The Debate of Solid vs Liquid

[sevenperforce]

6 minutes ago, Canopus said:

I don't think they can be sure to still have control over the booster in such a severe event. So even in case of a liquid booster the launch escape system would most likely be designed to escape from a stage running at full power.

Designed, yes. The whole idea of a LES is to give you all-envelope, all-failure contingency escape. Design assumption should be that everything which can go wrong, will go wrong.

That being said, the capacity to simply shut down the booster engines (something which can be accomplished in a wide range of ways) makes liquid-boosted aborts far safer than aborts involving SRBs. In an abort involving SRBs, you have to make a decision about when to fire FTS. Too soon, and you could trap the capsule inside the blast radius; too late, and you have a booster-capsule impact. The whole situation must be carefully simulated with the AFTS having a complex decision tree to deal with it. With a liquid booster, on the other hand, there's no decision; you simply MECO your engines simultaneously with abort, and then you can fire FTS at your leisure after the capsule is well-clear.

[PB666]

1 hour ago, sevenperforce said:

Designed, yes. The whole idea of a LES is to give you all-envelope, all-failure contingency escape. Design assumption should be that everything which can go wrong, will go wrong.

That being said, the capacity to simply shut down the booster engines (something which can be accomplished in a wide range of ways) makes liquid-boosted aborts far safer than aborts involving SRBs. In an abort involving SRBs, you have to make a decision about when to fire FTS. Too soon, and you could trap the capsule inside the blast radius; too late, and you have a booster-capsule impact. The whole situation must be carefully simulated with the AFTS having a complex decision tree to deal with it. With a liquid booster, on the other hand, there's no decision; you simply MECO your engines simultaneously with abort, and then you can fire FTS at your leisure after the capsule is well-clear.

With a dual SFR system and one booster faultering MECO simply isn't an option, 'destroyed by extreme aerodynamic forces rings a bell'. Lets say for some reason one SFR is packed improperly, and there is break in thrust on one side then the main engines have to correct all the rotational motion, but in doing that the two engines, now with a offset center of thrust is now creating its own windsheer. About the only thing you could do is to kill release the SFRs and then kill the main engine.

One has to beg the question, given that liquid boosters capable of RTLS are available, with ~300 ISP (325 with methane) why exactly is it a good idea to stick with SFRBs on manned launches.

[DerekL1963]

2 hours ago, sevenperforce said:

You mistake my point. Cutting the engines on a liquid booster immediately kills thrust on the firing stage

No, it doesn't.  Thrust takes time to decay.
 

2 hours ago, sevenperforce said:

An SRB engine cannot be immediately turned off without FTSing the whole stage,

Completely false.  You can blow the bottom off or blow open exhaust ports without destroying the entire stage.

So, essentially, your entire position is based on two entirely mistaken beliefs.
 

2 hours ago, Canopus said:

I don't think they can be sure to still have control over the booster in such a severe event. So even in case of a liquid booster the launch escape system would most likely be designed to escape from a stage running at full power.

Yep.  Every LES I've ever seen discussed is based around three standards:  First, sufficient thrust to pull away from the booster even at maximum acceleration.  Second, the capability to pitch over and pull clear of said booster even if it continues to thrust.  Third the ability to clear the fireball and debris fields resulting from flight termination.

1 hour ago, sevenperforce said:

That being said, the capacity to simply shut down the booster engines (something which can be accomplished in a wide range of ways) makes liquid-boosted aborts far safer than aborts involving SRBs. In an abort involving SRBs, you have to make a decision about when to fire FTS. Too soon, and you could trap the capsule inside the blast radius; too late, and you have a booster-capsule impact. The whole situation must be carefully simulated with the AFTS having a complex decision tree to deal with it. With a liquid booster, on the other hand, there's no decision; you simply MECO your engines simultaneously with abort, and then you can fire FTS at your leisure after the capsule is well-clear.

