Propulsion (PROP)

Definition

A propulsion system expels mass to change the velocity, and therefore the orbit, of a spacecraft. In small satellites its main uses are holding altitude against drag, phasing the members of a constellation, formation flying, collision avoidance and lowering the orbit at the end of the mission. Dumping momentum from reaction wheels is a secondary job.

Technology families

Cold-gas systems are simple, mature and safe, but their total impulse is modest. Electrothermal thrusters (resistojets) heat the propellant electrically to raise the specific impulse; some simple designs run straight from the spacecraft bus. Electrospray thrusters accelerate ionic liquids, or in FEEP thrusters low-melting metals such as indium and gallium, in an electric field and reach specific impulses of thousands of seconds. Propellant safety is one of the main reasons propulsion remains rare on low-cost CubeSats[1].

Flight examples

• MarCO (6U, 2018). Trajectory corrections on the way to Mars used eight cold-gas thrusters fed from one common tank of R-236fa, the compressed gas also used in fire extinguishers[2].
• FEEP on a 3U. Enpulsion’s FEEP thruster flew on a Planet Flock 3p CubeSat in an orbit of about 491 × 510 km; flight data agreed well with the commanded thrust of about 220 µN[1].
• Water resistojet. The ARM-A system of AuroraSat-1 provides 70 N·s of total impulse from 70 g of water through six resistojets[1], an effective specific impulse of about 100 s.

Propulsion and PocketQubes

Propulsion is regarded as the main enabling technology for future PocketQubes, and the constraint is quantitative[3]. MEMS fabrication is the main route to fitting a thruster into 50 mm boards[4]. TU Delft developed two water-based micro-resistojet concepts, a vaporizing-liquid microthruster and a low-pressure microthruster[5], and allocated Delfi-PQ a propulsion payload with a wet mass below 75 g, a peak power of 4 W and an envelope of 42 × 42 × 30 mm[6].

At the smallest end, the open-source AIS-gPPT3-1C pulsed-plasma module (34 g, 40 × 38 × 24 mm) produced an impulse bit of about 0.65 µN·s, roughly 0.2 µN at 0.33 Hz, before development stopped because sub-joule pulsed-plasma thrusters could not deliver useful thrust[7]. AMSAT-EA’s GENESIS PocketQubes, meant to demonstrate a miniature pulsed-plasma thruster, were caught in both Firefly Alpha anomalies[3].

The rocket equation

The velocity change a propulsion system can give depends on its propellant mass and specific impulse through the Tsiolkovsky equation[8]:

where $m_0$ and $m_f$ are the initial and final masses and $g_0=9.80665$ m/s². Ten times the specific impulse gives ten times the Δv for the same propellant, but electric thrusters pay for it in power and time.

Technologies compared

Technology	Isp (s)	Thrust	Δv (m/s)	Years of drag make-up at 450 km
Cold gasN₂, butane, R-236fa	40–110	10 µN–3.6 N	16–44	0.2–0.4
ElectrothermalResistojets: water or gas heated electrically	20–350	0.1 mN–1 N	8–140	0.1–1.3
Alternative mono- and bipropellantAF-M315E (ASCENT), LMP-103S	150–310	50 mN–22 N	60–124	0.6–1.2
Electrospray and FEEPIonic liquids; indium or gallium in FEEP	225–3,000	20 µN–20 mN	90–1,201	0.9–11.6
Pulsed plasma, vacuum arcAblation of PTFE or a metal cathode	87–3,200	4–500 µN	35–1,281	0.3–12.3

Ideal rocket equation, propellant only (tanks, power and thrust level ignored). Holding a 1P at 450 km under high solar activity costs about 104 m/s a year. Ranges from NASA’s State of the Art of Small Spacecraft Technology.

Electric propulsion typically delivers less than 75 mN per kilowatt[1]. A spacecraft that can spare 10 W gets at best about 0.75 mN, so accelerating a 6 kg 3U by 1 m/s takes hours of continuous firing. Reviews of small-spacecraft propulsion give the wider landscape[9][10].

What holding altitude costs

To stay at constant altitude a spacecraft needs a continuous thrust equal to its drag, $F=\frac{1}{2}\rho v^2{C}_{D}A$. With the density model of the PocketQube review, a fully loaded 1P needs[3]:

Altitude (km)	Drag force (µN)	Δv per year (m/s)	Total impulse per year (N·s)
350	1.15–4.64	145–586	36.2–146.4
400	0.33–1.91	42–241	10.4–60.2
450	0.10–0.82	13–104	3.2–26.0
500	0.033–0.37	4.2–47	1.1–11.7
550	0.012–0.18	1.5–22	0.4–5.6

Ranges run from low to high solar activity; tumbling mean area A_tot/4, drag coefficient 2.2. Solar activity: low F10.7 = 70 sfu, Ap = 6; moderate 140 sfu, Ap = 12; high 210 sfu, Ap = 18.

Ten grams of cold gas, about as much as a 1P could plausibly carry, buys 16–44 m/s: a few months of station-keeping at 450 km under high solar activity, and one to three years when the Sun is quiet.

Lowering the orbit

A single retro-burn that drops the perigee to 250 km brings a spacecraft down within weeks:

Altitude (km)	Perigee to 250 km (m/s)	Perigee to 180 km (m/s)	Low-thrust spiral to 250 km (m/s)	Propellant for a 1P, Isp 65 s (g)
500	71	91	142	26.3
550	84	105	170	31.0
600	98	118	197	35.6
700	124	144	251	44.3
800	150	170	303	52.4

Single impulsive burn from a circular orbit; a 250 km perigee brings re-entry within weeks. Propellant from the rocket equation for a 250 g 1P with cold gas (Isp 65 s).

Launch safety

The CubeSat Design Specification requires propulsion systems to be designed, integrated and tested to AFSPCMAN 91-710 Volume 3 and to carry at least three inhibits against activation[11]. Pressure vessels and hazardous propellants also bring extra analyses and documents from the launch provider, which is why low-pressure propellants stored as solids or liquids are preferred in small satellites.

Power and heat

The power demand of electric thrusters is comparable with the orbit-average power of a small satellite with body-mounted cells: about 2.4 W for a 1U and half a watt for a 1P. Thrusters therefore run in short windows fed from the battery. The heaters of resistojets and FEEP thrusters also load the thermal design.

Cold propellant

The bulk temperature of a 1P falls below 0 °C in eclipse, so water feed systems need thermal isolation or heater power to avoid freezing; silicon chips with thin-film heaters must survive repeated heating from that cold start, and wetted seals must meet the outgassing limits[6].

Thrust direction

A thrust vector that misses the centre of mass produces a disturbance torque. As propellant is used the centre of mass moves, so the ADCS must be able to hold the thrust direction during a manoeuvre.

Contamination

Plumes can contaminate solar cells, optics and thermal coatings. For thrusters that use metals or ionic liquids, plume geometry and placement belong in the early design.

The tension at PocketQube scale

Debris mitigation pushes PocketQubes towards low orbits, where they can only operate for long with micronewton-class propulsion that fits in a few tens of grams and draws a few hundred milliwatts. That combination has not yet been demonstrated[3]. The five-year rule article compares propulsion with the alternatives: low release altitudes and drag devices.