A NEW WAY OF GOING FAST

Technical FAQs:

How does the Jet-Gun projectile remain stable during launch without a barrel to guide it?

Short answer: Just like a rocket.

Long answer: As the projectile tilts to one side, it rotates about its center of mass. Thrust from the jet, acting through the center of thrust, attempts to reinforce this motion, and would succeed in the absence of aerodynamic drag. Thrust is destabilizing because its direction follows the direction of the projectile. As the projectile tilts to one side, thrust tilts in the same direction. Drag is a stabilizing force because it acts opposite to the direction that the projectile was traveling before it started to tilt. Naturally, the magnitude of the drag force must be less than the thrust force or the projectile wouldn’t go anywhere. Drag is still able to compensate for a destabilizing thrust force because the center of drag is behind the center of thrust, so it has a longer lever with which to stabilize the projectile.

Why are you developing the macron beamer for space launch? Why not just use a Jet-Gun?

The Jet Gun is limited to launch accelerations above 1000g because viscous dissipation of the jet limits the acceleration path length. A macron beamer can transmit kinetic energy over a vastly longer path because jets of solid particles are self-organizing rather than dissipative. This allows the launch acceleration to be reduced below 10g. Limiting our market to payloads that can handle over 1000g would make us non-competitive with conventional reusable rockets.

How do jets of solid particles self-focus?

When a hypersonic particle travels through the atmosphere, it carves out a low-density path of gas. Trailing particles experience lower drag forces in the vicinity of this path and are drawn into it while carving their own paths and attracting their own trailing particles. This behavior is seen in cases where an explosive is surrounded by a layer of dust-even when the dust layer is initially uniform, it organizes itself into many jets.

Why do you think that your launch system will be cheaper than conventional reusable rockets?

First, supplying energy and propellant from the ground allows launch vehicles to have a structural mass fraction of 25% instead of 5%. The development and operating costs of a launch vehicle can be vastly lower when it has comparable structural margins to a container ship. Second, the energy required to accelerate a rocket to a given velocity scales as e^(v/v_exh), while the energy required to accelerate a launch vehicle with a macron beamer scales as v^2. A rocket and macron beamer which have the same per-kg/payload cost to reach LEO will have a 2x-3x energy difference to reach the lunar surface.

How does the efficiency of your systems compare to rockets?

A Jet-Gun or macron beamer reaches approximately the same peak efficiency as a rocket, but it occurs when the vehicle is travelling at 40% of the jet velocity instead of 100%. This means that the macron beamer must achieve much higher exhaust velocities to efficiently propel vehicles to the same speed as a conventional rocket. On the other hand, the rocket must carry propellant as well as payload, so the overall energy consumed per unit mass of payload is much higher than a macron beamer.

What’s the fastest speed that Wave Motion has launched to?

700 m/s at a jet velocity of 2400 m/s.

What’s the heaviest projectile that Wave Motion has launched?

15kg

Can you comment on your technology's possible application to projectile-based nuclear fusion concepts?

One of our definite development goals is to be able to reliably launch projectiles to orbital velocity, which would put us close to the speed ranges needed for impact fusion. From a business standpoint however, we believe there is more opportunity in first applying this acceleration method to munitions and then space launch since the fusion market is rapidly filling with new players every day. But perhaps one day we may pursue a program to make a fusion reactor, our COO knows a thing or two about fusion!

Would be interesting to use a "parachute" made of graphene attached to the top of the spherical or bullet shaped projectile to catch the jet of gas. Should be strong enough to take the stress of a very forceful blast, be extremely lightweight, and if it folds around the projectile in flight, would not cause much drag.

Right, using a completely tensile structure to transfer thrust from the jet would be a good way to cut down on parasitic mass. We haven't been able to try it because the projectile stabilizes like a hobby rocket or arrow rather than a parachute-the center of mass must be in front of the center of drag for the projectile to be passively stabilized by aerodynamic forces. We would need to purchase some pretty powerful control systems to actively stabilize an unstable projectile. Our current generation of launchers puts out about a meganewton of thrust.

Where can I find information on your tests/trials?

We've got two conference papers in the Publications section of this Technology section which describe the tripwire velocimetry results of early tests. Our more recent testing is at a sufficiently large scale that we can measure velocity directly from video recordings of the launch. We'll be posting annotated video and screengrabs on our Twitter account (@WMLaunch) and on our website. We'll also update our website with some technical FAQs like "how does the projectile remain stable during launch without a barrel to guide it?"