I was watching the launch of a rocket on TV the other day, a real rocket, bringing a satellite into orbit.
It seems to ME that what I should see as the rocket climbs away is a foreshortening rocket, and eventually just the bottom of it, a bright ball of “fire”. That is, it should be climbing away perpendicular to the tangent plane at cape Canaveral (or wherever it was being launched from).
However, what I did see what the rocket go up and then go horizontal with respect to the television camera. That is, a camera – presumably stationed on the ground – was zoomed in on the rocket, and it’s orientation was a few degrees above parallel.
So, what’s up? What’s the path to orbit? I’d think they would try to fly the thing up a couple hundred miles through the thinnest path through the atmosphere, then start moving it towards its destination.
Or, do they just send it up a couple miles, and then start flying it relatively horizontally?
Basically, what’s the straight dope ona rocket path to orbit?
Since they need to acheive about 27,000 km/hr in a direction parallel to the surface of the earth in order to maintain orbit, best to get started as soon as possible.
They want to get the rocket out of the lower atmosphere as quickly as possible. That reduces drag and the aerodynamic stresses on the rocket. Once the atmosphere is thin enough, they can also jettison the shroud that protects the payload, which is useless weight.
Part of trajectory planning is range safety. They have to ensure that in the case of an engine failure or major malfunction, the rocket, or its debris, falls into a safe area, minimizing any threat to life or property.
At what altitude should the rocket start tilting toward the horizontal, for most efficient acceleration into orbit? At what altitude should it be nearly horizontal? I ask because I have a spaceflight simulator that I play with from time to time, which includes a Space Shuttle based on realistic physics. I always seem to reach LEO altitude with too much vertical and not enough horizontal velocity, so I fail to achieve a stable orbit. Instead, my Shuttle travels along a high parabolic arc before plunging back into the atmosphere.
This is true, but the main portion of the impulse goes to increasing the momentum of the vehicle so that it can stay in orbit; hence, once they get up high enough, you tilt over to start getting vectored into your orbital path. (In order to stay in any particular orbit, you have to have a specific velocity at a specific point; if you have less than what is required, you’ll fall back down to Earth in a ballistic trajectory, like Shepard and Grissom in Mercury-Redstone 3 and Mercury-Redstone 4, respectively.) You could go straight up and then straight over, but that would take extra energy; instead, you pick some trajectory that “cuts the corner”, balancing aerodynamic load and drag with optimal track to make orbital injection into whatever orbit you’re trying to achieve.
Aerodynamic forces are a concern, which is why the U.S. Space Shuttle almost immediately rolls into the supine orientation as it pitches over (without doing this it would show some negative structural margins on the wing joints) , but they aren’t the most significant one. Drag costs you somewhere between 5-10% of your total energy budget (see Sutton, Rocket Propulsion Elements, pg. 106-7), so it’s not insignficant but also not the key driving factor, which is, again, to get enough orbital velocity to stay in orbit.
Range safety for large boosters is usually assured by launching from a coast or island location and putting the immediate abort trajectory over uninhabited water., hence why the Shuttle launches from Canaveral or, hypothetically, from Vandenberg’s SLC-6 in polar orbit, though in practice this was never done. (The Soviets/Russians handled this by putting over sparsely inhabited tundra and not really giving a care if it fell on some shepherd’s head. Nice.)
Shroud jettison, where applicable, doesn’t generally occur until the booster has already finished final orbital boost phase. This is because blowing the shroud free is one of those events where things tend to go wrong, and trying to time it with staging and/or blow it while under boost adds to the complexity and likelyhood of failure. Some weapons systems–particularly anti-ballistic devices–will perform a shroud jettison under boost because they don’t have time to stop and take off the top, but it’s a high risk endeavor that requires precise timing. Shrouds are pretty lightweight, usually made of a thin skin of aluminum covered in some kind of thermal protection (often natural cork or thermal resistant fabric) and given stiffness by a light honeycomb structure sandwiched inside. Once the shroud is gone, you can commence with post-boost maneuvering to circularize the orbit, make lateral adjustments, orient and position your MIRV bus, dock up with your Agena target, et cetera.
With solid rocket boosters (where you can’t throttle back or easily terminate thrust) a method is used called GEMS (General Energy Managment System) which intentionally intruduces a mild lateral oscillation into the trajectory to burn off extra energy so as not to overshoot the target velocity.
So, in summary, most of the work you do is to get up to orbital speed, so it’s important to kick over and get that going as soon as possible. This is why schemes to merely lift a booster up to high altitude via airship are not particularly practical or useful.
This will totally depend upon what your target orbit is and how your vehicle performs at various altitudes. (A nozzle which is optimized for performance at sea level pressure will be significantly less efficient at high altitude.) It’s also going to depend on your thrust profile; for instance, the STS has an initial high thrust (3 Shuttle Main Engines plus 2 Solid Rocket Boosters), then the SMEs are dialed down in preparation for SRB separation, after which they’re throttled back up. Once fuel/propellent mass drops signficantly, I[sub]sp[/sub] goes up, requiring a gradual throttle down, then SME shutoff and EFT separation, with the Shuttle coasting up to orbit (circularized by the OMS system) and the EFT tumbling to destruction toward the Indian Ocean. So the Shuttle pitches over (and upside-down) early on and rides an almost tangent trajectory all the way to orbit. In general, the higher you want to go, the more you’re going to have to lean over and thrust “sideways” into an approximation of a spiral. It seems totally counterintuitive, but that’s orbital mechanics for you.