Date: Thu, 15 Sep 1988 15:20-EDT From: space-tech-request@CS.CMU.EDU To: "~/st/lists/stdigest" Subject: Space-tech Digest #4 Contents: Roger Arnold Re: low inclination EML Roger Arnold orbit raising for EML vehicles Paul Dietz Re: orbit raising for EML vehicles Roger Arnold Re: orbit raising for EML vehicles Jenine Abarbanel Senior projects Jenine Abarbanel Laser Rockets ------------------------------ From: telesoft!roger@ucsd.edu (Roger Arnold @prodigal) Date: Wed, 14 Sep 88 10:43:17 PDT To: bell@cs.unc.edu Subject: Re: low inclination EML Cc: space-tech@cs.cmu.edu Andrew - Gotcha! You are hereby found guilty of thinking in terms of passive structures. Taking your points in order: 1) My back-of-the-envelope numbers for the flying catapult assume an average wing loading of 50 kg/m^2, an average chord of 5 m, a length of 2 km, for a mass of 500 T, launching 10 one ton payloads into a single orbital plane over the course of a two hour flight. Assuming a launch velocity of 9000 mps, the "recoil" delivered to the catapult by each launch will be 19 mps (~ 35 mph). The recoil acceleration is about 4 gees, for just under half a second. 2) Straightness, in this, does not depend on passive characteristics of the structure. There is a laser beam running the length of the wing. Any deviation from straightness generates signals to the engines, control surfaces, and control tendons in the affected section of the wing. So long as the magnitude and rate of change from external purturbations don't exceed what the control systems can handle, the structure stays very straight indeed. 3) The laser beam defines where the catapult wants to point, and the active control system sees to it that that's where it does point, regardless of what's happening with the wind. (That's a conscious oversimplification, and again, presumes that external purturbations stay within limits of what the control system can handle. But the atmosphere at 60,000 feet is very stable.) The limit on pointing accuracy is imposed by limits on orientation sensing. Using phase comparisons on Navsat signals received at one end of the wing vs. the other, combined with onboard inertial sensor data, stable pointing within arc seconds should be feasible. All this is not to say that the concept is without problems. There are a number of areas where it's vulnerable to attack. You simply haven't found them, yet. :-) - Roger Arnold ------------------------------ From: telesoft!roger@ucsd.edu (Roger Arnold @prodigal) Date: Wed, 14 Sep 88 00:26:42 PDT To: space-tech@cs.cmu.edu Subject: orbit raising for EML vehicles I notice that Paul's figures for perigee raising in an EML vehicle presume an apogee burn. This is not optimum. To match a given orbit with minimum delta vee for the insertion burn, you want to use a pre-apogee burn that both lowers the apogee and raises the perigee. It's not too hard to see why this is so. If the burn is at apogee, it will be in a direction tangent to the orbit; the vertical component of the delta vee will be zero. Now change the angle of the burn by a small increment; the change in the horizontal component of the delta vee is zero, in the first order (derivitive of cosine of small angle is small). But there is a change in the vertical component that is linear in the angle. So a small residual vertical component in the velocity vector could have been killed with no cost--in the first order--to the horizontal delta vee of the burn. But if there is a residual vertical velocity component (meaning that the apogee of the transfer orbit prior to the insertion burn is higher than that of the final orbit), then there is also a larger horizontal velocity component, so that the horizontal delta vee required for the burn is less. Now the $64 dollar question: for a given launch elevation and given values for apogee and perigee, what are the formulas for the optimum launch velocity and the insertion delta vee, and what will be the direction of the insertion burn? As an interesting spacial case, for insertion to a circular orbit, what will be the required delta vee for the insertion burn as a function of launch elevation for the optimum transfer, and how does it compare to the numbers for the Hoehman (sp?) transfer model? I know how to derive the answers from basic physics, and I'm willing to do it if nobody else volunteers. I suspect that the numbers will be interesting. But it will be a strain on my very out-of-training grey cells to make them work that hard. - Roger Arnold ucsd.ucsd.edu!telesoft!roger ------------------------------ Date: Wed, 14 Sep 88 14:10:57 EDT From: dietz@cs.rochester.edu To: telesoft!roger@ucsd.edu Cc: space-tech@cs.cmu.edu Subject: orbit raising for EML vehicles Interesting... I had not realized that. Two other questions: 1. How does this affect the problem of widening the launch window from a fixed EML to a given orbit? 2. Does making more than one burn at different times help? For example, would it be better to make one burn at a higher apogee to raise the perigee, then another at perigee to lower the apogee to the final orbit? Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: telesoft!roger@ucsd.edu (Roger Arnold @prodigal) Date: Wed, 14 Sep 88 23:19:25 PDT To: dietz@cs.rochester.edu, ucsd.edu!telesoft!roger@cs.rochester.edu Subject: Re: orbit raising for EML vehicles Cc: space-tech@cs.cmu.edu In regard to your first question, I don't see that the pre-appogee burn strategy would have much effect on the launch window. It doesn't make plane changes any less expensive. I don't know exactly how the strategy I described compares with a two-burn strategy. What I described applies to matching a specific orbit from an intersecting transfer orbit with a single burn near the point of intersection. If the match is achieved with an initial burn at the appogee of the transfer orbit, followed by a perigee burn to lower the appogee.. well yes, I do know how it compares. The two burn strategy will be cheaper. Because in the limit of a parabolic transfer orbit, the delta vee for the appogee burn will approach zero, and I know that the delta vee for the perigee burn to lower the appogee will be less than would be needed to go from the transfer orbit direct to the final orbit at any altitude above the perigee. If you can afford a long coast out and back, that's the way to go. - Roger Arnold ------------------------------ Date: Wed, 14 Sep 88 16:07:10 PDT From: jenine%priam.usc.edu@oberon.USC.EDU (Jenine Abarbanel) To: space-tech@cs.cmu.edu Subject: Senior projects Do you suppose we could build an EML for $120 and in 3 months? It's certainly orginal. Jenine my apologies for this, but I'm desperate and getting hysterical ------------------------------ Date: Wed, 14 Sep 88 16:03:07 PDT From: jenine%priam.usc.edu@oberon.USC.EDU (Jenine Abarbanel) To: space-tech@cs.cmu.edu Subject: Laser rockets Well, my proposal for my senior project here at USC has been [literally] shot out of the sky. Now it looks like I will be experimenting with Laser Rockets [rockets ignited by aiming a high powered lazer up the end of the rocket at the fuel]. If anyone has any information on these, references, experiences, etc. it would be greatly appreciated. This set-back has caused serious difficulty time-wise. Better yet, does anyone have any better ideas for a low budget, relatively quick, and original project [we have about 3 months in which to perform the experiment and a budget of $120]. Yes, it's ridiculous, but this is their last chance to torture us as undergraduates, after all! Thank you, in advance. Jenine Abarbanel jenine@priam.usc.edu ------------------------------ [ end ]