How does the Space Shuttle slow down during re-entry, descent, and landing?

Question

Yesterday, my little brother asked me to help him build his new Space Shuttle Lego set.

When we finally finished building it, he started to play with his new toy and asked me to be the "Mission Control". After some time playing, he told me that he had to re-entry and land, so I gave him "permission to re-entry". Then he started to play as if he was coming back to Earth and told me that the Space Shuttle was too hot and moving too fast to land. And that got me thinking:

• How does the Space Shuttle reduce speed during the re-entry process?
• Does the Space Shuttle have flaps, spoilers and reverse thrust capabilities?
• Can the Space Shuttle make a go-around?
• Where does it land?

I know that when it is on the ground the Space Shuttle can deploy parachutes to reduce speed.

Answer 1

If you're interested in a more visual, and less technical, explanation of Space Shuttle reentry and landing, I gave a talk titled How to Land the Space Shuttle... from Space at the Stack Overflow meetup in October 2016.

I didn't notice this question until a couple of days ago, but as someone with an unhealthy obsession with specifically the entry and landing phases of shuttle flights, I can say there is a lot of factually incorrect information in the other answers here. Let me see if I can explain it better.

First, the two easy questions, which were answered well by other questions, but I'll include here as well for completeness:

• Could the shuttle perform a go-around? No. The OMS engines are too weak to make a difference in the atmosphere, and the main engines (which would be powerful enough) are only fueled by the orange external tank which is jettisoned after launch.
• Where did it land? 78 missions landed at the Kennedy Space Center, 54 (including the first) at Edwards Air Force Base, and 1 at White Sands. There were other landing sites designated for emergencies, but none were ever used.

Now, for the really big question of how the shuttle reentered and landed.

The primary source I'm going to cite in this answer is the Entry, TAEM, and Approach/Landing Guidance Workbook 21002 which was a workbook used for training astronauts. Sadly, I don't have a link to it, but it can be obtained from nasaspaceflight.com via an L2 subscription if you're really interested. I'm going to abreviate this source as ETAGW.

Orbital Mechanics

First, a real quick lesson in orbital mechanics. In order to change the altitude of your orbit you make a change in velocity ($\Delta v$). If you increase velocity, you'll increase altitude, and if you decrease velocity, you'll decrease altitude. However, this effect is most pronounced 180° from the position where you made the change. After a complete 360° orbit, you'll be at approximately the same altitude you started at.

This illustrates the effect:

Starting from the circular orbit (black), if you slow down at point A, you might end up with something like the red orbit, and if you speed up you might end up with something like the blue orbit.

Deorbit Burn

Because of the nature of orbital mechanics, as described above, you want to perform your deorbit maneuver on the opposite side of the planet from your intended landing site. This typically occurred over the Indian Ocean for a landing at Kennedy Space Center in Florida.

The burn itself was performed with the shuttle flying tail first and white (top) side facing towards the Earth (heat shield/black side facing towards space). The two OMS (orbital maneuvering system) engines were used to accomplish the required $\Delta v$ (anywhere between 200 and 550 ft/sec depending on the starting altitude). The burn typically took around 2.5 to 3 minutes. This would lower the perigee (lowest point in an orbit around Earth) to within a few miles of the ground (having a hard time finding a source, but I seem to remember it being around 30-40 miles), which was enough to ensure the orbital path would take them into thick atmosphere.

^{Image Source}

The OMS engines are essentially larger versions of RCS (reaction control system) jets. RCS jets were used for rotation (attitude) and small translation (velocity in a given direction) changes. The OMS were used for making orbital changes.

Both systems burned the same hypergolic mixture (monomethylhydrazine (MMH) and dinitrogen tetroxide (N2O4)). If the OMS had failed, the RCS jets could have, in theory, been used to slow the shuttle enough for reentry.

Nose Flip

Some answers have said that the orbiter would pitch down 140° for the flip. This is incorrect. After the deorbit maneuver was completed, the orbiter would pitch up about 220° until it reached a 40° nose-up angle of attack (referred to as the "EI -5 attitude" because they must be in that attitude by at least five minutes prior to entry interface).

