Propulsion Flashcards
What is a double base solid rocket propellant?
Composed of two monopropellant fuel components - one high energy, unstable and the other lower energy, stabilising and gelling
Both oxidiser and fuel contained in molecule
Eg. Nitroglycerin and nitrocellulose
Why is it easier to calculate the thrust utilising the below eq for a GIT than HET?
T = sqrt(2Mion/q)Ibsqrt(Vb)
In a GIT, easy to estimate Vb and Ib from voltages applied and currents measured but in a HET there is no good method for estimating Vc. To measure T more accurately for HET, use a thrust balance
Explain briefly the role of the three grids in a GIT
Screen - attracts ions
Accel - prevents electrons being attracted to screen and decel
Decel - applies pd and accels ions out
An upper stage for a small satellite is being designed. There is a choice between 3 propellants;
- Liq hydrogen & LOX
- nitrogen tetroxide & monomethyl hydrazine
- green propellants based on ionic liq
Compare the adv and disadv of the 3, discussing performance, storage, safety and other concerns
LOX/LH
- LH is cryogenic, limited materials selection, extremely toxic and needs large tank (low density) unless cooled further. Non toxic
- LOX stable but shock sensitive in mixtures, requires an igniter, non toxic
- Isp 386 at sea level
Nitrogen tetroxide and monomethyl hydrazine
- hyperbolic
- ni has limited temp range and is highly toxic
- hydrazine is stable but very toxic and carcinogenic
- Isp 283
Green ionic
- not toxic, company owned and thruster needs to be heated to high temp before operation
- Isp ~ 240 and relatively high density
Describe a pressure regulated and a blowdown system and both their adv and disadv
Pressure regulated - adjusts to keep at optimum
Blow down - blow propellant out of tank. Pressure and force decrease over time
Explain the operation of an aerospike at high and low altitude and its primary adv
Aerospikes have no outer nozzle wall. Allows for near optimum altitude at varying altitudes as when the ambient pressure reduces the gas boundary expands out
Diagram
Solar sails that utilise solar radiation pressure are on occasion used on GEO s/c. Explain their function and why they are useful
The sunlights photons create an acceleration by imparting a momentum exchange when colliding and reflecting off sail energy is transferred. Momentum from each photon is tiny so a large area is needed and a very light structure.
Allows for significant mass reduction - no propellant of fuel system needed
Ideal for high dV missions and can move in variable orbits by tacking
Explain the 3 main types of solar sail
Square sail - max force, most surface area, no hotspots, simple
Spinning disc - rotate in space which changes design structure
Heliogyro - strong structure, uses angular momentum to reduce need of support. Blades can alter pitch
Describe briefly the difference between typical double base and composite propellants, including performance differences, burning characteristics and possible combustion products
Double base consists of 2 monopropellant fuel components - 1 highly energetic but unstable and one low energy, stabilising and gelling. Isp < 210s, increases with burning rate and low density propellants. Ie nitroglycerin and nitrocellulose
Composites have fuel and oxidiser separate ie aluminium and ammonium perchlorate. Greater performance to DB and cheaper. Isp ~ 240s, Isp decreases with burning rate
Describe the similarities and differences between an electrospray (colloid) thruster and Field Emission EP (FEEP)
Both are electrostatic thrusters using liquid propellant
FEEP uses liquid metals, needs a neutraliser,
electrospray uses organic liquids, doesn’t need a neutraliser and propellant is easier to handle
Explain briefly the adv and disadv of electrospray thrusters
Good for very fine attitude control - very controllable thrust
Reasonable Isp, scalable - can change size without limiting power
Limited power available
Issue with lifetime - electrons erode thruster
With the context of a GIT, describe what a charge exchange ion is
Between the screen grid and accelerator grid, a neutral atom passes through and gives an electron to a positive ion to become a positive ion - charge exchange. It has not been accelerated through the screen grid and therefore is much slower and produces less thrust
Describe the 2 methods which charge exchange ions can damage the grids of a GIT
Barrel erosion - charge exchange ions hit the accel grid gaps
Pits and grooves erosion - post accel grid, collide with the grid surface
What is Debye length and of what relevance is it to the operation of the grids within a GIT
Debye length shields plasma from grid, ions will only sense attraction when near grid
Greater acceleration, not all at once
Comment on the difficulties of operating a low power HET
Reducing the size increases the surface to volume ratio which reduces the lifetime of the thruster. The efficiency is also reduced due to thermal issues and a large amount of power is needed for the same thrust to be produced
