Photon Rockets for Interstellar Travel



Say you want to build an interstellar rocket. Cool beans. What’s the best way to go about it? Well, to get the highest possible delta-v, you want the highest possible exhaust velocity for your rocket propellant.

So what’s the fastest thing in the universe? Unless you’re living in a universe with FTL, that would be light, which travels at about 300,000,000 meters per second. We’ll use a jet of photons (otherwise known as a beam of light) to propel our starship.

The problem? Photons are the fastest things in the universe, but the thrust-to-power ratio of a photon rocket is pretty low. How low? In the words of the Atomic Rocket engine list, “three hundred freaking megawatts” for every Newton of force.

In other words, the maximum output of a typical nuclear power plant would (given a 100% efficient photon rocket) just barely be able to levitate a measly paper clip against Earth’s gravity.

Well darn.

The Future is Bright

Hey, we’re planning an interstellar trip, remember? If we’re building a starship, surely we’re not mucking about with anything as primitive as nuclear fission, right? Let’s assume some sort of total conversion drive: one that can turn 100% of its fuel directly into energy, in the form of a perfectly collimated beam of photons.

Danger: Do not stand directly behind the engine.

Never mind how such a total conversion drive might work. The typical answer is a matter-antimatter reaction, but even that won’t give us the laser-perfect beam of photons that we need.

We’re firmly in the realm of fiction with our ideal, 100% efficient matter-to-photon-beam drive.

How much energy is in matter? E = mc2. One gram of matter (say, a paperclip), dropped into the total conversion engine, will convert to 8.98 x 1013 Joules of energy, which according to the Atomic Rockets numbers, works out to about 300 kN of force (if the energy is exerted over one second).

Exhaust Velocity

Ok, 300 kN isn’t a lot of force, but we’re not going to fuel an interstellar spacecraft with a paperclip. We’re going to bring along an appropriate fuel supply. How fast will it end up traveling?

Again, thanks to Atomic Rockets, here’s the relation between exhaust velocity and delta-v.

delta-v = ve ln R

The third variable is mass ratio. This is more helpfully expressed as a propellant fraction, or the percentage of the total mass that is taken up by fuel. Modern rockets are virtually all fuel with very little payload, but I don’t see the starship Enterprise carrying around massive fuel tanks that dwarf the mass of the rest of the ship.

Let’s say that only a small fraction of our interstellar spacecraft’s mass is fuel. Say… 10%.

Pf = 1-(1/R)
R = 1 / (1 - Pf)
  = 1 / (0.9) = 1.11...

Plugging that into the delta-v equation, we get a delta-v of about 10.5% c. Not exactly a warp drive, but it’s faster than my old car.

Where To?

Our starship is built, fueled, and ready to go. Let’s take her for a spin!

If we’re calling our spacecraft “interstellar”, obviously we need to prove it will go from one star to the other. Departing from our own solar system, the closest star system is what’s commonly known as Alpha Centauri.

I vaguely recall that in his book The Millennial Project, Marshall T. Savage prefers to refer to this star by an alternate name: Rigil Kentaurus. I think this is a much cooler name: when our starship reaches its destination, we can inform the crew that we are “entering the Rigil Kent system”.

(Savage laments that the name Rigil is likely to be confused with another star, Rigel, or Beta Orionis. I trust that a competent starship crew will not accidentally navigate 800 light-years off-course to reach the wrong star system. If this does turn out to be a problem, we can use a third alternative name: Toliman. “Attention all crew. We are now approaching (dun-dun-dun) Toliman!)

At 10.5% c, our starship can make the trip from the Sun to Alpha Centauri / Rigil Kentaurus / Toliman in about forty years. It’ll take time to accelerate to speed, and just as much time to decelerate. A forty year journey in a starship isn’t quite like hopping into the Millennium Falcon and calculating a hyperspace jump to Alderaan, but it’s on the same order of magnitude as a human lifespan. Our starship might be an interstellar ark, or we may have perfected suspended animation. Or we’ve used the latest Kurzweil Mk. II scanner to upload our brains into a computer.

Of course, we could just fly-by the system and continue on our merry way. There’s plenty more galaxy out there.

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