Liquid Boosters vs Solid Boosters

[G'th]

So as I continue work on the Eyes Turned Skyward rockets, I've come to the last stop with the Saturn Multibodies, half the designs of which use liquid boosters rather than solid rockets.

Now, my question is what are the actual advantages and disadvantages between the two as it pertains to real life? And more specifically, why liquid would be chosen over solids.

Obviously the variable throttle an advantage (though in the case of the MB's, none of the liquid booster variants are meant to be crewed, where I think the variable throttle would be most useful), but what else is there? Short of better lifting capability (IIRC, one F1-A should technically have more power than the Titan solid rocket), in which case liquid boosters seem an unnecessarily expensive option, I can't seem to figure out why liquid boosters are necessary in these designs.

For reference, this is the Saturn family of rockets in ETS:

[Bill Phil]

Liquids provide control. You would NOT BELIEVE how important that is, especially for rockets.

Plus, liquid engines are capable of more efficient specific impulses, and that's also important.

[xenomorph555]

-Solids produce high G forces, bad for manned

-Solids are uncontrollable, once started they keep going also cannot change thrust

-Solids have low isp, bad for in space

-Solids have lower efficiency, only last for short time

-Solids industry is expensive and complex

[GoSlash27]

Many of the advantages mirror those represented in KSP. Solid boosters have higher t/w while liquid boosters have higher Isp.Solid boosters are cheaper, while liquid boosters can be throttled, and so on.

In addition to these, liquid boosters tend to be safer to handle and operate, since solid boosters are essentially "a trash can of boom". Liquid boosters require refrigerated storage of the fuels, while solid boosters can fail due to cracks in the propellant.

Also, you can't stop a solid booster once you light it, while a liquid booster has a lot more parts that can fail.

Best,

-Slashy

[StrandedonEarth]

Large solids are difficult to produce with consistent properties, although ATK did a good job with the STS SRB's

Solids also have a lot more 'rumble' (vibration). Shuttle astronauts have reported that the ride gets a lot smoother when the SRB's separate.

[Guest]

In my opinion, SRBs have only 2 advantages: Long shelf life and instant activation. Thus only good for military use. In KSP, I tend to avoid them, except septatrons.

In civilian space program, safety is very important, and as GoSlash27 mentioned, they cannot be rendered safe.

There is a lot of situations, when due same failure or external circumstances, like weather, launch needs to be canceled.

Rocket needs to be rendered safe, and, often, moved back into test facility. You can drain fuel and fill tanks with inert gas for safe handling and rigidness, but what you can do with SRB?

Worst thing that can happen with LF rocket is some pyrotechnics go off with consequences of someone losing hand. Or head.

Now imagine SRB suddenly activates in VAB, and there many of them stored there...

That's why, in shuttle era, real VAB was high security arsenal, and, honesty, I would not approach such a building...

[Moar Boosters]

Looking at that series of rockets, it's because they don't want to design a bigger SRB.

They've got 1 massive core, and 1 smallish SRB. The "m42 and m43" is as large as they can go using those two components. To go bigger they had to abandon the SRB's and use massive cores as boosters.

[K^2]

-Solids are uncontrollable, once started they keep going also cannot change thrust

Not entirely true, even though it works out that way in practice. You can vary thrust of the SRB by varrying internal pressure. Unfortunately, it's complicated, less reliable than with liquid rockets, and comes at the cost of efficiency. At that point, the core advantage of SRB, which is mechanical simplicity, is completely gone. So it's much better to just use a liquid or hybrid engine.

On the other hand, shutting down an SRB is relatively straight forward. You just have to relieve the pressure, which is sometimes as easy as opening up the top. I have no idea if Shuttle SRB used something like that, but I do know that some SRBs have emergency shutdown ability that works along these lines.

[GreeningGalaxy]

Compared to liquid rockets, SRBs are very simple - in a liquid engine, you need to have all kinds of plumbing - pumps, pipes, a heavy nozzle and reaction chamber, and all that stuff. Worth taking with you to orbit and beyond, but if you're just going to use it up to a few kilometers and then drop it in the ocean, what's the point? Solid boosters are just cans full of boom, so they're far easier to produce, and they don't have nearly as many delicate components to get broken when the splash down, so refurbishing and reloading them is a possibility. As things like SpaceX's latest endeavours (namely landing the liquid-fueled ascent stage intact after launch) become standard, I expect SRBs will see less use in the common space industry, but in the meantime, they offer an attractive means of cutting down that all-important cost per kilogram.

Meanwhile, what about composite engines? They have the advantages of being both throttleable and safable, and I think they're only a little more complex than a typical SRB (although I might be wrong about that), so while you'd still have to have to store a liquid in a tank and figure out how to pump it around (which isn't hard, but is harder than stuffing a bunch of solid stuff in a tube and lighting it), you also wouldn't have the heavy and expensive nozzles of the same variety you see on liquid rockets. Of course, it might be a little longer before composite engines see common use after the Virgin Galactic accident, but it seems like that could be another way to go.

