In 1987 a former NASA engineer, Robert Burruss, published an audacious concept: a design for an intergalactic transport (Sept/Oct 1987, “The Futurist” magazine.) Entitled “Galaxyship: Voyage to the Next Galaxy”, illustrated by the evocative art of James Colwell, Burruss outlined a vast 1,000 miles wide disk-shaped Galaxyship, designed to accelerate to 0.4 c over a 50,000 year period, then coast to Andromeda’s M31 for 5 million years, then brake at destination over 10,000 years, before dispersing like a seed-pod to colonize a whole other Galaxy.
The details are a bit sketchy. Or, perhaps, open-ended. The disk-ship components – a billion hexagonal units with their own drive systems – can move around relative to each other, allowing avoidance of intergalactic hazards. One side, facing the direction of travel, is a radar array, while the other is a matter-antimatter drive. The drive produces 30 gigawatts of thermal energy. Some powers the mini-biosphere within and technological sub-systems, while the rest produces a thrust of gamma-rays.
Burruss didn’t specify the exact efficiency of the drive system, but imagined that 99% of the vehicle would be matter-antimatter, to annihilate on the way. A perfect photon-rocket with such a mass-ratio would cruise at 0.98 c, then brake to a halt at destination [thanks Ian Mallett – see comments]. Since the stated cruise speed is 0.4 c, meaning an effective specific impulse of ~0.19 c, the efficiency seems excessively low. Especially since the by-products are high-energy gamma-rays, thus bathing the vehicle and inhabitants. So what are the alternatives?
A perfect photon-rocket might be available, if Fred Winterberg is right about the ability to collimate an annihilating ambiplasma via relativistic self-focus and the Meissner effect. In that case, with a mass-ratio of 3, the Galaxyship could cruise at 0.5 c. Burruss imagined it would start with a mass of a trillion tons, but would be frittered away as gamma-rays to *just* 10 billion. The 1,000 mile disk, some 2 trillion square metres, would have an areal mass-density of just 5 kg by the end of its odyssey. Alternatively it’d shrink to 100 miles wide, sacrificing hexagons to the ravening gamma-ray engines. With a perfect photon-drive the final mass-density would be ~333 kg per square metre. That *might* be enough.
Could the Galaxyship be propelled in a different fashion?
In 50,000 years, accelerating at a constant rate to 0.5 c, a vehicle travels 25,000 light-years. The acceleration is 1E-4 m/s2 meaning a force of 1E+8 newtons is needed. If we push it with lasers, then each newton needs 150 megajoules of laser energy per second impinging on the ship. A total laser-power of 1.5 exawatts. Fortunately the Sun radiates some 400 million exawatts, so tapping a tiny fraction of that flow will be sufficient to propel a Galaxyship. Millions if we felt so inclined.
To direct the laser, keeping it focused on a reflective disk 1,600 km wide at a maximum range of ~25,000 light years, means we’ll need an immense optical system. To decelerate a 100 km wide light-sail 10.5 light-years away at Epsilon Eridani, Robert Forward imagined a 1,000 km wide lens. The Galaxyship is a target 16 times wider, but 2,400 times further away. The optical system – presumably some immense solar-collector/laser-array combination – would need to be 150,000 km wide. While that sounds like an incredibly large structure, it needs to direct only ~1 watt per square metre to push the Galaxyship as desired. The pointing accuracy across 25,000 light-years will require some skill as the angular size of the Galaxyship is 8E-15 radians at maximum range.
Appendix: A Quick Guide to Photon Rockets
The venerable Tsiolkovskii equation:
V = u.LN(Mo/Mf)
…V being the delta-vee, u the exhaust velocity (measured in the rocket’s frame of reference) and Mo, Mf, the initial and final masses respectively.
Relativity means the equation remains the same, but the velocity has to be converted to the Galactic reference frame:
V = c.TANH[(u/c)*LN(Mo/Mf)]
If the rocket is a perfect photon rocket then it simplifies to
V = c.TANH[LN(Mo/Mf)]
So if we want work out the mass-ratio, for a photon rocket that brakes to halt after cruising at some speed, then the mass-ratio we compute above must be squared.
Mo/Mf = EXP[2*ATANH(V/c)]
Thus 0.98c, when we ATANH it gives us ~2.3, which EXP[2*2.3] gives ~100.