Satgen315   Getting into Orbit ( and staying there)  by GM4IHJ  8 April 95
BID of this msg is SGEN315    Use this BID if you retransmit msg on packet

On 28th March 1995 a Russian rocket carrying the Israeli Techsat ( Gurwin
1), and the Mexican Unamsat, suffered a staging failure. The satellites
appear to have reached the designated orbital altitude , but they fell
back to earth , because they were not travelling fast enough to stay in
orbit. As a result , the hard work and dedication of the two groups of
builders was totally negated.

Aside from the obvious sympathy felt by satellite users, there has been
surprise amongst some radio amateurs , that having reached orbital height,
the satellites did not stay in orbit.

The point being missed is that speed in orbit is the essential feature
which keeps a satellite aloft, not simply orbital altitude.

For orbits around the Moon or Planets , the necessary orbital speed varies
in accordance with the orbital height above the surface, and, with the
mass of the Moon or Planet. The bigger the planetary mass, the higher the
speed needed to remain in orbit. The higher the orbital altitude , the
lower the speed required to maintain that orbit. Typical figures are :-

 Height in                Orbital Velocity in Kms/sec
 Kms above 
 surface             Moon       Mars       Earth      Jupiter

   250               1.57       3.43       7.75       42.05
  1000               1.34       3.12       7.35       41.83
 10000               0.65       1.79       4.93       39.45 


Please note that achieving the correct speed entering an orbit is
important whether, you launch upwards from a Moon or a Planet, or, enter
>From interplanetary space in order to take up an orbit around a planet.
So there is double jeopardy here. Too little thrust on launch from a Moon
or Planet is disastrous - the satellite returns to the surface. By
contrast,too much velocity when entering a planets gravitational sphere of
influence coming from space , and you fail to enter a safe orbit and go
back into space, too little velocity and you crash onto its surface. So it
can be very tricky entering orbit around somewhere like the Moon , where
the safe velocity window is a small one.

However none of this will reinstate Techsat or Unamsat. Because, unlike a
road vehicle when you run out of thrust as IHJs did recently, rockets do
not come slowly and safely to a stop. Any reduction in thrust on a rocket
is almost always fatal. Gravity ensures that there is no such thing as a
small, inexpensive, rocket failure.
73 de GM4IHJ @ GB7SAN , or gm4ihj@delphi.com