Selenian Boondocks

guest blogger john hare

Once in a while there is a reference to pulse detonation engines for spaceflight. There is usually some claim that they offer major performance advantages, which have yet to be demonstrated. They also eliminate the need for high pressure pumps or pressure tanks because the propellant is injected during the off pressure cycle and the detonation wave pressurizes the propellants. While I don’t see the advantage of using an engine cycle that could be described as a series of hard starts, it is interesting to see if there is a better hardware layout than my references describe.

Fast acting reliable valves in a harsh environment seem to be the first thing that must go. Asking precision propulsion hardware to operate like a machine gun or internal combustion engine seems like a return to the nineteenth century. There have been some references to PDEs that muzzle loaded propellants so the valves would not be in the chamber. Starting from there, why not just have many chambers rotating past the injection point? Then the stream of propellants could be continuous and serve many chambers with each plumbing set. A 50 psi dual propellant spray would easily blast its’ way into the chamber with no high pressure gas to stop it. From there, a Rotary Rocket spike engine rolling past four injection points would come to mind. With no moving plumbing, the rotary part becomes a spike nozzle with many separate combustion chambers that have no interior plumbing.

Igniters that fire many times a second per chamber seem like the next thing to go. While spark plugs work well with cars and single chamber rockets, it carries a mass and complexity problem when applied to a hundred or so chambers rolling around a shaft that fire dozens of times per second each. A small tea cart class rocket  engine firing up into the chambers just past the injection points will guarantee sufficient ignition power every time without having to get fancy with electronics and timers.

The extreme noise should be a target as well. While a properly functioning PDE will never be as quiet (!?) as a properly functioning rocket, some steps to close the gap seem possible. With a hundred chambers or more rotating past at least four  injection zones at six hundred rpm, the individual propulsive detonations will be at four kilohertz or more. This almost has to be quieter than the dozens per second I have read about, or at least more uniform with less power per pulse.

Thrust vectoring is simply valving the four injection units for two axis, with a separate thruster required for the roll axis. Throttling is by restricting the flow to all units. Since the individual chambers hard start forty times per second, they won’t mind if the detonations are less energetic.

If the proponents of Pulse Detonation Engines are to be believed, it is possible to get high pressure engine performance without high pressure engine plumbing. Some of the references even claim far superior performance to normal rockets. While I am sceptical, knowledge would be  preferred to speculation.