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Rockets are inefficient ways of accelerating from zero speed, due to the concept of propulsive efficiency. They also suffer from poor expansion ratio in the lower atmosphere. The Space Shuttle used about 30% of its propellant mass accelerating to 1000mph. This is a pitiful proportion of the overall potential and kinetic energy needed to reach orbit. But it accounts for a large percentage of total propellant expenditure. If this propellant cost could somehow be eliminated, we could reach orbit using an SSTO. This is the design goal of a launch assist system.
Could we use a very simple compressed air based propulsion system, to accelerate a rocket rapidly to a few hundred m/s? This would be a tank, with a pad on top that is actually mounted on guide rails on a launch tower. At launch, a valve would be opened on the tank and compressed air would expand adiabatically through a nozzle. This would provide high thrust, accelerating the pad and rocket upwards. The rocket stage would then ignite, propelling the rocket to orbit.
A very similar concept to this is the liquid air - water rocket. Liquid air and water are sprayed in fine droplets into an expansion chamber. This is done by gas pressurising the tanks where they are both stored as liquids. The liquid air vaporises into a high pressure gas by extracting the phase change energy from the water, which exits the nozzle as fine ice particles. Specific impulse would be about 80s. Which should be enough to accelerate a rocket to Mach 1.
Steam rockets are again a similar concept, which use phase change to produce high pressure steam propellant. The exhaust velocity of steam would be superior to air, but the storage pressure of water at 300°C+ is about 200bar.
Last edited by Calliban (2024-02-16 06:07:36)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban re interesting new topic....
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As an observation, submarine rocket launch systems may fall into the category of this topic. As I understand it, those systems are designed to move rockets rapidly from underwater storage to the atmosphere, where they are ignited so they may continue their journey under their own power.
Thus, I deduce that all the technology needed to implement your concept already exists.
What would be needed is leadership to persuade funders to take a chance on your idea.
Doing things as they have always been done is the mantra of older rocket engineers.
You'll probably have to look for younger ones to try your idea.
(th)
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For Calliban .... your new topic represents a new opportunity for old ideas to flourish once again...
An air gun (air rifle) is a technology that achieves remarkable performance using air as the energy transfer medium.
As mentioned earlier, existing submarine missile launch technology solves the significant problem of launching a ballistic missile through a layer of water, so that it can ignite it's engines and embark upon it's journey.
Your vision would appear to go well beyond existing examples of compressed air rocket launchers.
Please develop your idea at a scale that is relevant to the state-of-the-art of rocket launch on Earth in 2024.
You should be able to perform the calculations needed to put the existing Starship Heavy combination on an upward trajectory from some low value such as 1 meter per second to your target of 447 meters per second.
The launch tube needs to be sized accordingly, but it would all be below ground level, so the Earth itself can provide support for the walls of the chamber. The pressures will be significant. It will be a useful output of your work to see the actual values that would be required.
(th)
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In terms of maximum achievable exhaust velocities, these are ultimately set by the average speed of the gas in question at that temperature. This calculator allows three measures of average speed to be calculated for a number of gases.
https://www.omnicalculator.com/physics/ … e-velocity
For nitrogen at 0°C, average speed is 454.3m/s. At 300°C, this increases to 658m/s. For steam at 300°C, average velocity is 821m/s. This tells me that steam is a far more efficient propellant gas than nitrogen due to its low molecular weight.
If we are building the launcher underground, then we might as well build it as a cannon. This allows the launch platform to be much lighter as it only has to function as a sabot. To achieve a muzzle velocity of Mach 1 (343m/s), with an acceleration of 5g, the barrel length must be 1176m long. That is 3800'. If acceleration is 10g, the barrel length decreases to 588m, which appears more achievable.
Now for the pressure we would need. Starship second stage has a gross mass of 1300 tonnes, including payload. Its diameter is 9m, so lets make the launcher barrel 10m in diameter. To accelerate the vehicle at 100m/s2, we would need a force of:
F = ma = 1,300,000 x 100 = 130MN
Dividing that by cross sectional area gives the required pressure:
P = F/A = 130MN/pi x 5^2 = 1.655MPa (16.34atm(g)).
This is not a high pressure at all. We could make the barrel out of ordinary low alloy steel. It would be lined with grease to minimise friction and reduce corrosion. The sabot would be a steel frame with a plastic ring lining. I don't know if we would both reusing the sabot. Doing so means having a mechanism for either slowing it down dramatically before it exits the barrel or retrieving it. I don't think it will be an expensive component and it could be discarded with each shot. But we do need to consider where it will land.
Last edited by Calliban (2024-02-16 10:22:45)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban re #4
Thanks for this first iteration.
Boosting the Starship upper stage to 343 m/s is a good first step. The numbers are encouraging. Please continue to the logical next stop.
The Starship cannot reach LEO by itself, so please add the Heavy so the combination can put a large mass in GEO, or even larger in LEO.
If you are going to use steam, then a mixture of hydrogen and oxygen would seem well matched for the application.
here I am reminded that propellant for rifles (or any fire arm) is slow burning, so that the gases can be produced at a rate that does not burst the barrel.
Is there a way to moderate the combustion of oxygen and hydrogen. Can the ingredients be injected from ports as the vehicle rises in the barrel? Such a technique would moderate the combustion by supplying propellant as it is needed.
Please continue developing this interesting set of ideas.
(th)
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One unconventional launch assist system I have contemplated is a stratospheric light gas gun. Not having to exit into thick air should make things easier I think. The airship would be quite large and heavy in comparison to the rockets, but hopefully very very cheap with each shot.
There's a *slight* downside in that you're going to have to be very careful about where you point your stratospheric hypersonic cannon, since various countries will be interested in that...
An airship cannon could potentially use solar thermal and nitrogen, both of which can be sourced from the local environment, reducing the weight that needs to be transferred to the airship for each launch. We've talked about solar thermal airships to orbit before on this forum; this would flip that, so drag is far less of a problem, because we're not trying to accelerate the ship.
Re. nitrogen and water, I suggested that earlier as a heat source for liquid air energy storage. Both on ships and stationary. Let's you make up energy losses in storage using low grade heat to melt the ice. And it makes storing heat from compression far simpler than crushed rock systems.
Last edited by Terraformer (2024-02-16 11:22:43)
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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