launch rocket like bullet from steam/air rifle?

Question

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?

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?

Answer 1

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.

Answer 2

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.

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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.

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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.

Answer 3

The concept is sound. I believe this could work, but it would take considerable re-engineering of the launch vehicle. The pressures under the rocket would be immense so the launch structure could be compromised at ground level due to the shift from dirt to air pushing against the cylinder structure.

Pre heating the water to just under boiling would offer a faster reaction, and additional energy saved within the launch vehicle. I don't know the math, I'm just doing the logic in my head, so bummer.

If anything the sabot needed for the lauch would need to be developed from the ground up. We would need an ultra lightweight, cheap material that could withstand immense pressure. Need to really think about that one.

I theorize that the overall lauch structure could benefit from being completely built underground. That way the structure won't have any compromise points.

I also theorize that the overall design would benefit if the travel chamber was under vacuum at the event of launch. This could be achieved through a synchronize explosion event in a separate chamber connected via a narrow shaft to the travel chamber. I theorize an explosion that burns o2 could sucK the air out of the travel chamber empowering the potency of the steam.

In conclusion the small efficiency gained would expand the range of the vehicle; allowing for a broader mission capacity. A mere 3% increase in overall fuel efficiency could allow the rocket to carry a greatly expanded payload. A greater payload means we get more in space faster. This would be well worth the engineering, and construction cost of the project.