They have about 20 minutes between the deorbit burn and EI-5, so there's plenty of time to pitch in either direction. Nose up was likely preferred because the APU exhaust vents point up near the tail. This naturally causes the orbiter to want to pitch up when the APUs are running.

During this pitch-up, they would fire the forward RCS jets to dump all the forward fuel, unless it was required for center-of-gravity reasons. The forward RCS jets weren't used during reentry, and dumping the fuel reduces potential hazards to the astronauts.

Once in position, open-loop entry guidance would begin, holding the orbiter at 0° roll (wings level), 0° yaw, and 40° angle of attack (alpha).

Entry Interface

NASA defines the entry-interface (EI) as an altitude of 400,000 feet. There's no hard-edge to the atmosphere, but this is around the altitude where its effects start to become directly detectable.

S-Turns

Some answers have claimed that the shuttle used S-turns for the purpose of slowing down. This is a very common, but over-simplified and arguably inaccurate, explanation.

"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." - ETAGW 2.8.1 (emphasis is from original source)

As with any airplane, the shuttle's wings generate lift. As the atmosphere gets denser, the wings are going to generate more lift, and this upward lift will cause the descent rate to slow. In fact if the shuttle maintained a wings-level attitude, it would eventually start to gain altitude causing it to "skip" across the atmosphere several times until it was slow enough to fall through. And while technically a skipping reentry would be possible, it would be very difficult to control with any precision.

So, instead, when the decent rate starts to slow, the shuttle goes into a bank. By controlling the bank angle, they're able to control how much upward lift the wings are generating, and, by extension, control their descent rate.

^{ETAGW Figure 2-5 illustrating lift vector.}

Or course, with the lift vector pointed sideways instead of upwards, the shuttle is going to start to turn. Due to the incredible speed, the turn radius is enormous, but it does gradually turn nonetheless, and the orbiter develops an azimuth error (the difference between the orbiter's current direction and the direction to the landing site).

^{ETAGW Figure 2-3 illustrating azimuth error.}

To correct this azimuth error ($\Delta z$), the orbiter performs "roll reversals". In other words, it turns in the other direction. These turns create the distinctive S-turn reentry track.

The first reversal always occurs at 10.5° $\Delta z$. Subsequent reversals occur at 17.5° until Mach 4 when it starts ramping down to 10° at Mach 3.

Note: all mach numbers given anywhere in this answer, or referenced anywhere in Space Shuttle materials, aren't true mach numbers. NASA uses 1000 ft/sec as an approximation of Mach 1, and all mach numbers are multiples of that velocity.

Ranging

Obviously, the ultimate goal of reentry is to reach your intended runway at an appropriate speed for landing. While the orbiter doesn't have any engines to help accomplish this, it does have a tremendous amount of orbital energy. Therefore the goal becomes energy management, and more specifically, drag management.

The orbiter has two ways to affect drag during entry: changing angle of attack (alpha), and changing bank angle.

Angle of attack is the quickest way to increase or decrease drag, but the orbiter was only allowed to deviate ±3° from the nominal alpha (40° for most of entry). This is the "very small alpha envelope" referred to earlier. The limitation is designed to ensure proper heat protection and maintain vehicle control.

Changing bank angle, as described above, allows you to control your descent rate. A steeper bank angle will result in reaching thicker air faster, and will therefore result in increased drag. A shallower bank angle will keep the orbiter in thinner air for longer, and result in less drag. However, it takes a little longer to see the effect of a bank change than an alpha change.

There are also bank angle limitations because, again, you don't want to skip out of the atmosphere, and you don't want to fall into thick air so fast that it exceeds the maximum drag the vehicle can handle, but it is a larger envelope than the alpha limits. In fact, in early stages of entry, the orbiter could have even flown upside down (with its lift vector pointed towards Earth) if necessary due to an under-burn as the result of a malfunction during the deorbit burn.

Entry Guidance Phases

I'm not going to go into detail about entry guidance, but I will say that the primary considerations change as speed and altitude changes, and the entry guidance is broken up into phases to reflect this.

^ETAGW

In the chart above, the middle line represents the nominal profile. The lowest line is the "equilibrium glide" profile which is the minimum amount of drag that the orbiter must maintain to avoid gaining altitude and skipping. The line at the top left represents a thermal limit (if the orbiter exceeds it, it might burn up). The line at the upper right represents a dynamic pressure limit (if the orbiter exceeds it, it might break up due to aerodynamic forces).

