Randall Clague wrote: > At 11:02 AM 06/15/2002, Ian Woollard wrote: > >> However. all my rockets (under 10 tonnes all up) seem to have difficulty >> reaching orbit if they have a diameter much above 20 cm or so. Has >> anyone >> else found this, or am I messing up my aerodynamics > > No; it's a real scaling effect, the square/cube law. As the vehicle > gets smaller, because the area goes down less rapidly than the volume > (and mass), drag losses become more important relative to gravity > losses. It eventually reaches the point that you have to care about > drag.
> Shuttle throttle back isn't for efficiency, it's for aerodynamic load > management. Something about not wanting the wings to come off. > That's what they get for bringing along wings, if you ask me. :-) I always thought they should lose the wings, but not quite like that ;-) > As you've discovered, SSTO with a small vehicle requires trajectory > optimization, which in turn is a balancing act as you trade off drag > losses against gravity losses. There is probably a minimum size (for > a given shape, mass fraction, propellant type, engine, etc.) below > which a single stage vehicle can't make orbit. In theory you can make the rocket thinner, but that probably leads to structural issues- can a 20cm wide 6m long pole survive takeoff stresses- will it buckle, resonate? etc. etc. Mockingbird planned on a diameter of 80cm. I don't understand how it can be that wide at the moment, but I doubt my software a little- modelling drag, particularly supersonic drag its tricky to model. Anyway aerodynamics is quite critical. >> I think I prefer the Carter Copter architecture for an air vehicle >> provided it can be >> a) mass produced >> b) given more or less ensured stability.via computer control >> >> The aerodynamic efficiency should be higher; and the system is >> simpler and should >> be cheaper to maintain; and for air vehicles good aerodynamics is >> rather important. >> Also the failure modes of Carter Copter should be more benign in most >> cases.. > > I was thinking about this... A powered lift vehicle is always going > to have a harder time of it, regulatorily, than a helicopter, > autogyro, or other rotary wing aircraft. With a rotary wing, if you > lose power, you still have a wing. With powered lift, if you lose > power, you have neither a wing nor a prayer to save you. And FAA > doesn't like ejection seats in civilian aircraft... Still, IRC losing your engines on a 767 jet aircraft just after takeoff- you're screwed. Doesn't matter if you can make it back to the airport; the mass is too high to land without dumping the fuel- but you don't have time to do that. You're gonna have a bad day one way or another. The only solution; and this is the one used by 767, is multiple engines, as independent as possible. I'd like to think that regulations are ultimately based on commonsense. ;-) > -R _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
