Space Shuttle Gravity Turn

[736]

if you look at some rockets, after two minutes they're almost flying east. Its about 30 degrees from level for LEO, i think.

[Moar Boosters]

Side note: The Saturn V made the earliest pitch maneuver of any manned spacecraft ever. In the first few seconds after launch, the booster was within ten meters of the launch umbilical tower, and the inherent imprecision in the guidance system could have easily had it drift over into the tower in a vertical launch. Therefore, pretty much as soon as the engines were clear of the flame pit, the S-V was programmed to make a 2-3 degree pitchover *away* from the tower; it would then pitch back to vertical as soon as it had cleared the tower. This was internally known as the "lean," and was only there to decrease the odds of a catastrophic collision with the LUT, and is, to my knowledge, completely unique to the Saturn V (though I wouldn't be shocked if the Saturn I and IB also had a similar maneuver in their profile).

I think the space shuttle had something similar. Obviously it doesn't have the tower to clear like the Saturn V, but if you look at the launch from the side it didn't launch straight up, there's horizontal movement the moment it launches. (skip to 45 secs)

[rdfox]

I *think* that with the Shuttle, the horizontal movement at liftoff was more an unavoidable function of the net force vector not being perfectly vertical, due to the SSMEs being gimballed off the vertical to have their thrust vector pass through the stack CG; this meant that it was going to "slide" horizontally a bit at liftoff, until they could pitchover a little to adjust the net thrust vector to a vertical position. (Hooray, torque!)

[Galinette]

Actually, I think the real reason why the shuttle has a non vertical start was not cited here.

If you fly vertically, for each layer i of the atmosphere, you cross a certain thickness Ti of atmosphere.

If you fly with an angle to vertical theta > 0, then you cross a thickness Ti / cos(theta)

Since the cosine function is flat around 0, ( no first order term in the taylor series, 1-theta²/2 ), then having a small angle theta does not change a lot the crossed thickness. It's even almost free for the first degrees. For instance, with theta=5°, you cross a thickness of Ti + 0,38%

However, the horizontal speed gained is in sin(theta), linearly growing with theta for small angles. This means that the first degrees give a significant advantage as you are already gaining horizontal velocity. For 5°, you give 8.7% of your acceleration to horizontal velocity. That's a lot.

This means, that the optimal start angle is greater than zero. The actual one is complex to find, but apparently, for the shuttle, it was something like 20°.

Why isn't this used for rockets? Here I'm less sure, but maybe that oblique flight at rather low velocity is not easy to stabilize for a pure rocket with nearly no lift and control surface. For the space shuttle, it's easy.

Edited July 1, 2016 by Galinette

[Bill Phil]

The range of the gimbal on the SSMEs is over 10 degrees on two axes. Even if the angle of the vehicle is about 20 degrees, the gimbal of the SSMEs are pointing to the CoM.

[AdmiralTigerclaw]

This thread is ancient, but I was searching through for information on angles for gravity turns, and a thought occurs that nobody here ever considered, being focused on efficiencies.

Look at the space shuttle again.  It's a rocket stack, but what is hanging off the side of that rocket stack?

A GLIDER.  Not a pod with an L.E.T.  A giant, GLIDER.

Consider from a crew safety perspective, if you have to abort early in boost phase, you want to be able to separate that glider from the stack without suffering collisions.   Tests of the SR-71 blackbird launching the D-21 drone in the sixties showed that launching something from a piggy-back position on an aircraft tends to end in spectacular failure. 

If you want to escape from a very soon to be very much volatile rocket stack while stuck in a glorified glider, you want access to the best maneuver energy possible.  In an unpowered 'aircraft' with only altitude and speed available to it, that would be in the form of some kind of dive.  The only option for a dive would be from BELOW the stack.  In this case, you roll program and pitch onto your back as soon as you can.  That way, as soon as you blow the bolts on abort, not only does gravity pull you away from the stack, the pilot has immediate access to a Split-S maneuver, which will pull the glider down and away from the rocket, and if there's room in the maneuver, a full direction reversal from the stack itself.  The stack, with the SRBs still blazing away, continues on to whatever fate awaits.  Probably explosive.

 

If you attempt this in a more near vertical state, you stand a higher chance of collision since you don't have gravity to help pull you out of the slipstream of the stack.  Remember, as air passes quickly between two large, fast moving objects, its pressure lowers, pulling those objects together.  You an see this happen just watching two semi-trucks on the highway fight not to be sucked together.  The shuttle and it's booster stack are BIGGER.

