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.