Solar Sail First Stage and Then Impact Fusion to Get Towards Lightspeed

The Bussard Ramjet drive has been proven not to work, but laying out a runway of fuel could work. This has been examined in more detail.

Fuel is deliberately emplaced in the path of the spacecraft before the flight. Kare proposed that, at the velocities proposed for interstellar flight, nuclear fusion can be accomplished at the temperatures produced by impact, and proposed a propulsion system he tagged the “Bussard Buzz-bomb”. Analysis shows, however, that Kare’s initial concept is only plausible if the pellets can be pre-compressed before impact, a process which would have great difficulty to implement. A more detailed variant of the impact-fusion runway will be presented, and the major difficulties to this approach discussed. By going faster than 1245 kilometers per second first, we can start the passing over the fuel runway and have fuel explode at impact into fusion.

16 thoughts on “Solar Sail First Stage and Then Impact Fusion to Get Towards Lightspeed”

  1. Of all the interstellar rocket concept I’ve seen, the fission-fragment rocket and nuclear salt-water rockets are the best, most realistic.

    Instead of fusion pellets, have fission pellets feeding either of these. You get the advantages of bypassing the rocket fuel equation, but without all these problems of getting it to produce net energy/thrust by fusion.

  2. My first try would be an aerogel runway target impacting with a foil, neutral beam of atoms, or a combination of both.

    The concept might work better with pellets entering the engine from behind. conservation of momentum and all. The fusion products should spread out pretty fast. I suspect frequency of collision would be limited by materials constraints(temperature) rather than time to clear the reaction products from the pellet flight path.

  3. I think you will want multiple pellet dispensers station keeping along the path. A load of thousands of pellets. ultimately a generator that manufactures pellets from interstellar gases. The runway replenishes itself. So many vehicles can speed down the runway. Magsail deceleration or place laser deceleration systems on the other end.

    • The interstellar media’s extremely low density implies that collecting gasses there to produce the pellets is as much of a non-starter as the ramjet itself. For one thing, most of what you’d be collecting would be straight hydrogen, not deuterium, and thus almost impossible to ignite fusion in. (I withdraw this objection if you can find a brown dwarf or Kuiper object along the way.)

      You’re better off having a central pellet launcher in system, with access to condensed matter and plentiful energy, and using occasional “course correction” stations monitoring the motion of the pellets and correcting it to keep them on course.

      Remember, that while such course correction stations can be offsides, the launcher itself should be on axis,* and thus on the path the rocket will be following. So you really want to launch all the pellets at the start of the acceleration run, not midway.

      * It would be possible to launch pellets on a parallel course, and then use the correction stations to bring them on axis, or have the rocket “jink” over onto a new parallel stream, but why add complexity?

      • Perhaps have a system bound laser to push out ‘starchip’ like sails with fuel on them and an onboard spacecraft laser slows down each alternate sail so they impact at ionisation velocity or if high enough fusion temperatures. The reaction products are then funnelled out magnetically to produce thrust and power.

  4. I was thinking of this runway idea like 12 years ago when reading up on the ramjet.
    “How to get around the low density of interstellar hydrogen?”

  5. First you lay down the runway, with the pellet speeds optimized for the anticipated speed of the rocket when it reaches them. The faster the rocket will be traveling, the faster you want the pellets to be moving, because any relative impact velocity beyond that needed to initiate fusion is lost momentum in this application. So the more distant pellets have to be traveling faster.

    This may even imply that the pellets the rocket encounters towards the end of its acceleration were launched after the rocket departed.

    Then you use a pellet stream to push the rocket up to ramjet speed, with the impact fusion being delivered by the pellets arriving faster than the rocket. In this mode, you don’t bother slowing the pellets, because any excess speed is gained momentum, not lost. In fact, the fusion is just a bonus during this part of the procedure, it would work without fusion.

    Unfortunately, this scheme is not terribly applicable to slowing down at the other end of the trip.

