Solid Vs Liquid rockets

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kk434

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After reading tons of technical material about the subject I still dont know ANYTHING. ICBM's are mostly solid, satelite launchers are liquid. When looking at a solid rocket design you feel that it must be cheap, no turbopumps and other expensive stuff and yet no one launches satelites on solid boosters. I'm totally in the dark, why isn't solid rocket the cheap alternative?
 
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DarkenedOne

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kk434":2u5ra9yd said:
After reading tons of technical material about the subject I still dont know ANYTHING. ICBM's are mostly solid, satelite launchers are liquid. When looking at a solid rocket design you feel that it must be cheap, no turbopumps and other expensive stuff and yet no one launches satelites on solid boosters. I'm totally in the dark, why isn't solid rocket the cheap alternative?
Solid rocket boosters are used for in the military and for ICBMs because they are easy to transport, maintain, store, and launch. They are also very simple in design making them highly reliable. They are simply solid rocket fuel in a tube. I do not know if you have ever had a model rocket or a fireworks rocket before, but if you did you would know how easy they are to use. This fact is important to the military, because they have to be able to use them under pretty rugged conditions. However on the down side there is no way to control or throttle the rockets thrust once it has been ignited. I am also under the impression that solid fuel costs significantly more than liquid fuel. They also have much lower specific impulses compared to liquid.

Liquid fuel rockets on the other hand are far more complicated. Liquid rockets require a whole fuel handling system including fuel tanks. They also require rather expensive and complex rocket engines. On the other hand liquid rockets have 1.5 to 2 times the specific impulse of a solid rocket. That means that it requires a liquid rocket significantly less fuel to reach a certain speed. As a result liquid rockets can be much smaller than and equivalent solid rocket. Liquid rockets can also throttle their thrust.

For sending satellites into space the higher performance of the liquid fuel rockets is worth their extra complexity. In addition to that having the ability to throttle the thrust level of the rocket is also a highly desirable factor precise orbital control.

So there you have it. Solid rocket are valued because they are simple and easy to use. Liquid rockets are valued because they have much better performance and the ability to control thrust levels.
 
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kk434

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I have flown seveal Estes C-motor rockets and have tried to build a model 2 stage rocket, so I'm familirar with model rocketry. Thats why i think it's suposed to be quite cheap. It's not that i haved to file for a chapter 11 bankruptcy after building those rockets. They are so simple and hence cheap so why not build a big SRB? Sounds like a good idea to me.
 
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Polishguy

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Solid rockets are less safe than liquid rockets. You remember, of course, the Challenger disaster? There's no way to shut off a solid engine in flight.

Solid rockets are also less efficient. Their specific impulse (measure of thrust per unit of fuel) is just 269 seconds, compared to kerosene/oxygen's 300 seconds, and hydrogen/oxygen's 450-464 seconds. You need more fuel to put a pound into space with solid boosters. At a certain point, this balances out with the lower cost of fuel, leading to liquid being more cost effective.
 
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annodomini2

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Polishguy":2wvpwzqd said:
Solid rockets are less safe than liquid rockets. You remember, of course, the Challenger disaster? There's no way to shut off a solid engine in flight.
True, but if you rupture the tanks on a liquid rocket it's the same effect, no throttle is going to make a damn bit of difference at that point.

Safety wise they are about the same.
 
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alpha_centauri

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kk434":3u57d668 said:
After reading tons of technical material about the subject I still dont know ANYTHING. ICBM's are mostly solid, satelite launchers are liquid. When looking at a solid rocket design you feel that it must be cheap, no turbopumps and other expensive stuff and yet no one launches satelites on solid boosters.
I guess you missed Ariane 5, the rocket that has over 50% of the world's commercial satellite market...... :p

To be fair though if it was designed from the beginning with this in mind and not launching Hermes it probably wouldn't have solid boosters. Solid rockets tend to be best as boosters for the early stages of flight for relatively heavy loads, they provide a huge amount of thrust to get massive "dead weight" objects off the ground. Simply getting a comparatively heavy system up to speed can usually require a large amount of fuel.

For example by far the majority of thrust at launch on the Space Shuttle system is coming from the solid boosters. If you were to generate this level of thrust from liquid rockets you need a hell of a lot of engines!

