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Jerard Puckett mentioned in The Pod Bay yesterday that Shuttles did S-Curve maneuvers back and forth across their reentry trajectory to shed speed. After a bit of looking, I found some reference to this, but not much. In the Kerbal Space Program Forum this thread mentioned it:

Also, space shuttles would re-enter in a rolled attitude to steepen their trajectory (i.e. the wing's lift was directed sideways not up). The roll would result in a cross-track velocity component but roll reversal maneuvers were used to "sway" the re-entry trajectory back and forth across the target landing site.

And they were discussing the photo below, of Atlantis starting reentry, I guess taken from the ISS, which shows how the maneuver looks, seen from behind and above. Atlantis is the bright spot, engulfed in plasma, doing a turn:

Shuttle Atlantis reentry trajectory as seen from ISS

How much of their velocity did the Shuttles shed this way? Was it a critical part of the reentry process?

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    $\begingroup$ It is uncited, and doesn't really directly answer your question, but en.wikipedia.org/wiki/Space_shuttle#Re-entry_and_landing does say that "In a straight line, its 40-degree nose-up attitude would cause the descent angle to flatten-out, or even rise. The vehicle therefore performed a series of four steep S-shaped banking turns, each lasting several minutes, at up to 70 degrees of bank, while still maintaining the 40-degree angle of attack. In this way it dissipated speed sideways rather than upwards." If you get a good answer with citations, you may want to update Wikipedia. $\endgroup$
    – user
    Commented Apr 8, 2016 at 20:23
  • $\begingroup$ Yes it was very critical part of re-entry. See columbia STS-107 incident which would have survived without it. $\endgroup$
    – NikoV
    Commented Mar 25, 2020 at 19:40
  • $\begingroup$ Sharp turns at reentry speed are impossible anyway. Sharp turns at that speed would require huge forces and cause huge g loads. $\endgroup$
    – Uwe
    Commented May 20, 2020 at 17:39

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It's a bit of a misconception that the shuttle used S-turns to slow down. To quote the Entry, TAEM, and Approach/Landing Guidance Workbook by the United Space Alliance:

The next time you hear someone talk about the shuttle doing roll reversals to bleed off energy, do not listen. The shuttle does roll reversals because it has a very small alpha envelope.

What it's saying is that the ideal reentry strategy is to simply point the nose at the landing site and control how fast you're dissipating energy by controlling angle of attack (alpha). A higher alpha will result in more drag, and it's easy to make quick adjustments to get the level of drag you need.

However, in order keep the orbiter both controllable and protected by the thermal protection system, it had to be flown at a particular alpha (40° for most of entry) with no more than 3° of variation. This meant they had to use different means to control drag, and the way they did it was by controlling the lift vector.

A steeper bank angle meant that the lift vector would be more sideways than up, which would cause the shuttle to descend quicker, and, as you descend, the air gets thicker, which causes more drag. A shallower bank angle will slow your descent and keep you in thinner air for longer, which minimizes your drag.

But there's a problem with using bank... it starts to turn you off course. So the solution is to use roll reversals (aka s-turns) to keep you pointed towards the landing site.

Now, to sort of answer the literal question you asked (how much entry speed was bled off during the s-turns): most of it. The first roll would occur at nearly orbital velocity, and entry guidance ended at 2500 feet per second (Earth-relative velocity). After entry guidance ended, TAEM (Terminal Area Energy Management) guidance began, which primarily used alpha for energy management. Once the orbiter was subsonic, the speedbrake was also used for energy management.

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To expand on the answer already given, if referring specifically to the S-turns, the answer is some, but usually not that much. S-Turns were only performed in the last few minutes prior to touchdown below around 80,000 feet and 60 miles out, at speeds less than 1,700 mph. If this sounds contradictory to what is normally stated, it’s all about definitions.

First of all the banking turns that the Space Shuttle orbiter made during the high-speed phases of reentry were not referred to by NASA as S-turns. They were known as roll reversals. These back-and-forth turns were done on every mission. But they did not directly dissipate energy, as was explained in the other answer. S-turns on the other hand were performed much closer to landing when needed (i.e. not always). To the extent that they were used the S-turns did directly dissipate energy.

Speed can only ultimately and permanently be eliminated by converting the speed into heat. In the case of the Shuttle this conversion of kinetic and potential energy into thermal energy was done via drag, friction, radiation, and shock wave heating. This was accomplished directly by facing the blunt (bottom) end of the orbiter into the airstream at an extremely high pitch angle during the highest speed portion of reentry known as the entry phase, which began at entry interface with the atmosphere about thirty minutes after the deorbit burn.

For continuity I will repeat some of the important points made in the other answer. During the high pitch angle entry phase, the amount of drag could be directly controlled by adjusting the pitch, but only within very small margins. The roll reversal turns did not directly increase drag, all they essentially did was direct the lift in a different direction. However without any change in pitch this also increased the descent rate. A faster descent rate put the Shuttle into denser atmosphere quicker, which increased the energy dissipation. To increase the descent rate and thus the drag the turn angle would be made steeper. For less drag the turn angle would be made shallower. Thus indirectly the roll reversal turns were a way to control energy dissipation during the entry phase.

Turning in and of itself can also create some drag, but not as much as was caused by descending faster into the thicker atmosphere. To quote from the United Space Alliance Shuttle Crew Operations Manual page 2.13-61 regarding the entry phase,

“Drag acceleration can be adjusted by modifying the angle of attack, which changes the vehicle's cross sectional area with respect to the airstream, or by adjusting the vehicle bank angle, which affects lift and thus the vehicle's sink rate into denser atmosphere.”

