Launching Heavy Rockets with Gas Pressure
Rockets increase efficiency as their velocity increases. Rockets are the least efficient at the time of launch. You can think of this in terms of what fraction of the kinetic energy released as the rocket exhausts its combustion gas is going into the kinetic energy of the rocket versus the kinetic energy of the gas itself. For a typical rocket motor, the gas is exiting the rocket at about 3000 m/s and the rocket itself is standing still. Under this condition, nearly 100% of the kinetic energy released from the combustion of the rocket fuel is going into the kinetic energy of the gas, and therefore through turbulence, nearly all of it is being wasted as heat around the rocket. Essentially none of it is going into the kinetic energy of the rocket. Later on, when the rocket is traveling at 3000 m/s, the gas exiting the rocket is motionless in respect to the reference frame which is defined by where the rocket was launched from. At that point, all of the kinetic energy is going into the rocket itself. This occurs because thrust is based on momentum balance between the rocket and the exhaust gas. Considerations such as these are called the Oberth Effect, and this is why it is desirable to burn a rocket at the maximum velocity. For example, this is why it is best to burn the rocket motor at the perigee of an elliptic orbit rather than at apogee.
By giving the entire rocketship an initial velocity that is on the order of the speed of sound or slightly supersonic, it is possible to more than double the amount of mass that can be delivered to low Earth Energy orbit by a rocket. If the rocket is launched at slightly supersonic velocity, one avoids the vibration that occurs when going through the speed of sound. This is the point where most rockets go through their maximum vibrational dynamic load or Max Q. Getting the entire rocket supersonic before launch could potentially reduce vibrations that occur during launch.
Many science fiction stories have talked about launching rocket ships using electromagnetic mechanisms. However such mechanisms are not practical because they are not economical. The cost of an electromagnetic launch mechanism depends on the maximum power that needs to be delivered. By the very nature of the launch, all the energy needs to be delivered in about 10 seconds or so which requires very high power levels on the order of a gigawatt which makes the electrical launch mechanism very expensive. For a large rocket such as the Saturn V or SpaceX’s Starship, the maximum power is greater than 15 gigawatts. This implies enormous banks of capacitors to deliver the power and also the linear motors involve a lot of metal and insulator and cost; by comparison, a gun is much more economical.
I envision launching large rocket ships such as SpaceX’s Starship out of a launch tube which is approximately 12 m in diameter using gas pressure. The maximum acceleration that can be withstood by the rocket ship and also by potential passengers is on the order of 10 times normal Earth gravity. this is much less than the maximum acceleration that occurs in a gun, which can be on the order of 50,000 g’s.
The needed gas pressure to launch a 3 million kilogram rocket (corresponding to the Saturn V or the Starship) is only on the order of 30 atmospheres which is a modest pressure compared to a gun. I have a few patentable innovations around making this practical. In particular, it is important that the barrel of this rocket-launching gas gun not have any expansion joints. The means by which I propose to do this is proprietary for now. Also, it is highly desirable if the portion of the barrel in front of the rocket is evacuated, but this implies that the rocket must punch through some sort of a vacuum seal when the rocket enters the atmosphere. This is also a problem for which I have a solution.
SpaceX, The Boring Company, and the Hyperloop are all interrelated with this idea. Elon if you’re reading this, I’d sure like to talk to you about those interrelationships.
I wrote a blog post on this back in March of 2018 if you are interested in a more detailed discussion.