You seriously think they don't carefully simulate launch escape situation for liquid boosters to ensure they meet the standards outlined above?

[mikegarrison]

4 hours ago, sevenperforce said:

Designed, yes. The whole idea of a LES is to give you all-envelope, all-failure contingency escape. Design assumption should be that everything which can go wrong, will go wrong.

That being said, the capacity to simply shut down the booster engines (something which can be accomplished in a wide range of ways) makes liquid-boosted aborts far safer than aborts involving SRBs. In an abort involving SRBs, you have to make a decision about when to fire FTS. Too soon, and you could trap the capsule inside the blast radius; too late, and you have a booster-capsule impact. The whole situation must be carefully simulated with the AFTS having a complex decision tree to deal with it. With a liquid booster, on the other hand, there's no decision; you simply MECO your engines simultaneously with abort, and then you can fire FTS at your leisure after the capsule is well-clear.

Now wait a second. You aren't going to be using a LES if everything is going right. So something is going wrong. How can you be sure you can still control the engines? Maybe the reason you are aborting is because you lost control of the engines and you can't throttle them back or cut them off. You can't assume that you can always shut off a liquid engine rocket.

Also, liquid rockets are way more likely to explode. Even with the Space Shuttle, the solid boosters didn't explode when they burned through. The explosion was from the liquid tank.

Solids have different error modes than liquids, but that doesn't mean they are less safe.

[Racescort666]

22 hours ago, DerekL1963 said:

It's Popular Mechanics - it's not exactly written for discerning and educated reader.

Ha, fair enough, Cosmo for wannabe nerds. After learning to play KSP and spending a good amount of time on this forum, decent websites who write about spaceflight have been pretty hard to come by. It's kind of funny too, the people I know on social media have no idea that what they share is crap. Here, the expectation is that claims must be backed up with evidence and technical issues must be addressed for something to be feasible. On social media, you get insulted if you even point out that there are technical problems.

Edited April 12, 2018 by Racescort666

[YNM]

3 hours ago, mikegarrison said:

How can you be sure you can still control the engines?

If you explode a tank full of LOX, it'll go in one bang.

Try do that to an SRB.

 

The reason why LES works is also because the roman candle beneath is easy to explode and burn away at one moment. If you have an out-of-control booster still hunting you down after your LES is gone, you're in trouble.

Edited April 12, 2018 by YNM

[insert_name]

What is the average mass fraction of srms, it seems like it would be higher since the propellant is more sense. Also the article treats NGLV like a manned Mars rocket when it is being designed to lift USAF satilites

[Racescort666]

9 minutes ago, insert_name said:

What is the average mass fraction of srms, it seems like it would be higher since the propellant is more sense. Also the article treats NGLV like a manned Mars rocket when it is being designed to lift USAF satilites

There is some advantage to denser propellants when it comes to very large mass fractions and SRMs have good mass fractions. I’ve really only looked into it on upper stages but I suspect that a better designed liquid stage would still be superior.

As for boosters, I haven’t ran any numbers on pure performance but it seems that liquid is the best because you have the option to propulsively recover.

[DerekL1963]

25 minutes ago, Racescort666 said:

I’ve really only looked into it on upper stages but I suspect that a better designed liquid stage would still be superior.

Superior by what measure?  That's a serious question - because solid upper stages wouldn't be so common if they didn't offer advantages over liquids.  Most noticeably, they're far simpler and hence more reliable.

[mikegarrison]

1 hour ago, insert_name said:

What is the average mass fraction of srms

Keep in mind that the entire tube is the combustion chamber and thus the entire thing needs to hold high pressures. The tanks of a liquid fuel stage can be much lighter, but this is counterbalanced by pumps and turbines, etc.

[Racescort666]

13 hours ago, DerekL1963 said:

Superior by what measure?  That's a serious question - because solid upper stages wouldn't be so common if they didn't offer advantages over liquids.  Most noticeably, they're far simpler and hence more reliable.

