I'm Limited by the Technology of my Time

Aspire's deep space mission still needs an engine and the conventional options offered by existing technologies are a bust. Too much fuel, or too little thrust, or not enough power.

Today, I present to you a theoretical engine that solves all three of those problems: the Direct Fusion Drive. This is a real conceptual spacecraft propulsion system proposed by the University of Princeton. NASA commissioned a report to study how this drive might perform on a theoretical mission to Pluto and the results are fascinating.

To put it succinctly and simply: this drive uses a fusion reaction to heat a working fluid. The working fluid generates electricity. The fusion reaction generates heat and the reaction products can be channeled through a magnetic nozzle to produce thrust. This leaves us with a highly efficient engine (lower fuel mass required), capable of producing a lot of power and thrust. The only downside is extra waste heat that must be rejected.

I started with one of the drives proposed in the NASA report and sized it up tenfold into a 10 MW reactor. Based on the report, a 10 MW reactor generates 3 MW of electrical power and 2.5 MW of thermal power. Of The remaining 4.5 MW, 3.5 MW is expelled through the nozzle as thrust and 1 MW is recirculated to sustain the fusion reaction.

Now let's repeat some of the calculations we ran last time.

Using these numbers, a desired delta-V of 50 km/s, and a dry mass of 162,000 kg, let's give the rocket equation another go.

mass_wet = mass_dry * e ^ (delta_v / Ve)

This results in a wet mass of 267,000 kg -- and a propellant mass of 105,000 kg! Those are very workable numbers. But wait! We've been down this road before, only to be disappointed by burn time. How long does this engine need to thrust to provide the necessary delta-V?

We're good there, too! Only about 1219 days total burn time.

I want to highlight two more real-world problems with this solution that I completely ignored in my manuscript. 

• First, this theoretical fusion drive requires Helium-3 as a fuel. Helium-3 is exceptionally rare on Earth and would potentially be even rarer in a future where it's one of the primary fuels used in fusion reactors. 
• Second, a 100 N drive will never provide enough thrust to quickly leave Earth orbit... or re-enter Earth orbit at the conclusion of the mission. Theoretically, you can start in Earth orbit and spiral your way out to escape velocity, but that's not built into any of my fuel calculations (or my initial delta-V estimate). 
  • This is a scenario where a chemical rocket tug could pull Aspire from Earth orbit into a heliocentric orbit, where the fusion drive can take over.
  • Similarly, another chemical tug would be required on the return trip
• Finally, I handwaved away the problem of electrical energy availability. The Direct Fusion Drive thrusting for 1219 days and generating 3 MWe during that time actually provides way more than enough energy for the entire 16 year mission, assuming Aspire draws as much energy as the ISS at around ~100 kW. Some combination of high performance batteries and thrust timing might fix that.

So that's the final piece of the puzzle. Aspire's mission design is complete. Now that we've run all these numbers and assembled this intricate spacecraft, it would be a damned shame if something were to malfunction... in the first chapter, no less.