Entry interface began at 400,000 feet (75 miles, 122 km) and about 17,000 mph (27,000 km/h). About five minutes after entry interface the aerodynamic surfaces began functioning and the first banking maneuver began. Technically the first bank was not considered a roll reversal turn, it was referred to as an energy management roll.

Actually because of the high 40 degree pitch angle during the entry phase, what looked like a roll was as much of a yaw maneuver because the orbiter was yawing around the velocity vector. In the early entry phase the roll was performed by the thrusters, then as the dynamic pressure built up with lower altitude the elevons began to make a contribution to the roll maneuvers. Interestingly in the extremely thin atmosphere the control surfaces didn’t work like they did at lower altitudes. Lowering an elevon for example did not create lift, instead it created drag on one side which caused the orbiter to turn in that direction (Why were the Space Shuttle's elevons reversed, early in re-entry?)

As mentioned the bank angle would be adjusted as needed to change the descent rate and thus drag. At some point this would take the orbiter too far off course and thus a turn was made in the opposite direction, i.e. the “roll reversal”. Each reentry had a different range and cross-range situation, and so for a particular reentry the roll reversal turns did not always start at the same speed and altitude. The first roll reversal normally took place about fourteen minutes after entry interface, approximately nine minutes after the first energy management roll began. Each mission varied greatly as to what speed and altitude the first roll reversal began, ranging from 4,000 mph to 15,000 mph (Shuttle Crew Operations Manual page 9.3-4), with an average of around 10,000 mph at 200,000 feet altitude (40 miles, 60 km). This was approximately eighteen minutes prior to landing. It took about thirty seconds to roll the orbiter to the opposite banking attitude. As a side note Columbia began its first roll reversal twelve minutes after entry interface, about four minutes prior to the breakup.

It was quite a complex set of parameters for the 1970’s era computers to manage, as pitch and bank angle alone was used to control range, cross-range, speed, altitude, drag, descent rate, temperature, aerodynamic pressure, and wing loading. All of which had to stay within strict guidelines during a constantly changing and very dynamic flight regime.

FAA diagram
(Source: FAA, 4.1.7 Returning from Space: Re-entry page 313)

The roll reversals continued for a total of four turns, then the orbiter leveled off and began to gradually lower its nose as it prepared to transition from the entry phase to the Terminal Area Entry Management (TAEM) phase at around 80,000 feet and 1,700 mph. During the TAEM phase (pronounced tame) and until the Shuttle reached the HAC (heading alignment cone, often called heading alignment circle), S-turns were used as needed to dissipate energy, i.e. reduce speed.

Diagram HAC
WP = way point 1, NEP = nominal entry point
(source: NASA, United Space Alliance Shuttle Crew Operations Manual page 7.4-2)

As mentioned in the other answer the split-rudder speed brakes could be used for energy management once the orbiter became subsonic. Pitch could also be used to control the descent rate.

The final approach and landing phase began when the orbiter was lined up with the runway.

Diagram Orbiter Entry and Landing (Source: NASA, Space Transportation System Stack Assembly page 5

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I believe the S-Turns are primarily used to reduce altitude, not to burn off speed. I've got a little over 10 years experience flying gliders and have performed this basic maneuver many times.

The s-turns are used to manage the entrance into landing approach glideslope Way Point 1 at the correct altitude and velocity. This means managing total energy, both kinetic (speed) and POTENTIAL (altitude).

In the 2 excellent graphics shown by @StevePemberton it can be seen that Phase 2A (S-turn maneuver) is a somewhat loosey-goosey phase used for gross altitude adjustment so that the shuttle can hit Way Point 1 within the correct altitude/velocity window. If a lot of altitude must be shed the turns can be sharp and dirty and traverse widely (in terms of shuttle abilities). If little altitude adjustment is needed the turns can be few and mild. After WP-1 glideslope altitude loss is pretty much baked in (fixed within a very small range to compensate). If after WP-1 you found you needed to loose excess altitude beyond the ability of the shuttle to manage with flaps, drag etc. you'd either have to turn that excess altitude into excess speed and risk running off the end of the landing strip or you'd have to depart from the predefined and well understood flight path to get more distance from the runway. There is not much wiggle-room after WP-1.

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I herewith refer to the textbook Astronautics 3rd edition, Section 10.2 on Space Shuttle reentry.

A Shuttle reentry was a so-called lifting reentry (a.k.a. equilibrium glide), where the flight path angle needed to be kept strictly to -1.2° by its body flap. In addition, the Shuttle had to reduce drag early on by setting its angle-of-attack (AOA) to about 40°. However, this alone would counteract equilibrium glide by too much vertical lift. Therefore, the Shuttle (lift vector) was turned sideways (so-called banking) by about 80° still maintaining an AOA = 40°. This had the second advantadge of so-called cross-range capability, meaning the capability to steer sideways towards the landing strip. The resulting side-way motion is seen in the Atlantis reentry picture. If no or less cross-range was needed, the Shuttle performed so-called roll-reversals, meaning alternately banking to left and right.

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  • $\begingroup$ "where the flight path angle needed to be kept strictly to -1° by its body flap." If the textbook says that, it's wrong. $\endgroup$ Commented Aug 1, 2023 at 12:34
  • $\begingroup$ You might want to take a look at this Space Shuttle reentry data, column 6: de.scribd.com/doc/245906236/AeroDynamics taken from JSC/DM5 Flight Design and Dynamics Division, Descent Post-flight summaries. My flight was Miss Seq. No 55. $\endgroup$
    – astroflash
    Commented Aug 2, 2023 at 14:12
  • $\begingroup$ You might want to take a look at what the body flap actually was for. $\endgroup$ Commented Aug 2, 2023 at 14:37

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