1. Preentry: This is considered open-loop guidance because all it does it maintain 0° yaw, 0° roll, and 40° alpha until the total load factor becomes 0.132g (approximately 3 ft/sec² of drag), at which point closed-loop guidance begins.
2. Temperature Control: Begins at closed loop guidance and ends at a velocity of Mach 19. Tries to maintain a constant temperature within the design limits of the orbiter.
3. Equilibrium Glide: Simply provides a bridge between temperature control and constant drag phases. It ends when drag reaches 33 ft/sec². It's named as such because its shape is similar to that of the equilibrium glide profile.
4. Constant Drag: Maintains a constant drag rate of 33 ft/sec² until velocity reaches Mach 10.5.
5. Transition: Designed to transition from the high drag and high alpha of entry to the lower drag and lower alpha required for the orbiter to fly more like an airplane. This phase terminates at Mach 2.5 when TAEM begins.

TAEM

TAEM stands for Terminal Area Energy Management. The objective in this stage is to get the orbiter lined up with the runway with the correct amount of energy to make its final approach. Again, I'm not going to go into a ton of detail (feel free to ask other questions if you want), but here's the gist:

If all went well in the entry stage, TAEM will begin at about 82,000 feet and 60 nautical miles from the runway (intended ground track, not straight-line distance).

Whereas Entry guidance primarily uses bank angle to manage energy, TAEM primarily uses angle of attack. Below Mach 1, the speedbrake (a split rudder) also helps with energy management.

^{Source: NASA}

Phases of TAEM:

1. S-Turn: Usually not required, but the shuttle will perform S-turns if it is too high on energy at the start of TAEM (too high, or too close to the runway).
2. Acquisition: Turns the orbiter towards a point of tangency on the heading alignment cone (HAC) and then flys wings-level until it intercepts the HAC. The tangency point is referred to as "waypoint 1" (WP1). During this phase, the orbiter slows to below Mach 1, at which point the commander takes CSS (control-stick steering), which is the closest thing the shuttle has to a "manual" mode.
3. Heading Alignment: Guides the orbiter around a virtual "cone" (see diagram below) until it is in alignment with the runway. It's not really a cone, mathematically speaking, but it's the easiest way to visualize it.
4. Prefinal: Establishes the orbiter on the outer glide slope.

^{ETAGW Figure 3-13 illustrating the HAC.}

Approach and Landing

The final guidance phase is called "Approach and Landing". It begins when the orbiter is below 10,000 feet and established on the outer glide slope (OGS), but no later than 5000 feet regardless of glide slope.

The OGS was a 20° glide slope for "light" orbiters (gross weight less than 222,000 pounds) or 18° for "heavy" orbiters (by comparison, normal airplanes use a 3° glide slope). Heavy or light depended on what was in the payload bay. The nominal aim point for the OGS was 7500 feet short of the runway threshold, but there was also a "close-in" aim point at 6500 feet which was used in the event of a strong enough headwind.

The speed brake was used to maintain 300 KEAS (knots equivalent airspeed - it's effectively the same as indicated airspeed) on the OGS until 3000 feet, at which time it calculated how much speed brake should be required for landing, and moved to that position. It would re-calculate once more at 500 feet.

At 2000 feet, the orbiter would begin a "preflare" maneuver designed to transition from the OGS to the shallow 1.5° inner glide slope (IGS). This was accomplished with a circular pull-up followed by an exponential decay onto the IGS.

^{ETAGW Figure 4-8 illustrating the preflare geometry.}

In reality, the IGS is not followed for long, and is more of a guide for making sure you cross the runway threshold on the proper trajectory and get into position to begin the final flare, which is essentially the same as a conventional landing flare in a normal aircraft, except that the shuttle lands at a much higher angle of attack (about 8°) due to its delta wings (more similar to Concorde).

Landing gear was deployed at 300 feet, and if you're interested in landing gear, I wrote a whole answer on landing gear deployment once.

The targeted touchdown point was 2500 feet down the runway at an airspeed of 195 knots for light orbiters, or 205 knots for heavy (within +5/-10 knots).

