Shareware Grab Bag/Shareware Grab Bag.iso/010/orbit.arc/ORBIT.DOC

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                          ORBIT
                 A Spaceflight Simulator


                     Charles Goldberg
                       RD 2 Box 415
               Titusville, New Jersey 08560


ORBIT is a working orbital simulator similar to those used by
NASA to plan and monitor spaceflights in near-earth and lunar
trajectories.  Based on thrust and vector data which you pro-
vide, the program calculates and displays the resulting orbital
trajectory.



The program is based on a time-driven numerical integration
of the Newtonian three-body problem (Euler's Method).  It pro-
vides for the entry of an initial launch vector and any number
of subsequent burns.  It is capable of reproducing a full range
of orbital trajectories, including:

     o Suborbital flights
     o Parking orbits
     o Polar orbits
     o Deburn & reentry
     o Transfer trajectories
     o Lunar bypasses
     o Lunar orbits
     o Lunar landings
     o Geosynchronous orbits
       and more.

In fact, the simulator will reproduce almost any real-life tra-
jectory within a half-million miles of earth with sufficient
accuracy to permit serious research.

However, ...

BE PREPARED FOR A VERY SLOW PROGRAM!

Depending on the speed of your PC, ORBIT can take as much as two
hours to complete a six-day lunar mission.  Longer missions
(e.g. bisynchronous orbits) may take substantially longer.  For
the sake of your monitor:

IF YOU PLAN TO LET ORBIT RUN FOR MORE THAN A FEW HOURS, REDUCE
CONTRAST ON YOUR MONITOR OR EVEN SWITCH IT OFF TO PREVENT THE
IMAGE FROM BURNING IN.


                      STARTUP SEQUENCE

At the end of this document, a number of "recipes" are given to
get you started.  I recommend that you follow these recipes
carefully, saving your launch files.  You will find that these
launch files will save you an enormous amount of time in later
simulations.

After following a recipe, you might want to repeat the recipe with
slight differences in your vectors.  Observing the results of
minor variations will give you a feel for your spacecraft very
quickly.

When you start the program, you will be asked for a launch file.
If you enter a <Return>, the program will place you at Kennedy
Space Flight Center in Florida.  Later, as you build your launch
file library, you will be able to choose other places (and times)
for your starting point.

Next, the program asks for a launch date.  If you enter <Return>
it will set the launch date for July 1st at 00:00 UT.  Until
you are fully familiar with the program, the default setting
will be just fine.

Next, the program asks for a "Differential Velocity" in kilometers
per second (i.e. the increase in the spacecraft's velocity
as a result of the "burn").  Use your recipes initially, then
modify this value to produce alternate trajectories.

When you are making a ground launch from KSFC, the program will ask
for an azimuth (follow your recipes at first).  If you are executing
a "burn" in space, the program will ask for a polar vector and an
equatorial vector.  Once again, follow your recipes.

At this point, you are ready to launch.  Watch your telemetry (the data
printing out on the right hand side of your screen).  As you become
more familiar with these readouts, you will begin to "feel" the
spacecraft.  Also, in following the recipes, note your telemetry
readings at each orbital maneuver.  Particularly watch for perigee
and apogee, and keep an eye on your velocity.

When executing an orbital maneuver, check your telemetry to identify
where some of the recipe values are coming from.  Learn to recognize
direction-of-motion vectors, tangent vectors and compensating vec-
tors.  Note how they are used.


                         IN FLIGHT

Your control keys will be displayed at the bottom of the screen.

F2   PLOT    This key switches your display to geocentric plot.
             There are two plot options, polar or equatorial.
             The polar plot will usually be more useful, but
             you should get familiar with both.

F3   VIEW    Switches among the various geocentric plot options:

             E: earth-centered plotting
             S: spacecraft-centered plotting
             M: moon-centered plotting
             +: zoom in
             -: zoom out
             D: data switch (for full screen graphics)

F4   MAP     This is the Mercator plot, showing the spacecraft's
             position in latitude and longitude.  This is the
             display shown at launch.

These keys can be used at your option, at any time, to change the
display.  Changing these keys does not alter the simulation, only
the display.

F5   EPHM    This key provides an on-line lunar ephemeris for use
             in planning lunar trajectories.  When you finally
             understand how to use this table, you will be ready
             to select alternate dates for your launch!

F6   TIMER   This key works just like an alarm clock.  You set it
             for a specific time.  When the simulation reaches
             the set time, the timer "beeps" and the simulation
             pauses for a manual restart.  Use the timer for
             wakeup calls on long, boring sections of your missions.

F7   LOAD    Use this key to read in a previously saved launch file.
             After reading in your launch file, the program will
             give you the option of entering a burn.  If you
             <<return>> through the vector inputs, the program
             will pick up at the time and direction vector at which
             the flight was initially saved.

F8   SAVE    Use this key to save your current position in a launch
             file.  The program will ask for a file name, which
             can be any legal DOS name (including path and prefix
             if you wish).

