would we choose? I mean Kickback produces 670 kN Vac thrust and if we compare that to Twin Boar (2,000 kN Vac thrust), the ratio would be close to RD-180/AJ-60A. The problem is AJ-60A are small boosters, while Kickback is the largest we have. So there's no way to make something like SLS/Space Shuttle/Ariane V when SRBs provide most the power initially - you have to replicate those using liquid boosters.

TL:DR: I think we need 2.5m SRBs to do the heavy lifting. Maybe reduce the size of current ones a bit.

[Nori]

Yeah that's kind of what I was thinking initially. Maybe if SRBs had double the fuel and 50% more thrust or something.

[Beowolf]

I got curious enough to look up the density of liquid hydrogen once, and even braced by having seen many comments like these, it still surprised me!

A cubic meter of water contains 1,000 kg.

A cubic meter of liquid hydrogen contains 71 kg.

Yikes!

[regex]

As I understand it most SRM usage IRL is to increase the launch TWR to an acceptable level, especially for cryogenic launchers like Ariane V and the STS which have a wide variance between SL and Vac isp. For instance, the Vulcain accounts for roughly 8% of Ariane V's launch thrust. By the time the boosters are jettisoned, it is facing a much reduced load from fuel usage (not to mention the boosters being ejected) as well as higher thrust from being higher in the atmosphere. I'm not sure exactly what Ariane V's launch profile is, but the Vulcain might not even be enough for a 1.0 TWR when the boosters are jettisoned, so the boosters might even be a means to "pop up" the launcher so that the lower thrust, higher efficiency engine can do its work.*

*Incidentally, this is also how I launch my Angara 5 replica, using the lower stage to get the anemic upper stage to a higher apoapsis so it can circularize. The Saturn V used a similar launch technique, AFAIK, because the upper stages have crappy TWR.

[Hcube]

Still KSP's SRBs are very underpowered, since many liquid fuel engines are way more powerful than the biggest available SRB. Also their Isp has been trashed with 1.0 ...the burn time is sooo low ! They cannot be used for a Ariane 5 or Shuttle config...

[shynung]

Still KSP's SRBs are very underpowered, since many liquid fuel engines are way more powerful than the biggest available SRB. Also their Isp has been trashed with 1.0 ...the burn time is sooo low ! They cannot be used for a Ariane 5 or Shuttle config...

They can, if the main boosters are good enough.

[magnemoe]

As I understand it most SRM usage IRL is to increase the launch TWR to an acceptable level, especially for cryogenic launchers like Ariane V and the STS which have a wide variance between SL and Vac isp. For instance, the Vulcain accounts for roughly 8% of Ariane V's launch thrust. By the time the boosters are jettisoned, it is facing a much reduced load from fuel usage (not to mention the boosters being ejected) as well as higher thrust from being higher in the atmosphere. I'm not sure exactly what Ariane V's launch profile is, but the Vulcain might not even be enough for a 1.0 TWR when the boosters are jettisoned, so the boosters might even be a means to "pop up" the launcher so that the lower thrust, higher efficiency engine can do its work.*

*Incidentally, this is also how I launch my Angara 5 replica, using the lower stage to get the anemic upper stage to a higher apoapsis so it can circularize. The Saturn V used a similar launch technique, AFAIK, because the upper stages have crappy TWR.

Back in 0.9 my standard launchers was core with TWR a bit below 1 and SRB to get me up to 10 km, this should work better in 1.0 as the engines produces less trust at sea level.

This was the most economical rocket design, an rocket SSTO would be much larger and require much more fuel making it more expensive, the loss if it was not recovered would also be many times larger.

[Red Iron Crown]

The biggest thing missing in KSP SRBs IMO is gimballing nozzles. That and sane thrust profiles, real SRBs are tuned so that the TWR doesn't go as crazy high as they burn through propellant (through careful shaping of the cavity in the solid fuel).

[Nori]

The biggest thing missing in KSP SRBs IMO is gimballing nozzles. That and sane thrust profiles, real SRBs are tuned so that the TWR doesn't go as crazy high as they burn through propellant (through careful shaping of the cavity in the solid fuel).

This sounds interesting. Is it even possible in KSP though?

Agreed on the Gimballing. After playing KSP for a while I started looking at real rockets and was always intrigued to see gimbal on SRBs since KSP's don't have it.

[magnemoe]

This sounds interesting. Is it even possible in KSP though?

Agreed on the Gimballing. After playing KSP for a while I started looking at real rockets and was always intrigued to see gimbal on SRBs since KSP's don't have it.

I guess its posible, it would be an script who reduced trust over time, down to say 50% at burnout, might have a 1-0.1% trust afterward down to 0 over 15 seconds like real ones have.

Gimballing I don't miss much they are boosters after all.

[regex]

Still KSP's SRBs are very underpowered, since many liquid fuel engines are way more powerful than the biggest available SRB. Also their Isp has been trashed with 1.0 ...the burn time is sooo low ! They cannot be used for a Ariane 5 or Shuttle config...

KSP's SRMs are great for the lower delta-V requirements of Kerbin; I'm not sure why you would expect to be able to pull off a launch like Ariane V or the STS in stock anyway since your fuel is basically Aerozine 50 + N2O4. It's not like you have anemic lower stages to begin with.

[asmi]

I guess its posible, it would be an script who reduced trust over time, down to say 50% at burnout, might have a 1-0.1% trust afterward down to 0 over 15 seconds like real ones have.

Gimballing I don't miss much they are boosters after all.