Mass/dV. What I meant by that comment is that if you took something like the New Horizons mission, ditched the Star 48, and stretched the Centaur a little bit, you'd have a lighter upper stage and more dV. They didn't do that because they couldn't make changes to the upper stage. 

If you were to replace the Star 48 with a liquid stage, there isn't a huge advantage in mass/dV so you're right, go with the simpler solution.

[p1t1o]

Here's a thought -

Rockets are not designed for maximum dV efficiency.

Rockets are designed for maximum cost efficiency.

Those two different design imperatives result in different looking rockets.

Solids are WAY cheaper than liquids. WAY.

 

[mikegarrison]

On 4/11/2018 at 12:23 AM, Canopus said:

In a scenario where you'd need to shutdown your boosters i doubt you'd have a much better time with liquid fuel rockets.

Once you leave the pad, it's basically all the same. There is no safety advantage from liquids -- maybe a safety disadvantage, actually. More complications mean more chance of failure.

The one key safety advantage involved in shutting down rockets is before you leave the pad. Liquid engines can be fired up with the rocket still clamped to the pad. If anything goes wrong with that, they can be shut down and the crew can just open the doors and leave. But once you light solids, they are lit. So you have situations like on the shuttle, where they could still do a pad abort if they had to after lighting the SSMEs, but once they lit the solids they were committed to some kind of an attempt to land the orbiter.

Edited April 12, 2018 by mikegarrison

[Pand5461]

1 hour ago, p1t1o said:

Solids are WAY cheaper than liquids. WAY.

That's what KSP gets wrong.

From Wikipedia page on Shuttle design:

Quote

While a liquid-fueled booster design provided better performance, lower per-flight costs, less environmental impact and less developmental risk, solid boosters were seen as requiring less funding to develop at a time

Mind that a solid rocket motor is tonnes of highly volatile material that can set off from any spark, so handling SRBs is quite an issue. Compare that to kerolox - kerosene is easily stored and handled, and liquid oxygen is produced on-site.

Another issue is that solid rocket fuel consists of granules of oxidizer (typically ammonium perchlorate) and fuel (aluminum powder) mixed in a polymer binder. If those granules are mixed unevenly, the whole thing may go kaboom in flight. And even if mixing is OK, solid fuel does not burn as smoothly as liquid one, so SRBs are infamous for high vibration level.

On top of that, you cannot do a static fire test on an SRB and then put it on a launch vehicle - it needs at least be refurbished (or, basically, made from scratch).

So, nope, solid rockets aren't as cheap compared to LF rockets as KSP teaches us.

[DerekL1963]

15 minutes ago, Pand5461 said:

Mind that a solid rocket motor is tonnes of highly volatile material that can set off from any spark, so handling SRBs is quite an issue. Compare that to kerolox - kerosene is easily stored and handled, and liquid oxygen is produced on-site.

0.o  Kerosene is a highly volatile material than can be set off by a tiny spark.  Worse yet, it's liquid and flows and can vaporize - and both can lead to ignition far from the actual storage location.  Solids on the other hand, stay put and can't be ignited by a spark that isn't in contact with them.  Kerosene seems safe and familiar because we fill our cars with it on daily basis, but handling it in industrial quantities can be quite a headache.  (Ditto LOX, which is produced at a seperate plant and trucked to the launch site.)
 

18 minutes ago, Pand5461 said:

So, nope, solid rockets aren't as cheap compared to LF rockets as KSP teaches us.

Setting aside that none of your post actually addressed costs...  They don't have to be cheap - they only have to cheaper than the alternatives.  That's why the Shuttle (which focused on up front costs and largely ignored operating costs) went with SRB's.  That's why solids are popular for adding a little extra oomph off the pad or as the final stage.