F9   SLEEP   This is an important key.  Use it to switch between
             the simulators two time modes:

             Regular time (40-100 simulation minutes per minute of
                          real time)

             Sleep mode (150-400 simulation minutes per minute of
                          real time)

             Use the regular mode for near-earth orbits or at other
             times when you need minute-by-minute readouts.  Use
             sleep mode during less critical periods.  In sleep mode
             readouts are only displayed every 4 simulation hours.

F10   OMS    This is the orbital maneuvering system, the spacecraft
             engines.  They can be engaged at any time.  The simu-
             lation will pause, asking for differental velocity,
             polar vector and equatorial vector.  It will then
             execute the burn and continue the simulation.  Follow
             your recipes to start.  Then add your own variations.

In addition to the program keys, the following keys can be used at
any time:

            P: resets the perigee gauges
            A: resets the apogeee gauges
            +: zoom in
            -: zoom out


                          THE RECIPE BOOK

1. SUBORBITAL FLIGHTS

Using a standard launch velocity of 4 km/sec, complete a variety of
launches from Kennedy Space Flight Center, with varying azimuths
(0, 90, 180, and 270 degrees).  Observe the effects of earth rotation
on apogee and flight duration.

2. STANDARD ORBIT

Launch from KSFC at a launch velocity of 3 km/sec and 90 degree
azimuth.  At t-plus-5 minutes, tap F10 and enter a differential
velocity of 5.7 km/sec, polar vector 6.8228 degrees, equatorial
vector 95.5907 degrees (check your telemetry to see where these
numbers are coming from).

SAVE (F8) this orbit at t-plus-15 minutes.

Observe the orbit for several revolutions in each of your display
modes.

3. POLAR ORBIT

Launch from KSFC at 4 km/sec and 0 degree azimuth.  At t-plus-5,
enter a 5.5 km/sec burn at 127.7023 & 0.0000 vectors.

Observe how earth rotation (in Map mode) makes polar orbits use-
ful for mapping projects.  Observe difference in energy require-
ments as compared with a standard eastern launch.

4. EQUATORIAL ORBIT

Start from your standard launch file at t-plus-15 minutes.  At
t-plus-31 minutes, execute a 3 km/sec burn at 71 & 196.1490.

Save for later use in geosynchronous transfers and lunar
trajectories.

5. GEOSYNCHRONOUS ORBIT & APOGEE KICK

From equatorial orbit, place the spacecraft into a Hohmann Transfer
Orbit to Geosynchronous Apogee (at Radius Vector = 26270 miles).
Find the correct velocity by trial and error, using direction
of motion readouts for polar and equatorial vectors.

The objective of this part of the project is to use just enough
thrust to get your spacecraft to the proper RV.  In the process
you will be getting a feel for transfer orbital mechanics.

Once you have your spacecraft at the right RV, execute another
burn at apogee to bring your spacecraft into uniform circular
motion (UCM).  Calculate your velocity by comparing the UCM
readout with your velocity readout.  Execute your burn in the
direction of motion.

Confirm that you have achieved geosynchronous orbit by extended
tracking in map format (the spacecraft should move in a small
figure-8 VERY SLOWLY).

6. LUNAR TRAJECTORY

By now, you should have learned that you produce a transfer
orbit by burning (usually in the direction of motion) at the
point in the orbit where you want to place your perigee.
Apogee will then be located 180 degrees from perigee.  In other
words, you go where you want to go by aiming your apogee.  Then,
when you reach apogee, you kick your spacecraft into whatever
local trajectory you want.

Now, if you provide an additional 3.1 km/sec to your spacecraft
from equatorial orbit, you will produce a transfer orbit with
an apogee at about 238000 miles (i.e. at the distance of the
lunar orbit).  It takes about three days for the spacecraft
to make that journey.

Now all you have to do is aim your apogee at the place where
the moon will be in three days and you will get a close en-
counter.  The lunar ephemerides should help you to pick your
lunar window.

If at first you don't succeed...

7. THE ADVANCED STUFF

If you have run enough lunar trajectories, you will have dis-
covered rubber-banding, retrograde bypasses, lunar orbits
and lunar landings.   Now, you're ready for the interesting
stuff.

For example, in 1975, two Soviet astrophysicists, V. Ivashkin and N.
Tupitsyn published an article pointing out that (especially at
higher earth latitudes) a geosynchronous orbit could be achieved
with less energy by using a lunar bypass as an accelerator into
geosynchronous orbit.  In other words, instead
of firing a standard transfer orbit and apogee kick (like you
did in Project 5), they recommend a lunar trajectory which will
loop behind the moon and fall back toward earth.  When the returning
spacecraft reaches geosynchronous distance, you can execute a
burn to change the orbit into geosynchronous uniform circular
motion. They claim that such a procedure will require less energy
than the more traditional method.

Are they right?


Or try this one.

Suppose that we want to colonize the moon.  Large quantities of
materials and manpower need to be shuttled to the moon.  Now,
of course, we could transfer it all in individual spacecraft,
but that's expensive and unnecessary.

Suppose instead that we placed a cargo transport in an elongated
orbit with its perigee close to earth and its apogee at lunar
distance.  Can we develop a trajectory which will shuttle back
and forth between the earth and moon in a stable bisynchronous orbit?

If we could set up that type of trajectory, we could load the
transport here and offload it there with minimum energy.