Propulsion

Core idea
Convert stored potential energy into kinetic energy.

Momentum Exchange / Thrust

Gas enters a nozzle and then gets ejected out with a certain velocity.
The specific impulse is a function of the engine geometry and fuel properties.

I_{S P} = \sqrt{\frac{R T_{c}}{M W} \frac{2 γ}{γ - 1} (1 - {(\frac{p_{e}}{p_{c}})}^{\frac{γ - 1}{γ}})} + \frac{p_{e} - p_{a}}{\dot{m}} A_{e}

γ = \frac{C_{p}}{C_{v}} = 1.2 to 1.6

for complex molecules (like $NH_{4}$ ) with multiple degrees of freedom the value is smaller

\frac{γ - 1}{γ} = 0.16 to 0.38

Symbol	Description	Common Units
$I_{S P}$	Specific Impulse	$m / s$ (velocity) or $s$ (time)
$R$	Universal Gas Constant	$J / (m o l \cdot K)$
$T_{c}$	Combustion Chamber Temperature	$K$
$M W$	Molecular Weight (Molar Mass)	$k g / k m o l$ or $g / m o l$
$γ$	Ratio of Specific Heats ( $C_{p} / C_{v}$ )	Unitless
$p_{e}$	Nozzle Exit Pressure	$P a$ or $b a r$
$p_{c}$	Chamber Pressure	$P a$ or $b a r$
$p_{a}$	Ambient (Atmospheric) Pressure	$P a$ or $b a r$
$\dot{m}$	Propellant Mass Flow Rate	$k g / s$
$A_{e}$	Nozzle Exit Area	$m^{2}$

The Nozzle size is depended on the exit pressure and the ambient pressure
Propulsion Subsystem-1.png

Energy Source

Cold gas thrusters

Gas that is released from a pressurized supply through a nozzle without igniting anything.

Advantages:
- simple
- no thermal signature
- very low electricity/power requirement
- safe components
Disadvantages:
- very limited maximum thrust
- thrust level dependent on pressure in tank

Cold Gas	Molecular weight M (u)	Theoretical $I_{s p}$ (sec)	Density (g/cm³)
H₂	2.0	296	0.02
He	4.0	179	0.04
Ne	20.2	82	0.19
N₂	28.0	80	0.28
Ar	40.0	57	0.44
Kr	83.8	39	1.08
Xe	131.3	31	2.74
CCl₂F₂ (Freon-12)	120.9	46	Liquid
CF₄	88.0	55	0.96
CH₄	16.0	114	0.19
NH₃	17.0	105	Liquid
N₂O	44.0	67	Liquid

Monopropellant

A single propellant decomposes exothermally, typically in presence of a catalyst. This uses the chemical energy stored in the propellant.

Advantages:
- higher Isp than cold gas
Disadvantages:
- increased complexity
- difficult components
- lifetime limited by catalyst degradation

Often used propellant is Hydazine:

versatile
toxic
high freezing point
Isp ~230s (medium)

Bipropellant

Fuel and oxidizer are brought together and then ignited. Fuel and oxidizer can both be stored either as solid or as liquids.

Much higher Isp can be reached compared to monopropellant with both liquid phases, or the complexity can be reduced with solids.

Solid

Advantages:
- simple design
- high thrust
- long-term storable
- thrust curve can be adapted before launch
Disadvantages:
- Thrust curve can only be adapted before launch
- all but impossible to stop or restart
- typically toxic

Propulsion Subsystem-2.png

Hybrid propellant system

Combines solid fuel with liquid oxidizer. Widely used for student projects and sounding rockets.

Advantages over solid:
- higher Isp
- lower explosion hazard
- controllable thrust
Disadvantages:
- higher complexity

(Bi-) Liquid

Both fuel and oxidizer are in liquid form

Advantages:
- thrust control and optimal burn rate
- scales to the largest systems
- highest Isp
- refueling and static test burning is possible
- re-usable
Disadvantages:
- most complex plumbing
- mixture ratio needs to be controlled
- fuels move (slosh)
- cryogenic storage of oxidizer necessary
- significant preparation before launch necessary

Propulsion Subsystem-3.png

Electric Propulsion

System type	Propulsion principle	Typical propellant	Thrust range	Specific impulse ( $I_{s p}$ )	Typical missions / users	Flight heritage / comments
Resistojet thrusters	Electrically heats a stored neutral gas before expansion in nozzle	N₂, NH₃, H₂O, etc.	1 – 100 mN	150 – 400 s	Small-sats, ISS reboost systems, CubeSats	Simple, low-Isp but compact; often first EP step for small platforms
Arcjets	DC arc discharge superheats propellant -> nozzle expansion	Hydrazine, NH₃, H₂	0.1 – 2 N	400 – 1,000 s	GEO station-keeping (in 1990s–2000s)	Mature but mostly replaced by Hall / ion systems; higher power demand
Hall-effect thrusters (HET)	E×B field traps electrons -> ionize propellant -> ions accelerated by electric field	Xenon (increasingly Krypton or Argon)	10 – 400 mN	1,000 – 2,500 s	GEO comsats, LEO constellations (e.g. Starlink, OneWeb), deep-space (BepiColombo, DART)	Most widely used EP today; robust, efficient (~50–65%), 1000s of units in orbit
Gridded ion engines (GIE)	Ion extraction through electrostatic grids; separate neutralizer	Xenon	1 – 250 mN	2,000 – 10,000 s	Deep-space probes (Deep Space 1, Dawn, Hayabusa 2, BepiColombo), station-keeping	High Isp champion, lower thrust density, long heritage since 1960s
Colloid / electrospray thrusters	Charged liquid droplets or ions electrostatically accelerated	Ionic liquid (EMI-BF₄, etc.)	$μ$ N – mN	500 – 3,000 s	Precision attitude and drag-free missions (LISA Pathfinder, GRACE-FO)	Used for micro- and nano-satellites requiring ultra-fine thrust
Field-Emission Electric Propulsion (FEEP)	Field-emission of metal ions from sharp tips	Indium, Cesium	$μ$ N – mN	5,000 – 10,000 s	Attitude control for small, drag-free or formation-flying missions	High precision, low thrust, niche use
Pulsed Plasma Thrusters (PPT)	Electric discharge ablates solid Teflon -> plasma pulse	PTFE (Teflon)	$μ$ N – mN (pulsed)	600 – 1,200 s	Small spacecraft, early microsats, CubeSats	Oldest EP tech (since 1960s); simple, rugged, low efficiency

Overview of Propulsion Families

Propulsion Subsystem-5.png

Interaction with other Subsystems

Control Subsystem (ADCS)

Requirement on propulsion capabilities to maintain attitude

Structure Subsystem

Requirement on structure tu support weight of propulsion system on ground
Requirement to tolerate loads from propulsion

Power Subsystem

For electric propulsion: Provide power for thrusters
Tradeoff between attitude control and solar power generation
Placement of solar panels w.r.t thrusters (plume keep-out-zone)

Thermal Subsystem

Thermal loads from thruster operations
Indirect effect from attitude control

Plume Effects and Contamination

Depending on fuel combination, the rocket plume will be toxic. Residuals can impinge on solar arrays and sensors. Best practice is to have a 60° half angle keep out zone.