[mikegarrison]

15 minutes ago, DerekL1963 said:

0.o  Kerosene is a highly volatile material than can be set off by a tiny spark.  Worse yet, it's liquid and flows and can vaporize - and both can lead to ignition far from the actual storage location.  Solids on the other hand, stay put and can't be ignited by a spark that isn't in contact with them.  Kerosene seems safe and familiar because we fill our cars with it on daily basis, but handling it in industrial quantities can be quite a headache.  (Ditto LOX, which is produced at a seperate plant and trucked to the launch site.)

You fill your car with kerosene? What kind of car is it?

Kerosene is relatively safe at room temperature and pressure. Toss a match into a pool of it, and the match will probably go out. (But I wouldn't care to risk my life or property doing it.) The issue is the vapors -- they can mix with oxygen and reach an explosive air/fuel mixture, particularly if in a confined volume like a fuel tank. That's why TWA 800 exploded, and that's why all airplanes were later mandated to have nitrogen separators so that they can inert their center fuel tanks.

LOX is downright dangerous. It won't burn on its own, but it will certainly facilitate other things burning.

 

[DerekL1963]

Just now, mikegarrison said:

You fill your car with kerosene? What kind of car is it?

That was supposed to say "gasoline"...  meaning handling small quantities of familiar liquids isn't really relevant to handling industrial scale quantities.  Shouldn't have been writing one reply while thinking about another.
 

2 minutes ago, mikegarrison said:

Kerosene is relatively safe at room temperature and pressure. Toss a match into a pool of it, and the match will probably go out. (But I wouldn't care to risk my life or property doing it.)

Yup.  Had a friend in high school end up in the emergency room because trusted in the old saw that "diesel was relatively safe and hard to ignite".  (Thankfully his eye healed.)  I'd been using diesel to burn garbage on the farm, knew better, and backed way the hell away.  I don't care what it is, if it's volatile and flammable I'm going to treat it with respect and caution.  And folks handling it industrial quantities will too. 

[mikegarrison]

13 minutes ago, DerekL1963 said:

Had a friend in high school end up in the emergency room because trusted in the old saw that "diesel was relatively safe and hard to ignite".  (Thankfully his eye healed.)  I'd been using diesel to burn garbage on the farm, knew better, and backed way the hell away.  I don't care what it is, if it's volatile and flammable I'm going to treat it with respect and caution.  And folks handling it industrial quantities will too. 

Yeah. When refueling a plane everything gets grounded and all ignition sources are cleared from the area. Kerosene is only *relatively* safe compared to a more volatile hydrocarbon like gasoline.

[lajoswinkler]

Solid or liquid, hmmm... Well, I know some people prefer not to do solids upstairs.

 

[kerbiloid]

On 11.04.2018 at 11:11 PM, DerekL1963 said:

Completely false.  You can blow the bottom off or blow open exhaust ports without destroying the entire stage.

To shutdown ICBM - yes. Is it safe for manned rocket?

On 12.04.2018 at 2:12 AM, mikegarrison said:

You aren't going to be using a LES if everything is going right.

Unless you fly on Clipper which was to be using LES engines to reach the orbit if everything goes OK.

12 hours ago, p1t1o said:

Solids are WAY cheaper than liquids.

PBAN, pure aluminium and ammonium perchlorate are cheaper than hydrocarbons and oxygen?

11 hours ago, mikegarrison said:

There is no safety advantage from liquids

If you crash your turbopump, the fireball stays behind, while a sick SRB keeps chasing you from below. See the Challenger video. Both SRB required demolition, while the orange tank disappeared in flames.

10 hours ago, DerekL1963 said:

Kerosene is a highly volatile material than can be set off by a tiny spark. 

Even in nitrogen?

Solid fuel is a mix of ammonium perhclorate (!) and aluminium powder (!).
What do you prefer to keep in your garage?

Spoiler

Liquid-fuel engines are by definition better because you can refill the fuel tanks in KSP right in the flight mode.
While you can only put solid boosters in VAB.

P.S.
With liquids you have no problems with cracks. Because liquids are solid.