No need. RealFuels mod already have this functionality built-in for quite some time. The only thing that's missing is GUI to edit thrust curve (you gotta do that via config).

[Hcube]

I wish SRBs had thrust curves and thrust vectoring...

Edited July 23, 2015 by Hcube

[Motokid600]

I love seeing jettisoned SRBs in RSS/RO still burning propellant. And it's a nice little challenge to decouple them at the right point. Makes for a really cool effect.

Yes the stock SRBs desperately need some love.

[kujuman]

No need. RealFuels mod already have this functionality built-in for quite some time. The only thing that's missing is GUI to edit thrust curve (you gotta do that via config).

Yeah, I do wish I could pawn off AdvSRBs to someone else to maintain. A thrust curve GUI is actually really easy to have work in game*, and stackable boosters are really awesome (though procedural SRBs make stackable ones a little less important).

* Even if one doesn't want total control, it's pretty easy to write a small function to automatically configure SRB burn profiles to suit one's needs; constant excess thrust, start/end thrust, duration, etc.

I guess one of the issues with SRBs in KSP is that setting up burn profiles (if we could do it the "right" way) is a little bit of a chore, and that's fine for players who use common lifter rockets, but it can be a little annoying for those who always build custom rockets for each payload.

[Richmountain112]

I'm wondering why there are no Vacuum SRBs, ones that are optimized for Vacuum operations, and (maybe) can be shut down. It would be kinda useful for Tylo landers.

[magnemoe]

7 hours ago, Richmountain112 said:

I'm wondering why there are no Vacuum SRBs, ones that are optimized for Vacuum operations, and (maybe) can be shut down. It would be kinda useful for Tylo landers.

SRB has low isp so they are not very practical as upper stages. Its solid fuel kick stages however, the space shuttle used it as it was not safe or practical to have an liquid fueled stage in the cargo bay.

And wow that was an deep necro. 

Edited May 8, 2023 by magnemoe

[Hannu2]

12 hours ago, Richmountain112 said:

I'm wondering why there are no Vacuum SRBs, ones that are optimized for Vacuum operations, and (maybe) can be shut down. It would be kinda useful for Tylo landers.

What is the point of make difficult planet to game and then give tools that makes it simple and easy?

Also that kind of properties is not typical to SRB. It can be programmed in game but why they should call it "SRB". In my opinion things does not have to work exactly like in real life but there should be certain consistency  between game and real life objects and their operation. If there are real world named like SRB's, liquid propellant engines etc. they should work so that I can guess how they work based on general knowledge of rocket echnology. If game developer wants to give totally fictional ways to work, it can be interesting experience too, but they should be named with more scifi-ish names. Quantum enhanced gravitomagnetic antimatterturbine can do whatever magic or inconsistent tricks.

I am sure that there are loads of mods which give more powerful and/or simple ways to visit all bodies in KSP system, if you really want to go to see how planets look without too much experimenting and learning. But on the other hand, if you learn few simple ideas how space flight work it may give much more joy. KSP is quite easy game at default settings, after all. Not anything like real world's terror of rocket equation.

[sevenperforce]

On 7/21/2015 at 8:01 PM, Beowolf said:

I got curious enough to look up the density of liquid hydrogen once, and even braced by having seen many comments like these, it still surprised me!

A cubic meter of water contains 1,000 kg.

A cubic meter of liquid hydrogen contains 71 kg.

Yikes!

There is more hydrogen in a gallon of liquid gasoline than in a gallon of liquid hydrogen.

6 hours ago, magnemoe said:

wow that was an deep necro.

Shockingly deep.

6 hours ago, magnemoe said:

14 hours ago, Richmountain112 said:

I'm wondering why there are no Vacuum SRBs, ones that are optimized for Vacuum operations, and (maybe) can be shut down. It would be kinda useful for Tylo landers.

SRB has low isp so they are not very practical as upper stages.

In real life, there are plenty of vacuum SRBs. The Star 48 is one of the most popular:

Vacuum-optimized nozzle which gives it 286 seconds of specific impulse -- better than the sea level impulse of a Merlin 1D. Because they are so dense, they work well as terminal kick stages to fit inside a fairing with a payload. The Star 48 was the final stage of the Delta II launch vehicle and it was also used on top of the Centaur on the Atlas V 551 that launched New Horizons to Pluto.

These kick stages can either be spin-stabilized or they can have TVC.

I've always thought it would have been cool to have segmented SRBs in KSP. Choose a segment size, a number of segments, and a nozzle size/design. That way you could have a dense, low-thrust, high-efficiency kick stage, if so desired. 

[Hannu2]

1 hour ago, sevenperforce said:

I've always thought it would have been cool to have segmented SRBs in KSP. Choose a segment size, a number of segments, and a nozzle size/design. That way you could have a dense, low-thrust, high-efficiency kick stage, if so desired. 

I would like realistic thrust curve, like real SRBs.  For example Shuttle's SRB begins at full thrust, decreases during max Q, increases again and then decreases smoothly to keep acceleration in limits.

https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#/media/File:Srbthrust2.svg 

[sevenperforce]

Just now, Hannu2 said:

I would like realistic thrust curve, like real SRBs.  For example Shuttle's SRB begins at full thrust, decreases during max Q, increases again and then decreases smoothly to keep acceleration in limits.

https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#/media/File:Srbthrust2.svg 

I believe that you can set thrust curves for KSP SRBs using the KAL controller.

It would also be cool to be able to construct thrust curves using multiple segments in your booster with different burn rates.