[B787_300]

Most of the time rockets use Solids for the Umph they give more than the finesse. but liquids do have the ability to be throttled and have better ISP

[Kryten]

Worst thing that can happen with LF rocket is some pyrotechnics go off with consequences of someone losing hand. Or head.

http://en.wikipedia.org/wiki/Nedelin_catastrophe

http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=2543

http://www.nytimes.com/1989/09/28/world/1980-soviet-rocket-accident-killed-50.html

[Nibb31]

Solids are used by the military because they have long duration storage and instant availability.

Solids are used for civilian rockets because of the synergies with military rockets. Part of their cost is subsidized by military development, which makes them cheaper. It is also in the Government's interest to keep solid rocket manufacturers in business, for strategic reasons.

[GoSlash27]

http://en.wikipedia.org/wiki/Nedelin_catastrophe

http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=2543

http://www.nytimes.com/1989/09/28/world/1980-soviet-rocket-accident-killed-50.html

Kryten,

WildLynx was referring to transport and vehicle assembly, not when it's fueled.

Best,

-Slashy

Edited January 11, 2015 by GoSlash27

[magnemoe]

Solids are used by the military because they have long duration storage and instant availability.

Solids are used for civilian rockets because of the synergies with military rockets. Part of their cost is subsidized by military development, which makes them cheaper. It is also in the Government's interest to keep solid rocket manufacturers in business, for strategic reasons.

To some degree this is correct, large SRB is not easy to make, only US and France use them as far as I know.

Russia has had a lot of problem getting an new solid ICBM to work.

However once you have the capabilities they are cheap to make.

Note that the SRB are very different from solid fuel ballistic rockets so they can hardly share much except the knowhow and the fuel itself.

[Kryten]

To some degree this is correct, large SRB is not easy to make, only US and France use them as far as I know.

ISRO has ~200 ton SRBs as part of GSLV Mk. III, and China have an all-solid launch vehicle and are working on a larger one.

[B787_300]

Ah but SRBs get unstable when they hit a certain size... too many small imperfections add up. There is a reason why there is no SRB larger than about 4 m in dia

[magnemoe]

ISRO has ~200 ton SRBs as part of GSLV Mk. III, and China have an all-solid launch vehicle and are working on a larger one.

Sorry, then my knowledge is out of date

[Kryten]

Ah but SRBs get unstable when they hit a certain size... too many small imperfections add up. There is a reason why there is no SRB larger than about 4 m in dia

Aerojet successfully ground tested a 260 inch (~6.6m) diameter solid stage, intended to replace the entire Saturn I first stage.

[B787_300]

Hrm i did not know that Kryten... I forgot that there are two designs of making motors.. I think my comment is still valid for segmented motors though

[kujuman]

Another factor is that for SRs, the entire casing is the chamber, whereas the chamber in a LFR is pretty small (it's part of the engine). A large, powerful SR needs relatively more structure than a LFR of similar performance.

I don't know what development costs are for different variants, but I'd guess adding SRBs to a lifter is a pretty simple to get a small increase in mass to orbit compared to adding an LRB for the same performance.

Solids are used by the military because they have long duration storage and instant availability.

Solids are used for civilian rockets because of the synergies with military rockets. Part of their cost is subsidized by military development, which makes them cheaper. It is also in the Government's interest to keep solid rocket manufacturers in business, for strategic reasons.

A very large number of launches in the US are to support military (or otherwise GOVT) operations, so there's a subsidy for liquid fuel rockets as well.

[Pipcard]

Solids are used by the military because they have long duration storage and instant availability.

Solids are used for civilian rockets because of the synergies with military rockets. Part of their cost is subsidized by military development, which makes them cheaper. It is also in the Government's interest to keep solid rocket manufacturers in business, for strategic reasons.

Japan uses solid rocket boosters but they're not making long range ballistic missiles.

Edited January 12, 2015 by Pipcard

[zarakon]

A couple of notes about throttling solid vs liquid engines

Real-life liquid engines typically can't throttle very much, if at all. The way KSP allows you 100% throttle range and infinite completely-smooth shutdowns and restarts is very unrealistic.

Real solid motors can't be actively throttled in flight, BUT the propellant grain pattern can be designed to generate different thrust vs time curves. The SRBs on the space shuttle decreased in thrust through the burn (http://upload.wikimedia.org/wikipedia/commons/3/31/Srbthrust2.svg). This can partially get around the KSP problem where solid boosters end up providing way too much thrust toward the end of their burn. Since real flights are planned out in much more detail than most KSP flights, the lack of dynamic throttle control isn't such a big deal. They aren't flying by the seat of their pants the way we do, so a predefined curve can work just fine.

[mythbusters844]

This might not be relevant in real life, but while playing KSP, I was trying to stay under the mass limit of 18 tons to launch fairly large payload. Using an SRB was much cheaper than using a liquid-fueled stage with the same amount of ∆v, but was also much heavier (which got me over the mass limit ) and was also larger (due to its lower Isp I would guess). I guess that's an aspect of using SRBs instead of LFBs: more mass and size in exchange for lower cost.

Did any of that even make any sense? I really need to get more sleep at nights...