1. The war that outlasts the engines
Joe Haldeman’s The Forever War sends soldiers across light-years in ships that burn fuel for years to reach relativistic speeds. The engines are efficient enough to make the trips possible but slow enough that the war changes while the soldiers travel. The desktop’s problem is the opposite scale: it needs small, quick burns in LEO, not a decades-long acceleration. But the same trade applies. High thrust and high efficiency do not usually come in the same package.
Entry 254 estimated the delta-v budget at roughly 300 m/s. This entry picks the thruster technology.
2. The trade space
Thruster choice is a trade between thrust, specific impulse, complexity, and operational readiness.
| Technology | Thrust | Isp | Complexity | Notes |
|---|---|---|---|---|
| Cold gas | millinewtons | 30–70 s | Low | Simple but inefficient |
| Monopropellant hydrazine | 0.1–1 N | 220 s | Moderate | Mature, toxic ground handling |
| Green monopropellant | 0.1–1 N | 200–230 s | Moderate | Lower toxicity, newer heritage |
| Bipropellant | 1–10 N | 300 s | Higher | More performance, more complexity |
| Hall-effect electric | 10–100 mN | 1,500 s | Higher | Very low thrust, high efficiency |
| Gridded ion | 1–50 mN | 3,000 s | High | Extremely efficient, very low thrust |
For the desktop, the choice is between green monopropellant and electric propulsion, with cold gas possibly used for fine attitude control.
3. Why not cold gas alone
Cold gas thrusters use an inert gas such as nitrogen or xenon stored at high pressure. They are simple, reliable, and easy to test. Their specific impulse is very low, typically 30–70 s. For 300 m/s of delta-v, a cold-gas system would need a propellant mass comparable to or greater than the dry mass of the desktop. They are useful for fine pointing and momentum dumping, but not for orbit maintenance or disposal.
4. Why not electric propulsion as primary
Electric propulsion, such as Hall-effect or gridded ion thrusters, has very high specific impulse. A Hall thruster at 1,500 s would need only a few kilograms of propellant for 300 m/s of delta-v. The problem is thrust. A 50 mN thruster on a 120 kg desktop produces an acceleration of roughly 4 × 10⁻⁷ g. A 100 m/s disposal burn would take weeks or months.
For collision avoidance, electric propulsion is too slow. A conjunction warning is usually hours to days away. Electric thrusters cannot respond in that time. They are attractive for future versions with large delta-v needs and no urgent manoeuvres, but not for the first desktop.
5. Green monopropellant as baseline
Green monopropellants such as AF-M315E and LMP-103S offer a middle path:
- Specific impulse around 200–230 s, close to hydrazine.
- Thrust levels suitable for 1 N class thrusters, allowing burns of minutes to hours.
- Lower toxicity than hydrazine, simplifying ground operations.
- Growing flight heritage on small spacecraft.
A 1 N thruster on a 120 kg desktop produces an acceleration of roughly 8 × 10⁻³ g. A 10 m/s avoidance burn takes about 2 minutes. A 100 m/s disposal burn takes about 20 minutes, split into several passes to avoid thermal or power constraints. This is practical for LEO operations.
6. Hydrazine as fallback
Hydrazine monopropellant systems have decades of flight heritage and are well understood. They would work for the desktop. The downside is toxicity and the operational overhead of handling hazardous material. For a program that wants to iterate quickly and potentially manufacture or refill propulsion modules in multiple locations, green propellant is preferable.
7. Bipropellant for growth
A bipropellant system using monomethylhydrazine and nitrogen tetroxide, or green alternatives, offers higher specific impulse and thrust. It is also more complex: two tanks, two feed systems, precise mixture ratio, and more failure modes. Bipropellant makes sense for a larger desktop or for missions that need frequent large manoeuvres. The first desktop does not need that capability.
8. Recommended architecture
The baseline propulsion attachment for the first desktop is:
- Primary: green monopropellant thrusters, four to eight 0.5–1 N thrusters arranged for translation and attitude assist.
- Auxiliary: cold gas or very small monopropellant thrusters for fine attitude control and momentum dumping, if the ADCS needs them.
- Future option: an electric propulsion module that can replace or supplement the monopropellant system for high-delta-v missions.
This architecture gives the desktop the ability to manoeuvre quickly when needed and to perform disposal at end of life, without the complexity of bipropellant or the long burn times of electric propulsion.
What this changes
- Cold gas is rejected as the primary propulsion because its efficiency is too low.
- Electric propulsion is rejected as the primary because its thrust is too low for collision avoidance.
- Green monopropellant is the baseline for the first desktop.
- Hydrazine is a proven fallback if green propellant heritage is insufficient.
- Bipropellant and electric propulsion are held as growth options.
- The next entry can size the propellant tanks and feed system.