guest blogger john hare

Anytime a mission, manned or robotic, is proposed to destinations beyond Mars, travel times of years to decades are involved. Efforts to reduce trip times depend on some propulsion system well beyond the capabilities of chemical propulsion. The problem with most proposed systems is either the Isp is still not good enough, or thrust/weight is too low, or both. What we want is some propulsion system that has either relativistic exhaust velocities, or uses no propellant at all while having a high thrust/weight ratio. Other than solar and laser sails, nuclear in some form is required. With fission requiring bomb or at least radioactive type material launched from Earth, it pays to think of fusion solutions that use material that is more benign until triggered in some way.

I seem to recall suggestions that if two pellets of fusible material impact each other at enough velocity, fusion will occur. The problem is that the impact velocities are in the hundreds of km/sec range. Then you don’t get a sustained reaction, you get a really energetic boom. If we can get that energetic boom from a couple of pea sized pellets, Isp goes off the scale, if we can harness it. An Orion variant seems to be the most likely way to harness the boom for propulsion purposes. How do we get the boom?

I suggest that pellets are deployed in the path of a vehicle such that the closure rate is hundreds of km/sec. The external pellets impact the ship carried pellets to create a fusion explosion behind the pusher plate. The navigation and propulsion package for the external pellets becomes the reaction vapor that impacts the pusher plate at relativistic velocities.

With the external pellets supplying half the fusion mass and all of the vaporised material to drive the ship, effective Isp goes to the millions in a deep space fusion ramjet. Accelerating for a day at one gee gives about 850 km/sec of ship velocity. If Isp is in the millions, then mass ratio becomes less than 1.1. At 73 million kilometers per day, Mars is a day trip and Jupiter, Saturn and company are just weeks. All without launching fission material from Earth.

All we have to do is get the spacecraft up to 200 km/sec or so to get the reactions working. Getting perfect alignment between the ship and pellet stream for perfect impacts will be a bit of a problem. Some people get a bit upset when their vehicle is smacked by an object with TNT equivalent in the thousands, no sense of humor most likely.

We do have the technology to get the ship up to 200 km/sec relative to the pellets. It involves braking from the Earth’s orbit to a very close pass at the sun. Inside the orbit of Mercury, the ship can hit sun relative velocities of over 100 km/sec. The pellets have been braked all the way to a retrograde solar orbit that is the exact counter to the ship’s trajectory. At perihelion, the relative velocities of the ship and pellet stream is over 200 km/sec.

As the ship accelerates up the pellet path, the pellets have to have less velocity delivered from Earth with the ship supplying the impact energy. For interstellar probes, a month of acceleration at one gee gets up to about 8% of light speed. Here’s your safe, simple, and soon fusion drive, see you on Rigel 6.