|
Written by Szymon Ender |
| Wednesday, 26 July 2006 |
| Rocket Science for Dummies - Lesson 2 :
Climbing up and falling down
Playback srenario avaliable
here (zip format) or
here (7zip SFX)
The controllable Shuttle-A (SH-02) is parked on a similar orbit, 7 km away
if you want to do the tutorial yourself
Ok. Got the .scn? So *let's get it on!*
( Scenario files should be extracted to the tutorials folder of your
Orbiter installation)
 |
Point the shuttle prograde - either by steering manually or by using the
prograde autopilot.
|
|
Observe Orbit MFD and fire the main thrusters until the number labelled
"Ecc" reaches around 0.5 - then kill the engines with [*]. For a better
precision keep decreasing thrust gradually by holding [Ctrl] all the
time and pressing [Num -]
|
Right - time for some more theory.
The graphic for your orbit should now be quite elongated - you can modify
the display of OrbitMFD to make it easier to read by using [right shift M]
or MOD button repeatedly.
When changing the shape of our orbit there are two points of interest...
Periapsis : The point of closest approach to
the body we are orbiting. It is marked with a round filled dot on OrbitMFD.
Apoapsis : The point where our orbit takes us
the furthest from the cental body - marked with a round empty dot - exactly
opposite of periapsis.
And there are two numbers listed on OrbitMFD directly related to these
two points.
|
PeR : Periapsis Radius -
the distance of the periapsis from the center of the central body in
metres.
|
|
ApR : Apoapsis Radius - I
bet you can't guess what this is...
|
Trap for beginners : PeR and ApR are measured from the center of the
body, not from it's surface. If you don't pay attention, you may create a
new crater on the moon... I recommend changing the distance display mode to
Altitude [RShift D] or DST button what will change above values to PeA and
ApA - altitudes from surface. The only thing you'll have to know now are
atmosphere heights for planets and some moons but we don't care now since we
are over Moon.
Two other number of interest are :
|
PeT : The time in seconds
until you next reach the periapsis.
|
|
ApT : The number of
seconds until apoapsis.
|
Almost every manouver related to changing the shape of your orbit is
performed at one of these two points. OK, let's get out of the classroom for
a bit.
|
Coast around to the apoapsis - your current location is marked by the
solid radial line of OrbitMFD. You may wish to accelerate time for a bit
- use [T] to increase the rate, and [R] to decrease it. Don't exceed
1000x.
|
|
Once about 40s to apoapsis, turn prograde and fire the main engines
until the number labelled "Ecc" reaches zero (or as close as it will go)
- watch the shape of your orbit as you do so. Decrease thrust as before
([Ctrl] and [Num -] and kill with [*]).
|
|
We'll complete a quarter of orbit to enjoy the view... we're going back
to the classroom soon.
|
A summary of what we have done so far : A prograde burn at periapsis
increases ApA, and a prograde burn at apoapsis increases PeA. So to climb
from a low circular orbit to a higher one, we need to make two burns - one
to increase ApA to the required value, and one to bring PeA up to the same
value.
Three more numbers of interest on OrbitMFD:
Take a note of your current velocity and period - they should be around
970m/s and 34,000s (that's about 9.5 hours).
Right - how do we get back down again? It should come as no surprise that
we will be doing retrograde burns.
|
Coast around to your apoapsis - actually, we don't really need to wait
if our orbit is very nearly circular...every point on our orbit is
roughly the same distance from the Moon anyway.)
|
|
Turn retrograde, and fire the main engines until PeA is 40 km. If you
overdo and make Pe negative, either jump to 2d cockpit [F8] turn on both
AUX PODs' levers or switch RCS to translation mode [Num /] and use short
burst with [Num 9] key (retro) and [Num 6] (pro). Another [Num /]
returns to rotation mode.
|
|
Coast around to the (new) periapsis (start turning retrograde 45s before),
and perform another retrograde burn so that your eccentricity falls to
around zero - be careful. Use all precise methods described before.
|
|
Do a few orbits and enjoy the sense of speed...
|
Take a look at your new velocity and period - 1656m/s and 6700s (around
1.9 hours).
Even though to lower our orbit we used retrograde burns (which means we
slowed down...), we are now going much faster...this result is very counter
intuitive, but will be quite important later on. |