    • At the other end? Well, you would certainly need a lot of prep time to get the pellets at that end in place. I don’t know how much momentum could be shed through gravity braking (using the stars and planets at the destination to slow down) or how much fuel would be needed even to do that.

      Another problem is that the interstellar medium isn’t precisely empty. Supernovas and such have been throwing their debris out into it for a long time. Granted the density is very low, but a ship traveling light years through it passes through a lot of volume and therefore has a non-zero chance of hitting something, and at those speeds, it doesn’t take much of anything to hit very, very hard.

      If we do send manned ships between stars I anticipate a good-sized ship might send out thousands (or many more) smaller devices ahead of it, say up to a light hour or so, so that that if they detect something (or hit something) the real ship has at least a few minutes to react.

      Conversely, it might be preferable to send several ships on each voyage, each with identical payloads, to include copies of the crew and passengers stored in memory or residing in a virtual space, while bodies might be constructed, printed, regrown, thawed out, woken up, or even rehydrated from anhydrobiosis. Whether you would reinstantiate any of the surviving travelers at the other end, after those in the first successful shipload, is a moral and ethical question with no firm answer currently available.

      • Actually, we know that the overall density of interstellar space is very low, because it’s not stable against gravitational collapse at higher densities, and because we can see distant stars.

        And we know that there’s a power law; Hydrogen molecules and atoms are the most common particles, as the size goes up the frequency drops very rapidly.

        So, the odds of hitting anything larger than a dust speck on an interstellar trip are really very low indeed. Interstellar gas is really more of a radiation than a collision hazard; At the speed you’re traveling the gas is encountered as particle radiation.

        It’s certainly *possible* to hit something massive on such a trip, but it’s very much like a not so nice version of winning the lottery; You’re aware it can happen, but it’s not sensible to guard against it, because it’s a very rare and difficult to defend against event.

        Your best bet for protection from dust specks is just levitating a solar sail ahead of you; The speck explodes on contact, leaving a microscopic hole in the shield, and expanding to a particle shower by the time you encounter it. The solar sail doesn’t have to be much larger than your ship, since your velocity is so much higher than the transverse velocity of the dust.

        • It’s a bigger problem than commonly understood. There’s 10e-6 dust grains per cubic meter of space. An interstellar ship of say 1000 m^2 cross-section and travelling at 25% c, travels through 7.5e10 cubic meters of volume per second. This is 75000 dust impacts per second of travel. Even the smallest most common 10e-16 kg grains at that speed, produce 0.28 J of impact energy. Bigger grains can produce far larger impact-explosions. This is PER SECOND.

          It’s going to have to cleared out first. Marshall Savage in “The Millenial Project” has laser beams doing this, and the craft are streamlined.

          “In the Solar System, interplanetary dust causes the zodiacal light. Solar System dust includes comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System.”

          “Thousands of tons of cosmic dust are estimated to reach the Earth’s surface every year,[3] with most grains having a mass between 10−16 kg (0.1 pg) and 10−4 kg (0.1 g).[3] The density of the dust cloud through which the Earth is traveling is approximately 10−6 dust grains/m3.[4]”

          https://en.wikipedia.org/wiki/Cosmic_dust

          • Perhaps a sail ahead of the craft held there by a laser would help, the laser would be aided by the multi bounce process so thereby reducing energy consumption. The impact on the sail disrupts the dust into a plasma that is easier to manipulate. The disrupted sail and impactor material can then be processed and added to the sail again so no holes develop.

    • “Unfortunately, this scheme is not terribly applicable to slowing down at the other end of the trip.”

      I would expect that the devices, mentioned in the previous post, for interacting with the stellar wind & interstellar gas with a drag force would be the way to slow down at the end of the trip.

      We will want some experience with using them for interplanetary travel to determine the limitations of those devices.

    • Maybe you could fire large pellets at very high velocity for the ship to rendevous with?
      Would probably be a statistical hurdle to hit, but an idea

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