This link gives some pretty good short descriptions of some of the maneuvers done when the Shuttle decides to land. My questions, based on info in the link:
The Shuttle executes a series of four banks during re-entry to bleed off speed. I understand why, but could the Shuttle have gone on a ‘straight’ flight path if it had started the de-orbit burn from farther away, and thus have a longer time to reduce speed?
As it closes in on the runway, the nose is pitched downward, until shortly before touchdown. Is this because it is has at that point lost too much speed, and it needs to regain some for a safe landing?
The reason they make four banks is to allow for the possibility of making course corrections.
Remember, the Orbiter doesn’t use its engines during landing. It is essentially a glider. The approach to the landing field is intentionally too high, set to overshoot the runway by many miles. If they come down exactly as planned, they execute four banks to reduce speed and reach the runway.
But what happens if the Orbiter comes down too fast and too close? Then they can execute five banks instead of four. And what if it comes down too slow and too short? Then they can execute three banks instead of four.
Without engines, if your approach path didn’t include any opportunities to dump velocity, you’d be up a creek if you found yourself going slightly slower than predicted.
It banks (banked I guess) 80 degrees during the speed reduction maneuvers? I never knew that! Quite the ride it must have been, more so if you get a window seat.
You can’t fire the main engines during descent, because you don’t have the fuel for them. That’s what that big external tank is for. If you’re still connected to the tank during re-entry, something has gone horribly wrong.
The smaller orbital maneuvering engines (the ones on the “pods” on either side of the vertical stabilizer) have an onboard fuel supply, but they’re only for use in space, and would be pretty close to empty on re-entry.
The tiny attitude control engines, which are all over and so small you’d never notice them, might be used during descent, but they don’t give any usable amount of thrust for flying the vehicle.
These have been used during decent, if the control surfaces do not make the needed adjustments these will fire to do so. This was noted in the shuttle that broke up and was destroyed on reentry, the control surfaces first tried to compensate for the wing that was failing, then the RCS activated and fired as it went further out of "alignment’. But yes not for thrust, just angle adjustments.
Not sure how much the USA should be patting itself on the back with regards to the “incredible” part. Instead of building such a hugely complex reentry system, they could have brought the crew back in much smaller capsules. I have read that a reentry capsule can in principle be nothing more than a sphere weighted such that the side with the heat shielding tends to end up facing down. The russians have had some rather massive failures with their reentry capsules and almost always got everyone back. Even one time when they didn’t, the capsule made it back to the ground fine, the crew were dead from lack of oxygen.
The real shame about the shuttle was the whole platform blasts 100 metric tons into orbit (the mass of the orbiter) yet only 26 tons is payload. Sure, you get the orbiter back…but a much better idea would be to use the same rocket that can lift 100 metric tons, and only send home a small capsule with the crew. The rest of the payload, you leave in orbit.
Unless the payload is an orbital facility not engineered for permanent independent orbital residence. We have a fairly permanent space station now, but not back then. If you wanted to do science on-orbit, for instance, you had to take your experiments up with you and return them (unless they were going to be throw-away).
ETA: But I’m sure we’re not debating the wisdom of the Shuttle program, just seeking facts about the STS’ re-entry profile.
Let’s not completely derail the thread with this hijack. If you wish to discuss the shuttle vs. traditional rockets and capsules, please do so in another thread.
No, all aircraft do this maneuver during landing, its called rotation. In fact the Shuttle touches down on the runway at about the highest airspeed of any aircraft, around 215mph! It also descends at an extremely high rate for a glider, 9800fpm.
Besides the main engines (and the SRBs) the Shuttle has OMS/RCS thrusters. The OMS (Orbital Maneuvering System) are two powerful engines located on the rear of the Shuttle. They have engine bells like the main engines but much smaller. They do the bulk of the major maneuvering like changing altitude by speeding up or slowing down (firing with the orbiter facing backwards). This is also how the de-orbit procedure begins, with the OMS engines firing with the Shuttle upside-down and facing backwards. I don’t actually know if there’s any contingency to fire the OMS for thrust in the atmosphere if the Shuttle is somehow hopelessly short on approach (interesting question), but I doubt it.
TheRCS (Reaction Control System) are much smaller thruster ‘jets’ placed along the Shuttle to allow for precise pitch/roll/yaw control like an aircraft (but in the vacuum of space).
I can’t tell which part of the approach & landing process you’re talking about. Is this pitch-down maneuver 1 second, 10 seconds, or 5 minutes before touchdown? Is there some source that describes (or vid that shows) what you’re talking about in more detail?
Once we understand what you’re asking, we can probably get a good answer on what’s really going on & why.
I think the 19° pitch down is just around the optimum glide angle, no? [del]The glide ratio for the shuttle is 4.5, and tan(1/4.5) = 22.6°, so for final approach it would just be gliding with a 3°-4° angle of attack.[/del]
Trig was a long time ago. Trying again: arctan(1/4.5)=12.5°. So the 19° pitch down would be a bit steeper than the optimal glide ratio, perhaps giving a bit more of a margin for error.
IF the pitch-down maneuver the OP is talking about happens a minute or so before touchdown, I’d expect that to be this:
Prior to that point the shuttle is descending at a flatter-than-sustainable angle. Which means they are losing speed. In other words, they’re slowing down towards their final approach speed.
Upon arriving at final approach speed, they need to steepen the descent to maintain that speed. Hence the pitch down. They are now in a steady-state glide which could continue indefinitely until the altitude runs out.
We do something conceptually similar in jets: approaching the airfield we’re descending at idle and more shallowly than will maintain airspeed. So the speed is slowly bleeding off. At some point we intercept the desired descent path and steepen the descent. We add more flaps to offset the increased steepness and keep the airspeed slowly dropping off. Eventually we’ve slowed to final approach speed. And here’s where we part company with the shuttle. We cheat by adding thrust to maintain speed and path to touchdown. They don’t have any thrust to add so they nose over & pick up a steeper path instead.
At about 300 feet the shuttle also did what they called a pre-flare, reducing the descent angle (and therefore descent rate) by about half. And at that point they once again began burning off airspeed from final approach airspeed to touchdown airspeed.
The goal of the pre-flare was to reduce the vertical descent rate to a more moderate number so the actual touchdown flare maneuver would be doable; trying to round out perfectly from their hellacious dive would be very hard. Taking it in carefully calibrated bites leaves the final finesse-ful flare-to-touchdown a lot easier maneuver to perform. To be sure, you’re burning energy as you do that, and eating up horizontal distance. Which is why the whole process is massively computer-assisted and the runway is multiple miles long.
We did something similar when practicing engine failure landings in fighters. We’d dive at the angle necessary to maintain flying speed while aiming a little short of the runway. Passing a couple hundred feet we’d abruptly raise the nose to a much flatter pre-flare angle. Speed would begin dropping but now we were descending at a more typical landing rate; slow enough that we could manage the flare & touchdown by eye at a more or less typical landing speed.
Sailplanes and gliders use similar techniques to manage total energy so they arrive at the end of the runway at a speed, altitude, and descent rate that will result in a doable flare maneuver and a touchdown early enough on the runway to get stopped, but not so early that they land short.