The drag chute (an addition made in the early 90's) would be deployed shortly after main gear touchdown, but not faster than 195 knots, and sometimes they would wait until nose gear touchdown if there was a crosswind. It would be jettisoned at 60 knots to ensure the chute attachment mechanism didn't hit and damage the main engines.

Control

(e.g. how was the shuttle able to make attitude changes?)

In early entry, the orbiter is still controlled like a spacecraft, using RCS jets to control attitude. As dynamic pressure (q-bar) increases, the aerodynamic surfaces begin to become active and the RCS jets shutdown as follows:

• q-bar = 0.5 pounds/ft² (psf), the elevons begin to act as trim.
• q-bar = 2 psf, elevons begin to act as active control surfaces.
• q-bar = 10 psf, RCS roll jets are disabled.
• q-bar = 40 psf, RCS pitch jets are disabled.
• Mach 10, the speedbrake opens on a pre-programmed schedule to act as pitch trim.
• Mach 5, rudder becomes active, initially acting primarily as aileron trim.
• Mach 1, RCS yaw jets are disabled.

Navigation

(e.g. how did the shuttle know where it was at?)

Navigation, in NASA terms, basically means knowing where you are (guidance answers the question "how do I get where I want to go?"). During entry, the shuttle primarily used inertial navigation units, which were aligned using a star tracker prior to the deorbit burn. As it got closer to the landing site it could also incorporate GPS and/or TACAN signals into the nav data. OV-105 (aka Space Shuttle Endeavor) had a three GPS units installed and no TACANs, whereas the other orbiters had three TACANs and one GPS unit.

On the HAC, anywhere from about 15k to 20k feet in altitude, the orbiter would pick up the microwave landing system (MLS), which acts sort of like a very high precision ILS. From this, they're able to determine their position with high precision and accuracy. Once acquired, this becomes the primary source of nav data for the remainder of the flight.

On landing videos, you can actually see exactly when they pick up MLS because the altitude tape goes from being sort of jerky, then jumps a little bit and becomes smooth. For example, watch here at about the 20k mark (altitude tape is the one on the right).

Below 5000 feet, the orbiter was also able to use a radar altimeter for altitude information.

---

That's probably more information than you were looking for, but if you have questions about things I didn't cover, or didn't go into enough detail on, feel free to ask separate questions for those.

Answer 2

The procedure for the space shuttle for re-entering the earth's atmosphere is roughly as follows:

1. The shuttle is usually flying upside- down, with the vertical tail facing the earth and nose in the direction of flight.

2. The shuttle first de-orbits by turning 180$^{\circ}$ (in the yaw axis) and firing the thrusters, thus reducing speed, a procedure called retrofiring (or de-orbit burn).

3. Then, the shuttle 'flips' ~140$^{\circ}$ (in the pitch axis), so that it enters the atmosphere at around 40$^{\circ}$.

4. At this point, the atmosphere starts to thicken and the bottom of the space shuttle heats up. Due to the very high angle of attack, the shuttle generates a lot of drag, which helps in reducing speed.

5. Then, a series of steep 'S' shaped banking turns at up to 70$^{\circ}$ of bank are performed, while still maintaining the 40$^{\circ}$ angle of attack. This is done in order to reduce the speed.

6. After the completion of the final (banking) turn, the shuttle control is taken over by the commander (it is under pilot control till now), who 'flies' the shuttle (at a negative angle of attack before leveling off) and touches down on the runway.

Source: zlutykvet.cz

The flight control system of the shuttle consists of the following components:

• Maneuvering engines

• Elevons

• Body Flaps

• Split rudder (which acted as a speed brake).

Source: quest.arc.nasa.gov

There were no reverse thrusters. The shuttle usually landed at the Kennedy Space Center in Florida or its backup landing site at Edwards Air Force Base in California; Re-entry was a one way trip. There was no go-around as the space shuttle was essentially a very high-tech albeit inefficient glider during the reentry and landing sequence.

After touchdown, the pilot chute is deployed after a second, which opens the main (drag) chute, which slows the shuttle down. After the shuttle stops, the drag chute is jettisoned.

