Design of an unmanned interstellar probe using near term technology

A Munich Ghost Team has submitted their concept for a space probe to Alpha Centauri.

The Ghost Ship uses one single fusion propulsion stage for acceleration and deceleration. Deceleration is further supported by a magnetic sail system, which uses the drag of interstellar hydrogen acting on a magnetic field which decelerates the spacecraft. The fusion propulsion system is based on Deuterium – Deuterium inertial confinement fusion. Inertial confinement fusion is based on compressing a tiny pellet of fusion fuel by an ignition system, in our case a number of lasers. The fuel is compressed by these very high-power lasers to such a degree that fusion can occur. The team decided to use Deuterium – Deuterium, as Deuterium – Tritium would use large amounts of Tritium, which decays quite rapidly. This means that a prohibitively large amount of Tritium has to be stored on-board of the spacecraft. Deuterium – Helium 3 was discarded due to the difficulties associated with mining Helium 3 from the Moon or the gas giant planets.

The fusion ignition system is based on the fast ignition scheme. The beauty of this ignition scheme lies in the decoupling of compression and ignition of a fuel pellet. Without decoupling, a lot of energy is needed to create fusion conditions within the pellet purely by compression. It is like igniting a rod of dynamite by pinching it. It is possible but you need to pinch it very strongly. What you use instead is a “fuse”: a secondary high-power laser, which pierces the pellet and ignites it. In this way, you get the same amount of energy out of the pellet by using a lot less energy for compression and ignition.

However, in order to charge the lasers, you would still need a very large and heavy power source. So we used a “trick” to circumvent this requirement. We thought about using the fusion reaction itself as a power source. Thus, we would use the previous fusion reaction for igniting the next one. However, this is still difficult to do and large capacitor banks that store the energy would be needed. However, we found another trick. The Deuterium – Deuterium reaction produces a large amount of neutrons. Neutrons are uncharged particles. This means that they fly away in all directions. Thus, they do not contribute to thrust, as for generating thrust, particles would have to be ejected into one specific direction. So is there a possibility to use these neutrons for power generation? It would be ideal, as they would be otherwise lost. Surprisingly, there is a way to use neutrons for power generation: the nuclear-powered laser. Nuclear-powered lasers directly convert neutron energy into laser energy.

The neutrons enter a chamber filled with Uranium dioxide. This chamber has a ring-like shape and is located around the thrust chamber. The neutrons react with the Uranium dioxide, causing fission reactions. The fissile products then excite a fluorescent gas. The light flash which is generated by the gas is transmitted by a light pipe to the laser amplifier which generates the laser beam. This system has a very high efficiency of about 8%. Unfortunately, this system has to be fed with a Uranium dioxide aerosol and fluorescent gas. With these consumables, the total mass of the ignition system is a bit less than 1000 tonnes. Compared to alternative charging options, this is still a reasonably low number.

With a pellet mass of two grams and a pulse frequency of 150 ignited pellets per second, we need a total of 150,000 tonnes of fuel for our spacecraft. The specific impulse of our engine is about 540,000s.

The terrific power output of the engine creates waste heat that must be rejected. Otherwise, spacecraft elements will start to melt away quickly. We use a liquid droplet radiator, which has an extremely high heat rejection rate of 500kW/kg by using liquid Aluminium as the heat-conducting liquid. The radiators have a total area of 7.6km².

The main structure which holds the spacecraft together is a cylindrical truss structure based on Carbon Nanotubes. Carbon Nanotubes have an exceptionally high tensile strength at a very low density. Such a material is needed, as the engine thrust of about 1.6 MN, which is equivalent to about 160 tonnes of compression force, is acting on the structure for over 15 years. A structure subject to compression forces tends to buckle. A thin structure will buckle easier than a structure with a large cross-section. This is the reason why our spacecraft uses a structure with a diameter of 100m.

The spacecraft will accelerate for 15.6 years up to a velocity of 6%c. After 54 years of cruising, a magnetic sail is deployed, which decreases the velocity down to about 1%c. For the final deceleration, the fusion engine is used again. The spacecraft then enters the Alpha Centauri system to deploy sub-probes and collecting data.

Adding all masses and a reasonable margin together, our spacecraft has a total mass of 153,800 tonnes.

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