Solid boosters

A solid booster is an engine and its own tank in one sealed case. You cannot throttle it and you cannot shut it down — but before it ever reaches the pad you can cast it two ways, and the two dials do different jobs.

What a solid booster is

The Solid Booster and the smaller Solid Booster S are cases packed with a rubbery propellant — the grain — with a nozzle cast into the base. Light one and it burns flat out until the grain is spent. There is no throttle lever for it, no Shut down on its sheet, and no way to refuel it: no joint on a booster carries fuel, so it cannot be topped up from a tank and cannot feed one.

That is the trade. What you get for it is a lot of push for very little money and no plumbing at all — strap a pair to the flank of a stage on decouplers and you have a rocket that leaves the pad properly.

Note

Each booster burns its own grain and nothing else's. Two boosters on the same rocket are two separate cases running down two separate clocks; neither can reach the other's propellant, and one is not slowed down or held up by the other.

The two dials

Tap a booster in the hangar, then tap it again, and its properties sheet carries two sliders. Both are hangar-only: they describe how the motor was cast, and you cannot recast it in flight.

Length — how long it burns

The nozzle is cast into the base, so it is the same size whatever the case above it holds. Lengthening a booster packs proportionally more grain in behind that same throat, so the motor pushes exactly as hard and simply takes proportionally longer to get through it. A full-size Solid Booster at its base length of 128 pushes 67 kN for 60 s; dragged out to 288 it still pushes 67 kN, now for 135 s. The wider case also casts longer: the Solid Booster S stops at 288, while the full-size Solid Booster goes on to 576 and burns 270 s. Mind the weight: the thrust stays put while the grain piles up, so past about 420 a full-size booster weighs more on Earth's pad than it pushes at a 100 % rate. A case that long is a sustainer: let a core or shorter strap-ons lift it, or cast it faster.

Length is therefore the only dial that changes how much the booster is worth overall. A longer case holds more grain, weighs more, costs more and carries the rocket further.

Burn rate — how hard it pushes

Burn rate, from 50 to 150 %, is the shape the grain was poured in, and it works the other way round: it scales the thrust and the rate the grain leaves together, so the case holds exactly the same propellant and simply empties faster or slower. Half the rate is half the thrust for twice the burn. The total the motor is worth never moves.

So a 150 % cast is a shove off the pad that is over quickly, and a 50 % cast is a sustainer that keeps pushing quietly through the gravity turn.

Solid Booster	Thrust	Burns for	Costs
Length 64, rate 100 %	67 kN	30 s	90 cr
Length 128, rate 100 % (as sold)	67 kN	60 s	180 cr
Length 288, rate 100 %	67 kN	135 s	405 cr
Length 576, rate 100 %	67 kN	270 s	810 cr
Length 128, rate 50 %	33.5 kN	120 s	180 cr
Length 128, rate 150 %	100.5 kN	40 s	180 cr

The BURN TIME row on the part's info card, and on its sheet in the hangar, always shows what the two dials have settled on, so you never have to work it out.

Why this matters on the way up

Because the two dials are separate, boosters on the same rocket can be made to finish at different moments, and that is worth doing.

Staggered separation. A pair of short boosters that burn out early can be dropped while a longer pair keeps pushing, so the rocket is not carrying four dead cases up through the thickest air.
A long core with short strap-ons. Give the middle booster the length and the outer ones the burn rate: a hard kick off the pad from the sides, then a quiet push that carries on long after they are gone.
Matching a burn to a stage. If your first stage's engine runs 64 s, casting the boosters to last about that long means everything empties together and one press of STAGE is clean.

Stagger two boosters' burnout has the steps for the first of those.

Note

A spent booster is dead weight and keeps flying with the rocket until something lets it go. Put the decouplers holding it in a stage of their own, and drop it the moment the flame goes out.

Getting rid of one

Letting go gives the case a shove off the flank and no more, so a booster released beside a still-burning rocket can still drift back into the stack. The Separation Motor exists for exactly that: a thumb-sized solid on the case's flank whose one second of shove throws it clear. Throw a booster clear has the steps.

Once it is off, a dropped booster keeps flying on its own — and if its grain is not spent, it keeps burning. Debris and dropped stages explains what happens to it after that.