Source: spaceshuttleguide.com

Answer 3

To initiate the landing process, the shuttle executes a deorbit burn

When it is time to return to Earth, the orbiter is rotated tail-first into the direction of travel to prepare for another firing of the orbital maneuvering system engines. This firing is called the deorbit burn. Time of ignition (TIG) is usually about an hour before landing. The burn lasts three to four minutes and slows the shuttle enough to begin its descent.
_{^{All quotes sourced from NASA}}

To reduce speed once in the atmosphere

To use up excess energy, the orbiter performs a series of four steep banks, rolling over as much as 80 degrees to one side or the other, to slow down. The series of banks gives the shuttle's track toward landing an appearance similar to an elongated letter "S."

To control the shuttle within the Earth's atmosphere

Early in reentry, the orbiter's orientation is controlled by the aft steering jets, part of the reaction control system. But during descent, the vehicle flies less like a spacecraft and more like an aircraft. Its aerosurfaces -- the wing flaps and rudder -- gradually become active as air pressure builds. As those surfaces become usable, the steering jets turn off automatically.

To make a go-around

During reentry and landing, the orbiter is not powered by engines. Instead, it flies like a high-tech glider, relying first on its steering jets and then its aerosurfaces to control the airflow around it. ^{Emphasis mine}

i.e. it's a glider, no go around available.

Where does it land: On the ground, of course!
More seriously,

• The primary landing site for most shuttle missions was the Shuttle Landing Facility at Cape Canaveral
• Edwards Air Force Base on the Rogers Dry Lake Runways was an alternate. Edwards was also the primary landing site for the first several (10 or more?) shuttle missions.

Other alternate landing sites included:

• White Sands Missile Range (the only other site to be used)
• Vandenberg Air Force Base
• Lincoln Airport/Lincoln Air National Guard Base.
• Other locations were emergency abort destinations for use before the shuttle left the Earth's atmosphere.

Answer 4

Source: http://science.howstuffworks.com/space-shuttle7.htm

When a mission is finished and the shuttle is halfway around the world from the landing site (Kennedy Space Center, Edwards Air Force Base), mission control gives the command to come home, which prompts the crew to:

1. Close the cargo bay doors. In most cases, they have been flying nose-first and upside down, so they then fire the RCS thrusters to turn the orbiter tail first.
2. Once the orbiter is tail first, the crew fires the OMS engines to slow the orbiter down and fall back to Earth; it will take about 25 minutes before the shuttle reaches the upper atmosphere.
3. During that time, the crew fires the RCS thrusters to pitch the orbiter over so that the bottom of the orbiter faces the atmosphere (about 40 degrees) and they are moving nose first again.
4. Finally, they burn leftover fuel from the forward RCS as a safety precaution because this area encounters the highest heat of re-entry.

What's happening here is that the speed with which the Shuttle is flying around the Earth, which is the speed that keeps it in orbit, is reduced by firing rockets in the opposite direction. Once the speed is reduced enough, the Shuttle starts to fall out of orbit and back to the Earth. It has not yet started to encounter a significant amount of atmosphere at step 4. above.

Because it is moving at about 17,000 mph (28,000 km/h), the orbiter hits air molecules and builds up heat from friction [sic] (approximately 3000 degrees F, or 1650 degrees C).

This is the next phase of "slowing down". At this point, keeping in mind step 4. above, the Shuttle has no fuel and no way to power itself. It is now a glider falling down from space. As the Shuttle starts to hit air molecules, compression of the air generates heat (moreso than the friction - an error in the quoted passage), which we can view as a transfer of energy from kinetic to thermal. The loss in kinetic energy is a reduction in the speed of the Shuttle, so the Shuttle is now slowing down to atmospheric speeds. It's a bit tricky because the Shuttle needs air to slow it down, but hitting too much air too fast could cause a rapid heat buildup and destroy the Shuttle. The correct angle of incidence with the atmosphere is key to controlling the rate of heating versus slowing. This is how all manned spacecraft have gone from orbital speed to atmospheric speed since the very first manned missions.

When re-entry is successful, the orbiter encounters the main air of the atmosphere and is able to fly like an airplane [sic]. The orbiter is designed from a lifting body design with swept back "delta" wings. With this design, the orbiter can generate lift with a small wing area. At this point, flight computers fly the orbiter. The orbiter makes a series of S-shaped, banking turns to slow its descent speed as it begins its final approach to the runway.

