goldader@uhccux.uhcc.hawaii.edu (Jeff Goldader) (01/12/90)
Does anyone know what advantages the direct ascent launch trajectory gives? I've heard of it being used several times, but never really heard *why* it's used. I do know it makes the OMS-1 burn unnecessary, which leads me to believe it gives a higher apogee on the initial orbit, but that's about it. Thanks in advance... Jeff Goldader University of Hawaii goldader@uhccux.uhcc.hawaii.edu Institute for Astronomy UH and the IfA don't know what I say, and I don't care what they think, and we're happy that way.
henry@utzoo.uucp (Henry Spencer) (01/13/90)
In article <6124@uhccux.uhcc.hawaii.edu> goldader@uhccux.uhcc.hawaii.edu (Jeff Goldader) writes: >Does anyone know what advantages the direct ascent launch trajectory >gives? I've heard of it being used several times, but never really >heard *why* it's used. I do know it makes the OMS-1 burn unnecessary, >which leads me to believe it gives a higher apogee on the initial >orbit, but that's about it. I was hoping somebody who knew for sure would answer this, but since nobody has... The old OMS-1 burn was a result of cutting off the main engines on reaching a very low, in fact too low, orbit. This gave better control of where the external tank ended up, but required an immediate OMS burn to prevent the orbiter from reentering with the tank. Essentially this amount to flying a "dip" maneuver, which cost something in itself, and doing the last little bit of the ascent with the less-efficient OMS engines, which ran up the cost further. Going straight into orbit costs less in fuel and hence gives either greater payload or a higher orbit. I'd rather not be quizzed on the details, since I don't understand it very well myself. (In particular, at first glance I don't understand why the tank ends up in the Pacific instead of doing one orbit and going down into the Atlantic at perigee.) -- 1972: Saturn V #15 flight-ready| Henry Spencer at U of Toronto Zoology 1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
moe@nuchat.UUCP (Norman C. Kluksdahl) (01/14/90)
In article <1990Jan12.213536.7453@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >In article <6124@uhccux.uhcc.hawaii.edu> goldader@uhccux.uhcc.hawaii.edu (Jeff Goldader) writes: >>Does anyone know what advantages the direct ascent launch trajectory >>gives? > >to prevent the orbiter from reentering with the tank. Essentially this >amount to flying a "dip" maneuver, which cost something in itself, and >doing the last little bit of the ascent with the less-efficient OMS >engines, which ran up the cost further. Going straight into orbit costs >less in fuel and hence gives either greater payload or a higher orbit. > >I'd rather not be quizzed on the details, since I don't understand it >very well myself. (In particular, at first glance I don't understand >why the tank ends up in the Pacific instead of doing one orbit and going >down into the Atlantic at perigee.) I'm not absolutely certain of the details myself, but as near as I can figure, the direct ascent trajectory results in a highly elliptical orbit, which requires the OMS 2 burn to circularize. Now, from my reference on ballistic trajectories, the perigee and apogee can be uniquely determined from the flight-path-angle at burnout and the velocity. We know that the shuttle must do an OMS-2 to circularize, but the tank doesn't. Thus, from an approximation of the altitude of non-negligible atmospheric drag, one can determine the 'range angle' of the ET's trajectory. Simple ballistic missile calculation. What does all that mean? My interpretation is that the flight-path-angle at burnout is relativly near zero, giving the ET a very large range angle, and thus putting the ET down in the Pacific. Remember, just because the burnout is over the Atlantic, the perigee might be elsewhere, especially if the flight-path-angle is nonzero at burnout. (Ref: Bate, Mueller, & White, "Fundamentals of Astrodynamics" Dover Press, ISBN 0-486-60061-0. Copyright 1971. From the USAF Academy, hence the details on ballistic trajectories. Good book. theory and examples, unsolved excercises, but unfortunately no answers.) ===================================================================== Norman Kluksdahl ...!nuchat!moe
BARRY@RICEVM1.BITNET (01/15/90)
The OMS-1 maneuver is by no means obsolete. The need for an OMS-1 is determined by the flight design -- orbital characteristics, ET-impact footprint (hopefully it lands in the Indian Ocean), propellant loadings, etc. An OMS-1 is almost always required, regardless of the flight plan, if a MECO underspeed occurs or an Abort to Orbit trajectory is selected. OMS-1 is not performed with the ET attached (as I interpreted Henry's posting). In fact, for TAL cases, an OMS burn is performed in order to avoid the probable violent rupture of the ET when it reenters the atmosphere. This, however, is not an "OMS-1" maneuver. An OMS-2 maneuver is always required. In some problematic cases, the maneuver may be performed with the RCS, though it might be called an OMS maneuver. Consider the upcoming HST deploy mission, STS-31. The deploy altitude is 330nm, which requires protecting about 530fps velocity change in order to deorbit to "steep" targets using the OMS engines. The OMS-2 maneuver is on the order of 480fps to raise perigee to about 310nm. These two maneuvers require a full OMS load! Therefore, an OMS-1 cannot be performed, so the ascent must be a direct insertion. Post OMS-2, the orbit will be 330x310nm. To circularize the orbit, an RCS-1 maneuver will be performed a few hours after OMS-2 in order to achieve 330x330nm. Some folks refer to this maneuver as an "OMS-3" maneuver, though it uses RCS thrusters and RCS propellant. Hope this help clear things up. ------------------------------------------------------------------------- Matthew R. Barry Department of Mechanical Engineering Guidance and Propulsion Systems Rice University NASA/Johnson Space Center DF63 Houston, TX 77252 Houston, TX 77058 barry@ricevm1.rice.edu mbarry@nasamail.nasa.gov -------------------------------------------------------------------------