orbit manoeuvres Flashcards
what is orbital manoeuvring based on?
the principle that an orbit is uniquely determined by position and velocity at any point
what is needed to undertake a transfer or manoeuvre?
change in velocity vector using an external force
define burn time
finite time for which thruster is required to burn to achieve necessary change in velocity vector
why can we assume an impulsive burn?
if the burn time < orbit period
define single burn transfer
single impulsive burn of thruster, enabling transfer from one orbit to another
where do burns occur?
intersection of orbits
define general co-planar manoeuvre
non-tangential burn from an initial orbit velocity, to an intersecting co-planar orbit with final velocity
how many burns does a hohmann transfer require?
2
describe process of hohmann transfer
- first burn: required to change from initial circular orbit to elliptical transfer orbit
- second burn: made at apoapsis of transfer to give spacecraft sufficient energy to remain in final circular orbit
what are hohmann transfers used for?
used to reduce or increase orbit altitudes
define three-burn transfer
consists of three separate burns
when are three-burn transfers used?
when there is a large difference between intitial and final orbit radius
define fast transfer and when it is used
‘one tangent burn’, used when time is major constraint and fuel efficiency can be sacrificed
define rendezvous manouevre
spacracraft intercepts another spacecraft in the same orbit but a different true anomaly, intercepting spacecraft either speeds up or slows down
what orbit type is used to adjust the period of the spacecraft so it can meet the target spacecraft?
elliptical
to intercept a spacecraft which is ahead, should the inner or outer ellipse be used and why?
inner ellipse provides faster orbit to catch up with spacecraft ahead
how can we find the time required to synchronise with the target spacecraft?
difference in periods of the two orbits
what is a plane change orbit?
manouevre where a simple plane change is done or both inclination and plane change occur in a combined manoeuvre
why is it not favourable to change the plane of a low earth orbiting satellite?
very costly in energy terms, velocity in orbit increases as orbit altitude decreases, therefore energy required to make changes in a lower altitude orbit are higher than in a higher altitude orbit
when do we use a combined plane change and hohmann transfer?
when spacecraft transferring from from initial launcher parking orbit to final mission orbit
