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The thing about solids is that they can have awe inspiring amounts of thrust compared to liquid engines, as you can see in this collection of boosters and thrust levels:

  • Shuttle SRB - 2.8 Mlbs
  • Ariane 5 boosters - 1.5 Mlbs
  • Titan IV booster - 3.2 Mlbs (wow!)

Compared to liquid engines:

  • SSME - 660 Klbs
  • F-1 - 1.5 to 1.8 Mlbs
  • RD-170 - 1.6 Mlbs
  • RD-180 - 800 Klbs
  • Merlin 1D - 160Klbs

But their ISP usually sucks.

So with ATK (maker of US solid boosters) pushing to replace Antares first stage with a solid, and maybe even Atlas V first stage with a solid, the question I have is, do the numbers work?

ISP is less important than thrust (per se) during the first few seconds and minutes of flight. You have to get the darn stack off the ground before it matters about efficiency. Upper stages are ISP dependent, but less so the first stage.

The question I have is, how much so? I.e. Can a Solid first stage really make sense in competition with high-performance liquid engines?

Contrary examples would be:

  • Ares-1x flight (SRB first stage)
  • Ariane 6 - proposed solid first and second stage
  • Vega light launcher
  • Minotaur
  • Taurus
  • Pegasus

Of course Ares-1 was cancelled. And Ariane 6 is unlikely to end up as a solid based booster. Minotaur, Taurus, and Pegasus barely launch these days. Vega is very overpriced.

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  • $\begingroup$ The ISRO PSLV (Polar Satellite Launch Vehicle) has the following stages: <p> Strap On boosters and Stage 1 - Solid. <p> Stage 2 - N<sub>2</sub>O<sub>4</sub>/UDMH. <p> Stage 3 - Solid. <p> Stage 4 -N<sub>2</sub>O<sub>4</sub>/UDMH. $\endgroup$ – David Ratti Nov 12 '14 at 5:51
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    $\begingroup$ There are other factors than just thrust and specific impulse. Cost being quite far on the forefront. $\endgroup$ – SF. Nov 13 '14 at 9:08
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    $\begingroup$ A giant SRB was planned as a backup for the 1st stage of the Saturn V...now that would have been something. Not really something good, but something. $\endgroup$ – Organic Marble Jul 23 '15 at 13:40
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    $\begingroup$ @OrganicMarble a 7.5 million lb thrust Solid? That would have been 'something'. $\endgroup$ – geoffc Jul 23 '15 at 13:45
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    $\begingroup$ They got far enough to dig a giant test pit in Florida and fired some test articles. atlasobscura.com/places/aerojet-dade-rocket-facility More info: astronautix.com/engines/aj2602.htm $\endgroup$ – Organic Marble Jul 23 '15 at 13:47
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Solid rockets are cheaper, and easier to maintain. They also have quicker turnaround time, and are often used for missiles as such. They are considered safer, and overall have a lot of advantages.

As you stated, they have some significant disadvantages, namely poor ISP and less flexibility. I believe you could make some rockets that would work, but almost all serious rocket launches are liquid, because of the increased performance. Solids are saved for some specialty circumstances, or to give a slight boost at takeoff, where they can greatly assist in the launch of a vehicle.

Bottom line, Solid rockets are worth studying to see if they would still give the required performance, but liquids overall are more likely to give you the performance you want.

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This all depends on the grounds of the competition, whether it is launch vehicle development or operations costs, system safety, or just sheer performance.

Often solid boosters are quoted as being simpler to develop and fly, as they do not require plumbing systems and do not separate 'tanks' from engines. Liquid propellant stages will likely feature more complex engines - in many cases more than one chamber - and may also have to account for the issues of cryogenic storage. This is perhaps mitigated with pressure-fed liquid fuel designs (like Sea Dragon). Also, with breakthroughs from commercial companies, reusability may soon allow for liquid stages to simply be refilled and flown. SRBs must have their propellant cast again in a lengthy process involving some disassembly, if they are to fly again.

System safety is anyone's game in flight, though it appears more sway towards liquid fuels for their ability to shut off propulsion in an abort scenario. Solid fuel motors cannot throttle (but thrust profiles can be tailored before launch), and cannot shut down. However, we must remember that the increased complexity of many current liquid fuel launchers gives the possibility of many more potential failure modes.

