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I've included an image depicting how exhaust energy from a rocket launch can be recouped by turning it into a large scale pressure cooker by the use of some kind of platform=sabot (thanks @SolomonsSlow). Could this work? And more importantly, would it save a worthwhile amount of energy/fuel in, e.g., a Falcon 9 launch?

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The timings are based on a Falcon 9 starlink launch. I assume engines are not throttled up at launch, but since the total burn time of stage 1 is 170s, this might save a ballpark ~3% of fuel, assuming you obtain the same speed after 12s=200m but with the boosters working at a utopian 200% efficiency. Unfortunately, I don't know enough about rocket launches to do a meaningful calculation.

Also, one could easily devise other ways of jump-starting a rocket in a rocket fuel-independent manner (e.g., dump flywheel energy into a railgun-like propulsion mechanism) that further accelerate the rocket. Why aren't people doing this? Are such endeavours simply not efficient enough to pursue?

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    $\begingroup$ I can't answer, but FYI: Your "platform" somewhat resembles the thing that the firearms industry calls a sabot. That is, a lightweight "carrier" that seals against the inside of the barrel of the gun, and which falls away from the projectile shortly after leaving the barrel. "Sabot" (sounds like "sah*bow") is an old French word that means "shoe." $\endgroup$
    – Solomon Slow
    Feb 18, 2021 at 14:46
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    $\begingroup$ Assuming a rocket could be modified for this to actually work without any additional mass penalty (not a realistic assumption) and that you could actually achieve that 3% propellant savings for a modified Falcon 9, that amounts to about \$9000 worth of propellant. Propellant is literally cheaper than dirt, making things more complicated and adding infrastructure to save propellant is generally a losing trade. $\endgroup$
    – Christopher James Huff
    Feb 18, 2021 at 17:01
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    $\begingroup$ Related: What benefits can be gained from launching below ground? $\endgroup$
    – Russell Borogove
    Feb 18, 2021 at 17:04
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    $\begingroup$ Different question, same answer: Why aren't rockets launched in movement? $\endgroup$
    – Russell Borogove
    Feb 18, 2021 at 17:06
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    $\begingroup$ @PDiracDelta the base of the rocket is not built to withstand the forces you'd be exerting on it. The rocket as a whole is only built to be propelled by the thrust of the engines. Keeping it from emerging from the tube as a big fireball will cost additional structural mass. And no, you can't just replace the propellant with payload. The payload is hauled all the way to orbit, most of the propellant is expended in the first minutes of flight. Also, few flights are anywhere near the mass limit, and if it was a problem, it'd be cheaper to just make a slightly bigger rocket. $\endgroup$
    – Christopher James Huff
    Feb 18, 2021 at 18:31

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Your launch method would destroy an unmodified rocket by the heat and pressure of the exhaust. The water sprinklers would reduce the heat a little, but not the pressure.

The effect would be very, very tiny. The rocket has to reach a hight of 400 km and a speed of 8 km/s. If 100 m height is done in 8 seconds, the mean speed is only 12.5 m/s, a little more than a thousandth of the needed 8000 m/s. A thousandth of speed is equivalent to a millionth of energy.

It is like trying to speed up a race car (with running engine) by push-start for the very first meters.

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  • $\begingroup$ OK, so I understand that in reality this would either: a) not destroy the rocket but provide low added value, or b) have enough energy to make go at least somewhat fast, covering the 200m in a lot less than 12s, but also probably destroying the rocket. So in conclusion: for this to work, the rocket engines need to be modified to withstand the huge pressure, resulting in huge acceleration, that makes the rocket shoot like a bullet from a gun. I guess this puts unnecessarily strict constraints on the engine, as after 200m the external pressure is atmospheric again -> inefficient. Correct? $\endgroup$
    – PDiracDelta
    Feb 18, 2021 at 16:12
  • $\begingroup$ @PDiracDelta your thinking is along the lines of launching a rocket from a plane ie: using another device to save launch energy requirements. Generally know in rocket parlance as "the first stage". $\endgroup$
    – Robert DiGiovanni
    Feb 18, 2021 at 17:24
  • $\begingroup$ @PDiracDelta The large tanks of the rocket for fuel and liquid oxygen are much more sensitive to external pressure and heat than the rocket engine. If the 200 m are covered in 1 s instead of 10 s, you get 200 m/s instead of 20 m/s, but you still need 8000 m/s, that is 2.5 % instead of 0.25%. $\endgroup$
    – Uwe
    Feb 18, 2021 at 19:25
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would it save a worthwhile amount of energy/fuel in, e.g., a Falcon 9 launch?

As it stands? no. You'd need a whole new first stage capable of surviving the heat and pressure of launch (hands up now, who likes the idea of flame-grilled tanks of liquid and possibly rather cold rocket fuel?) as well as engines that can operate efficiently in a very hot and high pressure environment.

More importantly, you'd need to build a whole new set of launch facilities, cobble together some prototypes, blow a bunch of them up, get some working, then sell launch capacity at a discount to encourage people to use your unproven and risky new launcher, etc.

In that regard, whether or not it saves a few percent of fuel is rather counterbalanced by the enormous up-front cost and risk. It isn't a great trade-off.

Also, one could easily devise other ways of jump-starting a rocket in a rocket fuel-independent manner (e.g., dump flywheel energy into a railgun-like propulsion mechanism) that further accelerate the rocket. Why aren't people doing this? Are such endeavours simply not efficient enough to pursue?

There are already air-launched rockets of various colours and flavours... the first one that comes to mind that's used to put actual useful payloads into orbit is the Pegasus. There are more famous suborbital things like SpaceShipOne, of course, and also the likes of the ASM-135 anti-satellite missile, which was never used.

Ground-based launch assist systems seem to be perpetually academic exercises, but maybe one day we'll get some kind of maglev first stage. The MagLifter idea popped up in 1994. Non-maglev test tracks also exist.

Thing is, boring old-school staged liquid fuelled rockets work. They're reasonably priced, and reasonably reliable. There's not enough incentive to play with alternatives, yet.

For something that is a little bit like what you're thinking of though, you can look for a popular but fortunately little used style of rocket: the submarine launched ballistic missile.

To quote from the Titan II launch process described here:

TRIDENT missiles are launched from the submarine by a steam generator system. A small, fixed solid-grain gas generator is ignited and its exhaust directed through cooling water into the base of the launch tube. The missile is ejected from the tube, through the water, and to the surface. At that point, the missile's first-stage rocket motor ignites and sends the missile on its way

Note that the gas generator isn't part of the rocket itself; that saves a bit of mass and additional staging complexity.

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