Edited April 13, 2018 by kerbiloid

[GoSlash27]

On 4/11/2018 at 12:13 PM, sevenperforce said:

the capacity to simply shut down the booster engines (something which can be accomplished in a wide range of ways) makes liquid-boosted aborts far safer than aborts involving SRBs.

sevenperforce,
 SRBs can be shut down instantly.

https://charlesvono.com/minuteman-thrust-termination/

Best,
-Slashy

[kerbiloid]

17 minutes ago, GoSlash27 said:

SRBs can be shut down instantly.

https://charlesvono.com/minuteman-thrust-termination/

Can two parallel SRB of SpaceShuttle, SLS or another rocket simultaneously?

Edited April 13, 2018 by kerbiloid

[p1t1o]

16 hours ago, p1t1o said:

Solids are WAY cheaper than liquids. WAY.

 

14 hours ago, Pand5461 said:

That's what KSP gets wrong.

From Wikipedia page on Shuttle design:

Mind that a solid rocket motor is tonnes of highly volatile material that can set off from any spark, so handling SRBs is quite an issue. Compare that to kerolox - kerosene is easily stored and handled, and liquid oxygen is produced on-site.

Another issue is that solid rocket fuel consists of granules of oxidizer (typically ammonium perchlorate) and fuel (aluminum powder) mixed in a polymer binder. If those granules are mixed unevenly, the whole thing may go kaboom in flight. And even if mixing is OK, solid fuel does not burn as smoothly as liquid one, so SRBs are infamous for high vibration level.

On top of that, you cannot do a static fire test on an SRB and then put it on a launch vehicle - it needs at least be refurbished (or, basically, made from scratch).

So, nope, solid rockets aren't as cheap compared to LF rockets as KSP teaches us.

 

4 hours ago, kerbiloid said:

PBAN, pure aluminium and ammonium perchlorate are cheaper than hydrocarbons and oxygen?

 

Ok so some push-back, and on examination, perhaps I should not have typed "WAY" in all-caps, twice 

But solids *are* the cheap option. And I wasnt using KSP as my yardstick either.

Its not merely about price-per kilogram of fuel, its about how much it cost to get your payload into the desired orbit. How can anyone play KSP and presume that cost can be derived from a single value like price per unit of fuel?

Solid might have poor Isp, and they might be more expensive dollar-per-kilo (although I had a lot of difficulty confirming just exactly how expensive the average SRB is, nor is it particularly easy to find quoted prices on bulk amounts of what are essentially explosives....I may be on a few more watchlists) but they are cheap in terms of dollar-per-unit-thrust.

And some more things:

You really cannot discount the simplicity of their construction. Nor can you ignore the development costs for solids, which are certainly much less than that of liquid engines and does have an impact on the cost of launches even well after development is complete. Rocket scientists gots to gets paid.

Kerosene, one of the safer liquid fuels, is easily stored...only compared to things like cryogenic liquids or toxic fuels like nitric acid. Kerosene is still a hazardous liquid that requires specialised site safety measures. 

Producing liquid oxygen on site is also quite handy, but this isnt exactly cheap. The fact is it is produced on site because it is such a hassle to work with - its dangerous enough that constructing a specialised factory is preferable to storage or transport. And a liquid oxygen factory is not a safe place, requiring expertise and oversight. Do not ignore how dangerous LOx can be. Solid fuel is explosive. LOx can make your clothes explosive. LOx can make concrete flammable. Not that these are likely hazards - because of safety measures...which come at a cost.

Solid fuel *is* dangerous,  but for example, it wont spill or flow through gaps, it doesnt form clouds of explosive vapour, it wont flow into watercourses, it cant be easily inhaled or exposed to skin or eyes. Solid boosters can be locked up in a climate controlled shed and largely ignored until needed, transport costs for hazardous liquids as compared to solids are higher. None of these things individually are huge deal breakers but they all add up.

