[MobiusHorizons]

4 stages is certainly a lot more than most rockets, but most rockets don’t use solid rocket stages. Solid rocket motors are generally much more reliable/ simpler than liquid fueled stages, but they can’t be throttled or turned off early which makes it hard to achieve precise orbits. My guess is the extra stages allow for better control by carefully choosing when to light the next stage. The scout family of rockets are also 4 stage solid rockets and probably a good comparison.

[ggreer]

Solid motors are simpler, but I wouldn't say they're more reliable. It was the solid booster failure that caused the Challenger disaster, and solid booster failures that have caused ULA's Vulcan Centaur rocket to be grounded. As you say, solids can't be throttled or turned off easily. They also can't be test fired. Yes, you can static fire a solid rocket and reuse the housing/nozzle/etc, but solids often fail due to imperfections in the propellant. Pockets of air or fractures in the propellant can cause a sudden increase in propellant surface area, which then generates more pressure, potentially blowing up the booster. With a liquid rocket, the engine can shut down if there's an anomaly. But with a solid, there's no option but to let the reaction continue.

There's also the issue that a solid booster must be "fueled" before it reaches the pad, meaning you have ground crew working around a large quantity of explosive material. A Brazilian solid rocket exploded on the launch pad, killing 21 people.[1] Liquid rockets can be made inert until everyone is far away, then loaded with propellant.

A big advantage of solid rockets is that they can be stored for long periods and quickly launched. This is handy for use cases like ICBMs, but not particularly important for commercial launches.

1. https://en.wikipedia.org/wiki/VLS-1_V03

[londons_explore]

Don't you need at least a little thrust ~40 mins later to circularize the orbit?

Obviously solid rockets can't be relit.

Could that be the use of the 4th stage?

[numpad0]

Rockets are usually two-staged + satellites. The booster take it to outside of the atmosphere, then the upper stage puts it into a ballistic trajectory, and the payload does the circularization burn 45 minutes or so after the liftoff at the peak of the parabola.

Above is the basic semantics, and it can be further optimized, such as by extending battery power for the second stage to use it for circularization, inserting a single purpose satellite-like pusher device above the responsibility boundary at the top of second stage and payload satellite and calling it the third stage, or just adding actual third stage above second stage, etc.

Adding more and more stages improve performance per Konstantin Tsiolkovsky's rocket equation, but it'll add risk factors and also obviously add more dead weights in electronics and engines and support equipment, so 2-3 stages is usually the good balance between performance and risk/costs. You can have as many stages as you want if you think you can handle it.

[MobiusHorizons]

> Rockets are usually two-staged + satellites.

I think that only applies to liquid fueled rockets. At a minimum the Scout family of rockets (USA) and the Lambda 4s (Japan) both use 4 stages. That is what I was trying to say above. The sibling comment also points out that you at least need a third stage to circularize the orbit, since you can't re-light solid rocket motors. I'm not exactly clear on why its 4 and not 3, but that seems to be the standard with solid fueled rockets.

[numpad0]

I think it's for maneuvering. At least L-4S seemed to have had 3rd for the pitchover and 4th for the apogee kick. The first two were spin stabilized and unguided for political reasons. "Payload" stages built more like satellites are better suited for precise guidance.

(ISAS side of Japanese space programs is chock full of political BS, everything from the pencil rocket that continued on from IJN rocket researches to the "unguided" L-4S to the LUNAR-A probe with diameter of approximately 152-155mm to one-man laptop launchable Epsilon LV concept focusing on "civilian low-cost rapid launch demands")

[walrus01]

One example of a typical liquid fueled small third stage would be:

https://en.wikipedia.org/wiki/Fregat

[MobiusHorizons]

Good point! The final stage being liquid would also give them more control to fix any errors with the orbit.