The orbiter is really a glider at this point, not an airplane. It does not have control surfaces as sophisticated as many airplanes do, so the S-turns are used to slow it down.

When the orbiter is 2,000 ft (610 m) above the ground, the commander pulls up the nose to slow the rate of descent. The pilot deploys the landing gear and the orbiter touches down. The commander brakes the orbiter and the speed brake on the vertical tail opens up. A parachute is deployed from the back to help stop the orbiter. The parachute and the speed brake on the tail increase the drag on the orbiter. The orbiter stops about midway to three-quarters of the way down the runway.

So while the Shuttle has an airbrake system in the tail, it is generally not used until after touchdown.

As noted elsewhere, there is no ability to go-around. Once the de-orbit burn is past a certain point, the re-entry and landing are fully committed. Given that fact, and the fact that the whole speed management process, line up for landing, flare, touchdown, and roll to a stop have to happen correctly the first time with not a very wide margin of error, we can be impressed at how we've never lost a Shuttle in the landing process, and only one Shuttle during re-entry.

Answer 5

To give a less scientific version of what was already stated: there is no need to significantly reduce speed before re-entry as the re-entry is the way to reduce speed. And the Shuttle won't be hot before the re-entry, but rather the re-entry heats it up.

In some more detail: Yes, the Shuttle needs to slow down before landing. But the reason for its high speed is not that its falling down from such a high altitude. Rather, it's really not that high: AFAIK a typical Shuttle orbit is about 150km above ground. If you compare that to the diameter of Earth at more than 13000km, the Shuttle orbit is actually kind of close to the ground. The reason for its high speed (about 5 miles per second) relative to the ground is that it needs that velocity to stay in orbit and not fall down. That's why the Space Shuttle's sits on such a huge rocket on launch: it is necessary not so much to lift the Shuttle high up, but rather to give it that huge speed needed to stay in orbit.

But you can't land a Shuttle at that velocity, so the Shuttle needs to slow down before landing. How do you do it? Using another rocket booster would be an idea. But that would mean carrying huge amount of rocket fuel into orbit. And that in turn would require far bigger rockets at launch to lift and speed up the mass added to the Shuttle by those "brake" rockets. Instead, the Shuttle uses another approach: It is slowed down only slightly (the "de-orbit burn" explained in other good answers) using its thrusters. This way it cannot maintain its orbit and gets closer to Earth and its atmosphere. The de-orbiting burn does not significantly reduce the Shuttle's velocity, but in the atmosphere its velocity causes significant drag. And its that drag on re-entry that at the same time slows down the Shuttle and heats up its hull.

Answer 6

1. The space shuttle slows during re-entry using its underside, flying at a very steep angle of attack.. Here are some images that illustrate what it looks like. The re-entry generates a lot of heat and the underside has special heat-resistant tiles to cope with this.
2. During re-entry the shuttle isn't really 'flying' so much using its underside to slow down. At that stage the usual flaps, spoilers and reverse thrust wouldn't do much. Re-entry refers normally only to the portion where the shuttle is coming into the atmosphere. After a while, when the speed is reduced sufficiently, the shuttle starts to fly more like a normal plane, and uses normal controls. That phase isn't normally called re-entry. When it has actually landed it has a parachute that slows it down, as well as normal wheel brakes, that help slow it down. The engines are nor working during landing, so there is no reverse thrust.
3. The shuttle has no way to make a go-around.
4. The shuttle usually lands at Kennedy Space Center, while early missions and some later ones landed at Edwards Air Force base in California. One missions landed at White Sands base in New Mexico. Wikipedia has more information

Answer 7

A quick glance at the Space Shuttle's page on Wikipedia will answer all your questions.

By the way, Space Shuttle (more specifically, the orbiter was (last flight was in 2011) no different from any other spaceship/capsule/orbital vehicle/etc.: during re-entry, aerodynamic drag was used to slow down the vehicle.

The Orbiter was a lifting body, with ailerons/elevators and a rudder that also worked as speedbrake. The Orbiter was a "glider" during re-entry, with no propulsion, so a Go-Around was out of question.