But I commonly see the analysis of safety only extend to in-flight dangers. It is crucial to remember that a rocket spends a long time in preparation on the ground while awaiting launch, and here solid fuel motors lose. A motor casing must have its propellant cast well before launch, and this includes during vehicle assembly. As such, workers are placed in the presence of live stages (think Shuttle in the VAB). Liquid fuel staged need only be fuelled on the pad, just before launch.

Performance is liquid fuel's game for the most part, with its generally much higher specific impulse. However, the battle for Isp is not most important during the first portion of ascent. From reading papers on proposed Shuttle liquid booster programs, it is 'impulse density' that wins here. Rather than simply exhaust velocity, an engine must combine that with reaction mass to achieve optimum figures (or something like that - where is that paper anyway!?) As you may see, this appears to say high thrust and fuel storage are what you're looking for, and SRBs have it all there - dense propellant and massive burn rates. This holds true for many liquid rocket stages as well - the Saturn V first stage's heavy launch mass and low specific impulse were less of an issue, because combined with its massive thrust it could generate delta-V against resistance from gravity and drag losses. It is these losses that ultimately define the design of lower stages.

About the rockets you listed - the Shuttle, Ariane 5 and Titan IV. Do you notice any similarity between their designs?

All three feature two SRBs around a high-performance, and long burning, liquid fuel core stage. The solid boosters provide most of the thrust at liftoff and then separate after a short burn, while the core does most of the work in reaching orbital velocity. This makes the core into what is known as a 'sustainer.' Basically, it is what makes a two-stage rocket out of a 'stage-and-a-half' boosted design (Titan's first stage is perhaps less suited to this, but it's core is not a LH2 stage and is less efficient than the others - thus it has more upper stages). The first stage, according to this, is the two solid boosters AND the core, together. The second stage is that very same core, without boosters. The core features an engine with high Isp but lower thrust and fuel mass, great for an upper stage but not for a first. The boosters feature a low Isp but massive thrust and fuel capacity - so, when combined, the boosters lift the core and fuel through atmospheric resistance, while the core slightly increases the overall Isp of the first part of the ascent. It's like having a normal first stage that is slightly more efficient than the boosters themselves.

Long answer short: solids shouldn't have to compete - they're best when they work with liquids, not by themselves.

Just no messing around when the thing is on the ground...

Al.

(By the way, that 'impulse density' stuff has escaped me for the moment. I hope it's correct, but if not, please forgive my forgetfulness!)

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  • $\begingroup$ Actually, some solid-fuelled rockets can be shut down and restarted in flight; the ability to shut down a solid-fuelled rocket before it had run through all its fuel was absolutely vital to enabling the Minuteman ICBM to be solid-fuelled. $\endgroup$ – Sean Jun 11 '18 at 2:48
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A few more reasons against solid rockets:

  1. They're difficult to control once ignited. To my knowledge, it's very difficult-to-impossible to throttle, shut down or restart them. While ability to throttle might not matter for a first stage, the fact that you can't just stop the turbo-pumps to shut down the engine might pose a safety risk. (Challenger disaster)
  2. SpaceX wants to develop a rapid reusability capability on Falcon 9's first stage. Here, liquid fuel seems to be superior. Although Space Shuttle's SRBs demonstrated some reusability (by parachuting the booster to the ocean and then refurbishing it), it was a long, painstaking process. By contrast, re-use of a liquid fuel rocket could, in principle, be as simple as filling the tanks with new fuel.
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As you said, for the first stage, gaining thrust for the first few minutes is much important than efficiency, and solid propellants provide much more thrust than the liquid propellants. plus they are comparatively cheaper. Due to presence of high atmospheric pressure, lower stages of rockets are designed to give very high thrust, so that the rocket could climb to high altitude very fast where atmospheric pressure will be low. To do that either solid rocket engines or multiple stages of liquid or cryogenic engines are used. LVM3 launch vehicle used two S200 solid propellant booster for lift off. Specific impulse for these were 274.5 sec and the thrust provided was 9316 KN. The core stage of LVM3 consisted L110 stage, which uses two Vikas engines (liquid), with specific impulse of 293 sec and the thrust provided was 1598 KN. So for the first stage of a rocket, i think solid propellants are more advantageous to liquid ones in terms of thrust and serve as a better option but unlike liquid ones, they do not account for safety and possess a higher risk.

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