 

Thus, it can be surmised that part of the Pitch-over program happening so early is in part to put the shuttle orbiter into position for a quick abort from boost phase.  I'm even willing to bet that if the flame from the SRB that killed Challenger had been spotted in time, that accident would been exactly that abort mode.

[DerekL1963]

7 minutes ago, AdmiralTigerclaw said:

Consider from a crew safety perspective, if you have to abort early in boost phase, you want to be able to separate that glider from the stack without suffering collisions. 

Shuttle doesn't perform any abort maneuvers until after the SRB's burn out.  (As a simple google search on Shuttle abort modes would have made clear.)
 

8 minutes ago, AdmiralTigerclaw said:

  Tests of the SR-71 blackbird launching the D-21 drone in the sixties showed that launching something from a piggy-back position on an aircraft tends to end in spectacular failure. 

There were four such tests, only one of which resulted in failure.  So, no.  The tests did not show that it " tends to end in spectacular failure".  A worrying concern, certainly (which is why they terminated the M-21/D-21 program), but not a certainty or a tendency.
 

12 minutes ago, AdmiralTigerclaw said:

If you want to escape from a very soon to be very much volatile rocket stack while stuck in a glorified glider, you want access to the best maneuver energy possible.  In an unpowered 'aircraft' with only altitude and speed available to it, that would be in the form of some kind of dive.  The only option for a dive would be from BELOW the stack.  In this case, you roll program and pitch onto your back as soon as you can.  That way, as soon as you blow the bolts on abort, not only does gravity pull you away from the stack, the pilot has immediate access to a Split-S maneuver, which will pull the glider down and away from the rocket, and if there's room in the maneuver, a full direction reversal from the stack itself.  The stack, with the SRBs still blazing away, continues on to whatever fate awaits.  Probably explosive.

During an early abort, the stack won't be in a dive - and neither will the Orbiter be in a dive when it separates.  (It will still have considerable upward momentum.)  Much more important than gravity is the direction of the lift vector...  And I seriously doubt the Orbiter can pull a supersonic Split-S anyhow.   The Orbiter isn't exactly a fighter...
 

32 minutes ago, AdmiralTigerclaw said:

Thus, it can be surmised that part of the Pitch-over program happening so early is in part to put the shuttle orbiter into position for a quick abort from boost phase.  I'm even willing to bet that if the flame from the SRB that killed Challenger had been spotted in time, that accident would been exactly that abort mode.

But in reality, none of that matters - because the Shuttle cannot perform a controlled departure from the stack while the stack is under thrust.  It's going to tumble and get torn apart by aerodynamic forces (as Challenger was)... 

tl;dr - The Shuttle can't safely separate while the SRB's are burning, so...  No.  Rolling heads down was not done for crew safety reasons.  It was done to control aerodynamic loads and to aim the antennas on the Orbiter towards the downrange ground stations.

On 1/22/2014 at 5:36 AM, rdfox said:

Secondly, yes, the drag model in KSP is screwed up (and so is the *rate* of change in the atmosphere's thickness), resulting in the lower atmosphere being like soup compared to Earth. This means that you do want to start gaining downrange velocity as soon as possible (look, for example, at films of Polaris or Trident submarine-launched ballistic missiles being test-fired from submarines and starting pitchover almost immediately after ignition), though not as aggressively as the Shuttle did; that profile was almost entirely for range safety.

Reading through the whole thread...

SLBM's don't pitch over aggressively and early for efficiency reasons...  They do so to provide clearance for the remaining missiles.

[Gargamel]

On 11/6/2018 at 2:29 PM, DerekL1963 said:

The Shuttle can't safely separate while the SRB's are burning, so... 

Since a good portion of this thread is about KSP vs real life, try running an abort in KSP where you decouple live SRB's during flight.  It's usually very very messy.

And another side note, I believe this thread was written when we were still using the old soup-o-sphere. The current KSP version's atmo is much better. 

[Confused Scientist]

35 minutes ago, Gargamel said:

Since a good portion of this thread is about KSP vs real life, try running an abort in KSP where you decouple live SRB's during flight.  It's usually very very messy.

And another side note, I believe this thread was written when we were still using the old soup-o-sphere. The current KSP version's atmo is much better. 

Oh, okay. I was wondering...

I recall a book by a shuttle astronaut (Mike Mullane, I think) where he said that if the SRBs were separated while they were still burning, they would most likely be held in place against the side of the ET until burnout; if the shuttle separated from the ET at the wrong time they would collide. Also, it's important to keep in mind that some aspects of the shuttle's roll program (for example, the 'upside-down' roll attitude) are intended to minimize aerodynamic forces on the wing.

Edited November 8, 2018 by Confused Scientist