Of course you don't really need this if all you are launching is at most an approx ~10 tonne satellite. It's usually better to use more efficient liquid systems.

Saying all this ESA is developing the Vega small launcher which is almost entirely solid rocket,

http://www.esa.int/SPECIALS/Launchers_A ... CNC_0.html


Polishguy":3u57d668 said:
Solid rockets are less safe than liquid rockets. You remember, of course, the Challenger disaster? There's no way to shut off a solid engine in flight.
The ability or not to shut the engines off though was not really Challenger's problem IIRC. Could have happened on a liquid tank as well.
 
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kk434

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Yes this Vega rocket is very interesting, I hope they can make it cheap and launch very often, here in Sweden we launch a lot of Maxus(all solid) sounding rockets from Esrange Kiruna. I know that the solid rocket has a low specific impulse but you only need a tube filled with propelant, there must be a cheap way to build this! Those "rocket scientists" from Gaza build a lot of VERY cheap Quassam rockets(read on Wikipedia) and i think that a cheap and simple SRB is the way forward.
 
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RVHM

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Polishguy":252voiyo said:
Solid rockets are less safe than liquid rockets. You remember, of course, the Challenger disaster? There's no way to shut off a solid engine in flight.

Solid rockets are also less efficient. Their specific impulse (measure of thrust per unit of fuel) is just 269 seconds, compared to kerosene/oxygen's 300 seconds, and hydrogen/oxygen's 450-464 seconds. You need more fuel to put a pound into space with solid boosters. At a certain point, this balances out with the lower cost of fuel, leading to liquid being more cost effective.
That had very little to do with the solid rocket boosters. The same thing would have happened if they had been liquid rocket boosters, and probably earler in the flight since there wouldn't have been any aluminium to plug the gap in the O-ring until the Shuttle encountered wind shear.
 
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drwayne

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Polishguy":2v91fr4f said:
Solid rockets are less safe than liquid rockets. You remember, of course, the Challenger disaster? There's no way to shut off a solid engine in flight.

Solid rockets are also less efficient. Their specific impulse (measure of thrust per unit of fuel) is just 269 seconds, compared to kerosene/oxygen's 300 seconds, and hydrogen/oxygen's 450-464 seconds. You need more fuel to put a pound into space with solid boosters. At a certain point, this balances out with the lower cost of fuel, leading to liquid being more cost effective.
Note that comparing ISP does not tell the entire story. Solid fuel is much more dense than liquid, which has signficiant
advantages in terms of storage. That is in fact why there is a very useful parameter called density specific impulse that
is used.
 
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ZiraldoAerospace

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I think that solid fuel is more expensive because of them having to make up huge batches without any bubbles or imperfections that would cause it to explode. On a small sounding rocket, this isn't much of a problem, but on something the size of the shuttle boosters, you have to be extra careful, which costs money. The SRB fuel is like $20 per pound, and the hydrogen fuel for the shuttle in something like $1 per pound, and I think LOX is even less.
 
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Polishguy

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RVHM":tdixqa2s said:
That had very little to do with the solid rocket boosters. The same thing would have happened if they had been liquid rocket boosters, and probably earler in the flight since there wouldn't have been any aluminium to plug the gap in the O-ring until the Shuttle encountered wind shear.
I stand corrected on that matter. But I would also like to point out that there actually aren't O-rings on liquid rocket stages at all. Liquid stages are welded during construction, instead of in segments like the SRBs, no?
 
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pathfinder_01

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Solids don’t have to have O rings. The shuttle has them because the solid rockets are broken into segments for easier transport.

Solids however can be expensive because they are fully fueled. They are an explosive hazard and some of the compounds in the solids are toxic. In addition they are heavy (where as a pure liquid fueled rocket doesn’t mass much until fueled at the pad). This makes them cost much more to process and handle.
That being said they can be great source of thrust (although newer liquid engines esp. lox kerosene ones are less in need of this).

In terms of safety it is a complex question. Solids have less to go wrong, but you can’t test them before hand. Liquid engines can be tested before launch but are more complex.
 