Some people say "But they cant be shut down" - I say, for a rocket, this is often an advantage. A liquid engine can fail, and the rocket fall back to the pad - failures in the first 30 seconds are the most feared events. Light off a solid and there is very little on gods green Earth that will stop it from at least going very far away. Solids are extremely reliable, whilst liquid engines require complex active control measures and have many failure modes. Plus others and I, have pointed out that it is far from impossible to shut down an SRB. I will note though that safety concerns change dramatically when the vehicle is crewed, but then this changes many significant factors and man-rating a craft is hugely expensive for these reasons.

Some people say "solids cant be tested, only fired" - true, but then an SRB has 2 components: the fuel and the casing, it hardly needs testing. A liquid engine on the other hand can have hundred of components, many with very fine dimensions and tolerances, made of many different materials, often involving huge differences in temperature. They MUST be tested in case it EXPLODES. An SRB is far more predictable in nature.

Many of the above things are important factors in insurance policies which is a large part of the price of a space launch. Sometimes a rocket could even be more expensive, but if it gets you a lower insurance premium based on reliability and component failure rates....

Like, do you want the liquid upper stage on your one-off, very expensive, important satellite? Or do you want the very simple, solid-state booster that has worked on hundreds of launches. Even if it means a slightly larger lower stage, the solid is often the right choice.

 

 

I will admit though that after some research, the difference in cost is - as far as I could tell due to difficulties mentioned above -  not as much as I had thought.

Dont make the mistake either, of assuming that I am trying to say that solids are better/cheaper in all situations. Im only giving a rationale for why they are ever used at all.

 

 

 

13 minutes ago, kerbiloid said:

Can two parallel SRB of SpaceShuttle, SLS or another rocket simultaneously?

Manned and unmanned flight must be treated seperated. Could multiple SRBs be shutoff? Of course, certainly. Can they be shut off whilst saving an intact payload? Thats much more difficult - but it is not exactly easy with liquid engines either.

But multiple SRBs *can* be prevented from running out of control, though it may be easier/simpler to engineer the flight so that their maximum splashdown footprint is all within a safe area.

 

***

 

Why are LESs solid fuelled?

Because you can get monstrous thrust.

Because they can be relied upon to fire exactly when you want them to.

Because Isp efficiency is a tiny factor in this context.

 

Edited April 13, 2018 by p1t1o

[kerbiloid]

1 hour ago, p1t1o said:

Could multiple SRBs be shutoff? Of course, certainly.

Without a time gap making the rocket to turn?

1 hour ago, p1t1o said:

Why are LESs solid fuelled?

They are simple to ignite. You can run them while the booster is still burning and going to explode.

Edited April 13, 2018 by kerbiloid

[p1t1o]

40 minutes ago, kerbiloid said:

Without a time gap making the rocket to turn?

The answer to that is down to the engineers and scientists constructing the rocket. But for my given answer above, its not relevant. If all you want to do is stop a failing rocket from going off-range, asymmetric shutoff is less of an issue, and if we're talking crewed vehicles then there are already obvious safety concerns such as escape in the first minutes of flight. I agree that solids are less appropriate fro crewed launches:-

1 hour ago, p1t1o said:

Can they be shut off whilst saving an intact payload? Thats much more difficult - but it is not exactly easy with liquid engines either.

 

Can any pair of widely seperated boosters be shutoff in such a timely manner? Its not a trivial process to clamp down on enormous fluid flows either.

But for what its worth, the blow-out shutoff on the minuteman booster was extremely punctual as it was part of the aiming process - by modulating the impulse you modulate the distance the warhead flies and very small time errors - or residual thrust - could result in large aiming errors.

 

***

40 minutes ago, kerbiloid said:

They are simple to ignite. You can run them while the booster is still burning and going to explode.

Which is what I meant when I said:

1 hour ago, p1t1o said:

Because they can be relied upon to fire exactly when you want them to.

 

 

Edited April 13, 2018 by p1t1o