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At first glance, solid rocket launching makes more sense than liquid:

  1. Simple to scale. Once designed, it can be made perfectly the same, all the time.
  2. Storage - Keep for as long as you want, launch on demand.

And modern designs can be made to be precise - if they can be made to hit a house halfway across the world, they should be able to hit a precise orbit.

So why so few commercial solid launch vehicles?

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    $\begingroup$ @AndrewThompson it's possible to design a SRB with a thrust decreasing over time. What's (almost) impossible is to shut it down. $\endgroup$ – Antzi Sep 16 '16 at 5:35
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Solid rockets have advantages, but also drawbacks:

  • they have low Isp. Especially on upper stages, this means the stage has to be large.
  • they generate more vibration than a liquid stage.
  • they're not as simple to scale as you think. Casting the solid fuel is a difficult process that has to be tightly controlled to get repeatable results.
  • because the entire stage is the combustion chamber, the entire stage has to be built to withstand combustion pressure. This means their empty weight is high and fuel fraction is low (85% for the Shuttle SRB, liquid rockets get over 90%).
  • rockets have to be able to reach a variety of orbits. This is much easier with a liquid engine which can be throttled in flight, and shut off before fuel depletion. A solid motor can only be throttled in the design phase (by changing the casting pattern for the propellant), and can only be shut down by blowing holes in the casing (which is not advisable when the motor is still attached to the payload).

The precision of an ICBM has nothing to do with motor design. The solid motors just propel the missile in the general direction of the target. After the boost phase, accurate targeting is done with liquid-propellant maneuvering thrusters.

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    $\begingroup$ Not to forget the problems with an exact cut-off when the desired velocity is reached. The only way is to blow up a hole into the hull of the solid rocket using explosives. $\endgroup$ – Uwe Sep 16 '16 at 7:08
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    $\begingroup$ Not the only way. There are other approaches to extinguishing an SRB. I would like to stress the vibration problem. Finding people willing to put their delicate satellites on a purely SRB first stage is tricky. $\endgroup$ – Rikki-Tikki-Tavi Sep 16 '16 at 9:29
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Making a solid rocket, that is large enough to be useful for orbital launch is quite hard.

You cannot fuel it on the pad, so the fuel has to be cast into the booster where it is manufactured. Thus it is very hard to transport after that, since a large SRB is VERY heavy. And often very long. So you cannot ship it by truck or even train.

Thus the Shuttle SRB's are made in segments so they can be transported from Utah to Florida via train. Length is an issue since the curves of the train tracks limit it before it might impinge on a train on the parellel set of tracks.

There is a great story, how the size of the SRB's is directly related to the size of a horses arse.

The SRB segment in length and width cannot exceed the ability to transport by rail. This is dependent on the gauge of the rail (distance between the two rails. Narrow gauge (Russia) and wide guage (US/Europe in some places) are common. Toronto subways as a fun example use a third size just to be different) Rail gauge is based on the Roman roads that still exist in some places in Europe. The width of the road was determined by a wagon with pairs of horses pulling it. Thus the direct connection.

Solids can have awe inspiring amounts of thrust (SRB at 2.8 million lbs of thrust, compared to F-1 or RD-170 engines at 1.5 Million lbs thrust. They are sort of the largest liquid engines seriously built) but their ISP is usually lousy. This is why they are often used as lower stages where thrust matters but ISP is less relevant.

Which is why it is always interesting when you see a design for a booster with a solid upper stage.

And yet, conversely, kick stages, sometimes a baby 4th stage, or even 5th stage can be solids. The Star line of motors is pretty common in commercial use. There, solids scale better for the size, do not need huge performance, just need that last little bit of impulse.

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    $\begingroup$ The final Ariane 6 design does not use a solid upper stage. It will use a Vinci LOX/LH engine. $\endgroup$ – Hobbes Sep 16 '16 at 17:52
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    $\begingroup$ Russia actually uses broad gauge (1520 mm) as opposed to Europe's standard gauge (of 1435 mm). $\endgroup$ – grizzly Sep 16 '16 at 22:09
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    $\begingroup$ The size limit for rail transport is the structure gauge, but that is somewhat related to the railway track gauge. Track gauges seem to be based on multiples and fractions of imperial feet. The chronological gap between Roman roads and railways is a rather large, thus I would not suppose a link between them. $\endgroup$ – Dohn Joe Sep 17 '16 at 9:36
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    $\begingroup$ Railroad gauge, horse's asses and whether the story is true: snopes.com/history/american/gauge.asp $\endgroup$ – Hobbes Sep 17 '16 at 10:11
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Solid rocket motors seem conceptually simple, but making them large enough and reliable enough to put something in space is not easy. This is fortunate, because solid-fueled ballistic missiles have a lot of military advantages over liquids (such as storability, portability, and readiness), and their production difficulty has delayed Iran, North Korea, and others from fielding such missiles despite decade-long development programs.

As others have mentioned, the low Isp and mass ratio of solid rockets also make them a poor choice for space launch, which demands exceptional performance. They have been used, though.

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