From: jbh55289@uxa.cso.uiuc.edu (Josh Hopkins)
Subject: DC-Y trajectory simulation
Keywords: SSTO, Delta Clipper
Organization: University of Illinois at Urbana
Lines: 91


I've been to three talks in the last month which might be of interest. I've 
transcribed some of my notes below.  Since my note taking ability is by no means
infallible, please assume that all factual errors are mine.  Permission is 
granted to copy this without restriction.

Note for newbies:  The Delta Clipper project is geared towards producing a
single staget to orbit, reusable launch vehicle.  The DC-X vehicle is a 1/3
scale vehicle designed to test some of the concepts invovled in SSTO.  It is 
currently undergoing tests.  The DC-Y vehicle would be a full scale 
experimental vehicle capable of reaching orbit.  It has not yet been funded.

On April 6th, Rocky Nelson of MacDonnell Douglas gave a talk entitled 
"Optimizing Techniques for Advanced Space Missions" here at the University of
Illinois. Mr Nelson's job involves using software to simulate trajectories and
determine the optimal trajectory within given requirements.  Although he is
not directly involved with the Delta Clipper project, he has spent time with 
them recently, using his software for their applications.  He thus used 
the DC-Y project for most of his examples.  While I don't think the details
of implicit trajectory simulation are of much interest to the readers (I hope
they aren't - I fell asleep during that part), I think that many of you will
be interested in some of the details gleaned from the examples.

The first example given was the maximization of payload for a polar orbit.  The
main restriction is that acceleration must remain below 3 Gs.  I assume that
this is driven by passenger constraints rather than hardware constraints, but I
did not verify that.  The Delta Clipper Y version has 8 engines - 4 boosters
and 4 sustainers.  The boosters, which have a lower isp, are shut down in 
mid-flight.  Thus, one critical question is when to shut them down.  Mr Nelson
showed the following plot of acceleration vs time:
                  ______
3 G         /|   /      |
           / |  /       |          As ASCII graphs go, this is actually fairly 
          /  | /        |	   good.  The big difference is that the lines
2 G      /   |/         |          made by the  /  should be curves which are
        /               |          concave up.  The data is only approximate, as
       /                |          the graph wasn't up for very long.
1 G   /                 |
                        |
                        |
0 G                     |

             ^          ^
           ~100 sec    ~400 sec


As mentioned before, a critical constraint is that G levels must be kept below
3.  Initially, all eight engines are started.  As the vehicle  burns fuel the
accelleration increases.  As it gets close to 3G, the booster engines are 
throtled back.  However, they quickly become inefficient at low power, so it
soon makes more sense to cut them off altogether.  This causes the dip in 
accelleration at about 100 seconds.  Eventually the remaining sustainer engines
bring the G level back up to about 3 and then hold it there until they cut
out entirely.

The engine cutoff does not acutally occur in orbit.  The trajectory is aimed
for an altitude slightly higher than the 100nm desired and the last vestiges of
air drag slow the vehicle slightly, thus lowering the final altitude to 
that desired.

Questions from the audience:  (paraphrased)

Q:  Would it make sense to shut down the booster engines in pairs, rather than
    all at once?

A:  Very perceptive.  Worth considering.  They have not yet done the simulation.    Shutting down all four was part of the problem as given.

Q:  So what was the final payload for this trajectory?

A:  Can't tell us.  "Read Aviation Leak."  He also apparently had a good 
    propulsion example, but was told not to use it.  

My question:  Does anyone know if this security is due to SDIO protecting
national security or MD protecting their own interests?

The second example was reentry simulation, from orbit to just before the pitch
up maneuver.  The biggest constraint in this one is aerodynamic heating, and 
the parameter they were trying to maximize was crossrange.  He showed graphs
of heating using two different models, to show that both were very similar,
and I think we were supposed to assume that this meant they were very accurate.
The end result was that for a polar orbit landing at KSC, the DC-Y would have
about 30 degrees of crossrange and would start it's reentry profile about 
60 degrees south latitude.

I would have asked about the landing maneuvers, but he didn't know about that
aspect of the flight profile.

-- 
Josh Hopkins                                          jbh55289@uxa.cso.uiuc.edu
		    "Find a way or make one."
	             -attributed to Hannibal