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drwayne

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pathfinder_01":3prytiw2 said:
In terms of safety it is a complex question. Solids have less to go wrong, but you can’t test them before hand. Liquid engines can be tested before launch but are more complex.
And of course a conventional solid has issues with being gracefully shut down. (I put it that way because you can do
some things to "thrust terminate" a solid, but they are usually done towards the end of a burn profile to adjust flight
parameters.

And of course the long proposed "hybrid" solids which use a separate oxidizer are a different beast entirely.
 
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vulture4

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Solids save somewhat on development costs and are reasonably low in cost up to the size of the one-piece SRBs used on the Atlas and Delta medium variants. But the Shuttle/Ares SRBs are quite costly to reuse, heavy and hazardous to transport, and require the costly VAB, MLPs and crawlers. The Ariane also uses segmented boosters, although they are nonrecoverable and the fuel grains are cast onsite. However the Shuttle/Ares boosters are more than twice the mass of those on the Ariane. Solids produce more liftoff thrust, so for LEO launches where the payload is relatively heavy but doesn't require as much velocity, they have a relative advantage. For GTO satellite launches and planetary probes, where total acceleration is more important and liftoff mass is less, all-liquid high-ISP launch vehicles like the Delta IV heavy have a relative advantage. I suspect that the heavy versions of the Falcon will come out well ahead of the Ariane V in processing cost for comparable GTO launches, which may be why Ariane needs the Soyuz booster.

However solids are not practical to reuse, as Ariane decided even after adding a recovery system, whereas in theory at least a liquid can simply be refueled and reflown. Practical human spaceflight ultimately can only be achieved with fully reusable systems, so in the long run only fully liquid-propelled launch vehicles will be practical.
 
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scottb50

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vulture4":oluvx5au said:
Practical human spaceflight ultimately can only be achieved with fully reusable systems, so in the long run only fully liquid-propelled launch vehicles will be practical.
I think it would be easy to build a re-usable solid booster. All you need is a basic housing and a fully fueled, sealed propellant insert. Pull out the used insert and push in a new one. Inserts would be cleaned and re-used.
 
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vulture4

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scottb50":26hfsgbz said:
vulture4":26hfsgbz said:
Practical human spaceflight ultimately can only be achieved with fully reusable systems, so in the long run only fully liquid-propelled launch vehicles will be practical.
I think it would be easy to build a re-usable solid booster. All you need is a basic housing and a fully fueled, sealed propellant insert. Pull out the used insert and push in a new one. Inserts would be cleaned and re-used.
Easy on paper or with an Estes model, which is probably why it appealed to Mr. Griffin. I don't think it would work with a 500-ton SRB at 1000psi and 2000 degrees. Come to the Cape before everything is mothballed and see what's needed to do it in the real world with something the size of an SRB. The reality is that after ocean exposure and parachute landing the SRBs must be totally diassembled and magnafluxed, but before they can even be magnafluxed all the coatings and platings have to be removed with abrasive blasting, and the asbestos insulation with hydrolasing. All this creates tons of hazardous waste. Then the entire assembly process starts over again. And don't forget the ships and parachutes, the parachute inspections and repairs, the robots needed to spray the asbestos back on, the tedious crane lifts to stack the big segmented boosters, the hand operations to make sure the O-rings are in place, the maintenance on the huge VAB, mobile launch platforms the size of a ship, and the crawlers. The SRBs are one of the major reasons the Shuttle costs a lot more to operate than was originally anticipated. ESA has even recovered a few Ariane boosters but has made no attempt to reuse them.
 
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rockett

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scottb50

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rockett":mcmif1z7 said:
Why not this idea?
Buzz Aldrin already has a patent on it, it's called Starbooster.
http://www.starbooster.com/
Here's the patent:
http://buzzaldrin.com/files/pdf/2003.9.2.USPTO.PATENT.Flyback_Booster_Patent.pdf
If you consider 2 similar modules, slide into a much larger booster with multiple large Rockets and Jet engines. Quite a bit wider and many times bigger. But while I have never seen it before it helps explain my basic idea, for one part of a very large system.
 
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kk434

Guest
Someone here said thar SRB fuel costs 25$/ponud, but i dont know how accurate this number is. With this number 1 shuttle SRB would cost 25 milion $ just for fuel. Since a SRB is so simple, no moving parts it's must be cheap in the long run. Throwing away an expensive liquid engine every time sounds too expensive.
 
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