It seems that less and less new rockets coming out are relying on solid fuel motors and are instead opting for liquid. I am just wondering why this is? Besides the obvious safety issues what factors are causing this change---Costs? Environmental laws? Production issues?
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I read that NASA is considering an upgraded Shuttle SRB for launching the CEV. I definitely don't like this idea.<br /><br />Solid rockets should eventually be phased out completely, we don't need them. They can never be as environmentally friendly or safe as liquid engines. Just look at the Saturn V, Delta IV or the Falcon I/V rockets...
When a liquid motor blows it can cause at least as much havoc as a solid. I really don't see either being safer than the other, except there are a lot more things to go wrong on a liquid fueled engine than a solid motor. <div class="Discussion_UserSignature"> </div>
You can test fire a liquid fueled rocket motor prior to incorporating it into a launch vehicle. That makes it inherently safer than a solid rocket.<br /><br />A solid rocket's success is based almost completely on quality control practices within the rocket factory. A single small mistake by one employee, not caught during the manufacturing process, may kill the astonauts upon launch and the Gods only know who else.<br /><br />I'm no steely-eyed missle man <img src="/images/icons/smile.gif" />, but solids seem best suited to throwing nuclear warheads on short notice, and serving as economical booster rockets. Maybe I'm wrong. Those NASA folks would certainly know better than I would.
Liquid-fuel rockets are more economical and are probably better suited for missions with a smaller payload as opposed to solid-fuel rockets which are large, complex and expensive. <div class="Discussion_UserSignature"> </div>
...and there was me thinking it was the other way around... anyone got some actual figures for solid vs liquid?
><i>...as opposed to solid-fuel rockets which are large, complex and expensive.</i><p>There's probably no machine out there that gets so much output with such a lack of complexity - in it's simplest form a solid rocket has no moving parts, as opposed to a liquid rocket which, as a minimum, has to have valves to control the flow of propellants. A solid rocket as large as the Shuttle SRB has <b>way</b> fewer parts than a large pump-fed liquid rocket such as the SSME.</p>
I think the Chinese figured out a long time ago that solid rockets are much less complex. Liquid fueled engines came along 600 years later. Liquid engines do have the advantage of restarting, Delta and Centaurs with RL-10 engines. <br /><br />The Hybrid engine may be the best of both worlds. Other than the exit nozzle, for gimbaling, you would only need a few moving parts. <div class="Discussion_UserSignature"> </div>
Of course, technically, the SSME has (I think) fewer moving parts than the SRB, but that's only because the SRB includes the TVC actuators and plumbing while with the SSME they are part of the Orbiter. Excluding the control and recovery systems, the SRB has very few moving parts.
Errr, scratch-off complex. I have a tendency of lumping negative terms together. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>
Solids are great for ballistic missiles and various weapon applications where you have to sit inert in a warehouse or silo for years on end and and be ready to rock and roll in minutes. Not so great in applications where you have to control the thrust (which is very high) and not restartable. You can terminate thrust with them, but it's not an elegant procedure. As on-orbit kick motors, they also leave behind an awful lot of orbital debris. Liquids are great for throttlability and restarting. Hybrids are intriguing and if they ever get into decent performance ranges, they will be certainly worth looking at. I especially like the burn-through sensor and the ability to shut down on SS1.
Sustitute LOX and a hybrid, like SS1, would work great. If you could relight it two or more times. A rubberized, easily cast, and inspected, propellant, would work wonders. With multiple tanks you could have multiple LOX sources and multiple engines. The problem becoming the total energy needed. The only moving part is the valve that controls a tank. <div class="Discussion_UserSignature"> </div>
In summary: Solid rockets are cheaper and less complex than liquid engines, while liquid engines are higher performance and controllable.<br /><br />Kinda tough to reach orbital velocity with *just* solid rockets...even the Shuttle's SRBs, which are probably some of the highest performance solid boosters around, only have an Isp of around 250 or so I believe...shuttle_guy can correct me if I'm wrong here. Meanwhile, the shuttle's Lox/H2 main engines have an Isp of 454, according to this link. Thats nearly double! Even the Lox/Rp-1 engines of the falcon supposedly have an Isp of something like 325-350. Your not going to get performance like that from a solid motor.<br /><br />Hybrids: I believe the N2O engines of the SS1 had an Isp of 235; however they used a blow-down system which meant they were not getting the best performance they could have even for N2O. Basically, an economy rocket engine 
. As shuttle_guy mentioned, a LoX based hybrid would have much better performance...not as good as LoX/H2 by any means, but possibly as good as a Lox/P1 liquid engine. Add to this the comparative simplicity of operation of a hybrid.<br /><br />Supposedly Jerry Irvine & Co tested a small amateur LoX/htpb hybrid engine at the MTA, but the king of spam was not forthcoming with any hard numbers on the results of the static test. That engine ("motor") would have been a strictly low-tech affair, anyway, with a burn time of less than 15 seconds.
. As shuttle_guy mentioned, a LoX based hybrid would have much better performance...not as good as LoX/H2 by any means, but possibly as good as a Lox/P1 liquid engine. Add to this the comparative simplicity of operation of a hybrid.<br /><br />Supposedly Jerry Irvine & Co tested a small amateur LoX/htpb hybrid engine at the MTA, but the king of spam was not forthcoming with any hard numbers on the results of the static test. That engine ("motor") would have been a strictly low-tech affair, anyway, with a burn time of less than 15 seconds.><i>Kinda tough to reach orbital velocity with *just* solid rockets...</i><p>Minuteman and Polaris missles get a significant payload to a large proportion of orbital velocity with solid motors. Athena basically puts its payload in orbit with the solid stages and uses the liquid stage to target the desired final orbit.</p>
Actually, ISP doesn't tell the whole story - if it did, we wouldn't bother with solids.<br /><br />The reason you get so much more thrust with a solid that a liquid is that the liquid is constrained by the ability of the liquid rocket system to mix propellant and oxidizer together. You really have to pump the heck out of the system to get enough of those together to get high thrust.<br /><br />In a solid, the propellant and oxidizer are already mixed, and the only constraints on the thrust are the burning surface, and the aperture diameter.<br /><br />Solids also have the advantage of being very dense.<br /><br />While not throttle-able, the thrust profile can be set by the solid fuel grain - and is in the case of the shuttle.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
Is it possible (technically) to put some decent payload in orbit with only solid booster ? If yes, i would think that even if this is not efficient it would cost way less because of the simplicity of the solid motor. The ground support between flight would be much less too.<br /><br />Overall, it would be more economical to use only solid even if not efficient (total fuel quantity need to reach LEO) because of all the saves done on the infrastructure. Is anyone able to put some numbers on that idea ?<br /><br /> <div class="Discussion_UserSignature"> </div>
The Pegasus rocket puts medium-sized satellites into orbit with only solid propellants. It is air-launched, and so it's not practical to make it liquid-fueled. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>
There is a ground launched variant of the Pegasus, called the Taurus.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
I have been known to forget my own phone number!<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
Interresting.... i think i'm gonna surf tonight <img src="/images/icons/wink.gif" /> thank you all for those inputs! <div class="Discussion_UserSignature"> </div>
By the way, there are solid fuel combinations these days that eliminate Chlorine from the mix....<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>
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