The moment of truth has almost arrived. We've put in the leg work. We developed a mission concept. We calculated the delta-V required for our trip. We even figured out the dry mass (mass not including fuel) of the spaceship. Finally, it's time to pick an engine to get us to the edge of the solar system.
One of our primary tools throughout this selection process is Tsiolkovsky's rocket equation. We'll use this to determine how much fuel mass will be required to meet our delta-V target, given the exhaust velocity (Ve) of the engine as a measure of performance.
delta V = Ve*ln(mass_wet / mass_dry)
There are a few different categories of engines to choose from. There's trusty chemical propulsion, which the world has used since the 1950's. There's nuclear propulsion, an interesting concept that was explored in the 1960s-1970s but has never seen the practical light of day. And then there's electric propulsion, a more modern approach with a few different flavors.
Please meet our candidates:
And here are some basic characteristics for these engines:
| Engine Type | Fuel | Exhaust Velocity (m/s) | Mass Flow Rate | Isp (s) | Input Electrical Power (kW) |
| Chemical: RS-25 | H + O | 4436 | 514 kg/s | 452 | Negligible |
| Nuclear: NERVA | H | 8000 | 32.7 kg/s | 869 | Negligible |
| Electric: MPDT | Ar | 45800 | 60 mg/s | 4665 | 100 |
Recalling that the previously calculated delta-V for a round trip is 50 km/s and Aspire's dry mass is 162,000 kg, we can use the rocket equation to calculate fuel mass, with the following results:
| Engine Type | Wet Mass (kg) | Propellant Mass (kg) |
| Chemical: RS-25 | 1,547,000 | 1,384,000 |
| Nuclear: NERVA | 567,000 | 404,000 |
| Electric: MPDT | 202,000 | 40,000 |
Chemical propulsion requires too much fuel to be practical. Nuclear is a little more reasonable, but still the fuel mass alone more than doubles up the ship's mass. But electrical...wow, this is the clear winner. Onward!
I left out a key piece of information in that table of characteristics up above, which is the amount of thrust each engine provides. Interestingly, the rocket equation doesn't care about time, only the performance of the thruster in question. So let's fill in the missing information and add a sanity check--based on the mass flow rate, how long does it take to burn through all the fuel required to reach the delta-V target?
| Engine Type | Thrust (N) | Burn Time |
| Chemical: RS-25 | 2,279,000 | 8.97 hours |
| Nuclear: NERVA | 344,000 | 41.2 hours |
| Electric: MPDT | 2.75 | 20.92 years |
Oof. Well, with this extra context, maybe electric propulsion doesn't make as much sense as I hoped. 2.75 N is about equivalent to the weight you'd feel holding a grapefruit in your hand. And that burn time--my whole mission round trip is only supposed to last for ~16 years. However, this is science fiction, so I could just scale up the electric thruster size to suit my needs. I can bump up the thrust and power draw by a factor of ten. That ought to do it.
To generate a paltry 2.75 N of thrust with the MPDT will require 100 kW of electrical power. If I'm upping this by a factor of at least ten, I need 1000 kW of power. Where am I getting that kind of power to feed my engines for years on end? For that matter, where am I getting the power I need to feed my life support systems for years on end?
I've ignored the "Power" problem so far, but it's inextricably linked to using any sort of electric propulsion method.
Solar arrays aren't a viable option so far into the solar system; they're kind of useless past Mars thanks to the inverse square law that governs solar intensity.
One proven option for space power generation is to convert the heat from decaying radioactive isotopes into electricity using Stirling Radioisotope Generators. This is how the Voyager probes and the Curiosity and Perseverance Mars rovers get their power. I explored some options, but the short answer is that it requires carrying around thousands of kilograms of very toxic stuff--for example, 3500 kg of Strontium-90 or 2200 kg of Plutonium-238 to ensure at least 400 kW of electrical energy is available by the end of the mission. Those are numbers far above the critical mass for spontaneous nuclear fission. In short, not a good option for this amount of power.
The few nuclear reactors that have flown in space have provided on the order of single-digit kilowatts. Not nearly enough.
Is Aspire's mission finished before it even gets off the ground? Have the realities of science and engineering caught up with my fanciful mission design? Is it impossible to accelerate this much mass over this short of a time?
I need a lot of power. I need a lot of thrust. I need as little fuel mass as I can get away with. I need a technological leap.
So, like generations of engineers before me, I will look to the holy grail of near-future technologies--fusion--to solve my problems. After all, it's only twenty years away. Just like it was twenty years ago. And twenty years before that.