Vertical Mortar to Launch Heavy Rockets
Figure 1 shows a conceptual design for a vertically oriented gas-powered mortar capable of launching a large rocket ship at a velocity above the speed of sound. Figure 1 is not to scale in that the five-kilometer-long rocket launch tube and a one-half-kilometer-long combustion chamber below the launch tube are not shown to scale for the SpaceX Starship build 11.
Figure 1a shows a mechanism for having the rocket launch tube at atmospheric pressure during the launch. This figure shows the overall layout of the mechanism, including the elevator shaft needed to load rocket ships into the breach of the gun. Air is pushed out of the way as the rocket moves up the launch tube. When the rocket and the platform it is sitting on exit the gun, there will be a devastating muzzle blast. The silencer is not the correct design exactly, but the idea is to direct this muzzle blast upwards as to avoid damaging the rocket ships lined up to go down the elevator, and also to mitigate noise.
Figure 1b shows more detail on how the rocket is positioned in the launch tube sitting on top of a piston platform which has gas-tight sliding seals at the edges between the platform and the barrel. This figure also indicates the possibility that there is a partial vacuum in front of the rocket prior to launch. This would create some very serious engineering problems, and evacuating such a long and large-diameter launch tube is probably impractical. Figure 1b also shows that the combustible gas mixture may be ignited at multiple points along the walls of the combustion chamber and the launch tube.
The launch tube is 5 km long, and there is a 0.5 km long combustion chamber underneath the rocket. The barrel of the rocket launch tube has about three times as much volume as the combustion chamber located just below the rocket, which is sitting on its piston platform. The platform separates the barrel (which may contain a partial vacuum or atmospheric pressure) from the combustion chamber below, which has a higher pressure (for a 400,000 kg rocket + platform, the pressure differential between below the platform and above the platform would be at least 39,280 Pa, just to support the weight of the Rockingham platform). It is better actually if the pressure in the combustion chamber is exactly right to create a desired initial acceleration even before the combustible mixture is ignited. For the sake of being able to visualize the process better, consider the scenario where the diameter of the launch tube is 11.28 m. In this case, the cross-sectional area of the platform on which the rocket sets is 100m2. This means that the total volume in the lunch tube would be 500,000 cubic meters.
The elevator shaft needs to be far enough away from the gun barrel so that rocket ships can be queued up to go down the elevator without being damaged by the launch. There will be a conical-shaped silencer where the rocket comes out of the launch tube. The shockwave from The launch would otherwise be devastating.
There needs to be a large volume of high-pressure combustible gas beneath the rocketship for this idea to work. For Figure 1, there would be a larger 20.6 m diameter combustion chamber directly below the launch tube, which is 11.28 m in diameter (chosen so that area equals 100 m2). This means that the combustion chamber volume is approximately 1/3 the volume of the launch tube.
The initiation of combustion by the explosive unlatching is expected to only have a small effect on pressure initially. A ring of spark igniters at the transition zone between the combustion chamber and the launch tube guarantees that all the gas entering the launch tube has been ignited. The low velocity of the flame front in air/methane mixtures allows good control of the position of the flame front, and therefore also the pressure during the launch.
This air methane gas mixture has such a low combustion front velocity that if the bottom of it is ignited at launch, the flame front will not catch up with the rocket before the rocket exits the launch tube. This enables control of the combustion front by igniting the gas along the sides of the combustion chamber as the rocket moves up to the launch tube. The walls of the combustion chamber contain circular rings of spark igniters regularly spaced along the walls to allow for accurate location of the flame front propagating up from the bottom.
The rocket itself sits on a platform composed of truss structures that define a circularly symmetrical piston. The piston is held against the wall of the launch tube by sliding seals which are pushed against the inner surfaces of the smooth and seamless launch tube by inflation pressure and or the differential gas pressure across the seal (similar to the way that seals in conventional air or hydraulic cylinders work), which is capable to prevent leakage of combustible gas around said piston rocket platform.
At the end of the launch tube, there should be a silencer. Otherwise, each rocket launch will cause a truly devastating sound and blast wave as the rocket exits the barrel. I visualize a conical silencer. This will involve detailed calculations that I don’t have yet.
Although there could be a vacuum in front of the rocket ship as it moves down the barrel, that does imply some very difficult engineering challenges. It may be better to simply have the launch tube open at the end. To achieve an average acceleration between 50 meters per second to 100 meters per second given the geometry discussed here and a total launch mass of 400,000 kg, the pressure difference between below the platform and above the platform would be between 2 to 4 atmospheres.
The platform gets launched with the rocket and it’s either grabbed by cables or cylinders connected to hooks at the outlet of the gun, or it may return to Earth via parachutes.
There are Teflon (PTFE) or ultrahigh molecular weight polyethylene (UHMWPE) sliding seals where the outer perimeter of the rocket launch platform slides against the smooth inner wall of the launch tube. There probably also needs to be sliding surfaces between the upper part of the rocket and the launch tube for lateral stabilization.
This document is in effect written to get the attention of SpaceX and Elon Musk. Elon Musk has publicly stated that in order to establish a Mars base we will need to be able to launch one rocketship every 4 hours during the limited launch window we have every two years.
Consider a rocket launch where the rocket has an initial velocity of 500 m per second when it exits the launch tube. Even though this velocity is well below orbital velocity, having that velocity at launch (even if the launch is from sea level) greatly increases the mass that can be delivered to low earth orbit.
The efficiency of a rocket is nearly zero at launch, and increases as the rocket velocity increases. For the case of the Saturn 5 rocket, for example, 36% of the total mass of the rocket at launch (1.08 million kg) was consumed just getting the rocket up to the speed of sound (at about 20 km above the Earth’s surface). At that point, the rocket goes through Max Q which represents the maximum dynamic load on the rocket due to instability that occurs as the rocket goes through the speed of the sound barrier.
Numerous proposals in the past suggested accelerating a large rocketship using electromagnetic methods, but the flaw in these schemes is the very high capital cost of the launch mechanism, and the enormous electric power draw when launching a large rocket ship. The mortar launch method proposed here is a dramatically lower capital cost and does not require as many technical problems to be solved.
There are multiple ways of operating, as was described in my previous post on this subject. To reiterate, in what I consider being the best mode of operation, the launch will be powered by a combination of gas pressure which is not based on combustion as well as gas pressure arising from the combustion of a gas mixture behind the rocket ship. This gas mixture would be close to the lower combustion limit of natural gas in air. That would result in complete combustion of the methane, at a maximum temperature that is low enough to substantially prevent the formation of nitrogen oxides.
In the preferred methods, the rocket is sitting on a platform that works like a syringe plunger tip in that it creates a division between the gas pressure behind the rocket and the gas pressure in front of the rocket.
The means to create a smooth inner surface of the rocket launch tube without expansion joints is part of the intellectual property that I have protected in a provisional application which is now pending.
For very specialized launches at high velocity, hydrogen and oxygen may be used to achieve much higher muzzle velocity.
I want to thank Allison Brubach for the figures.
Come drop by my Twitch channel to discuss this with me or you can email me at grandpa@elpipe.com
