If a spacecraft is carrying a human crew, or if the craft needs to be retrieved, then plunging into a fiery re-entry like a meteorite is not an option. A safer return from low Earth orbit normally starts by retro-firing rockets to slow the craft down so it begins to fall into a lower energy, lower altitude orbit. There the higher air density starts to slow the craft further. At the bottom of LEO, orbital speed is nearly 7.8 km s–1. Orbiting spacecraft could not carry enough propellant to ease down to the surface. The crew has no choice but to head bravely towards the Earth at the correct angle, using only high technology and clever physics to protect them.
Safe re-entry is a balance between two forces: drag and lift. Drag is the deceleration force; lift is the force that keeps an aeroplane in the air. Air moving relative to the craft creates pressure differences. If pressure underneath is greater than that above, lift results. The shape and orientation of the craft and the re-entry angle all affect the ratio of these two forces.
Discuss issues associated with safe re-entry into the Earth’s atmosphere and landing on the Earth’s surface.
Identify that there is an optimum angle for safe re-entry for a manned spacecraft into the Earth’s atmosphere and the consequences of failing to achieve this angle.
1 minimising the effects of deceleration (g-force) 2 managing the effects of heating
3 landing the craft safely in the right place.
The first two issues lead to the existence of a narrow range of safe re-entry angles and speeds (Figure 2.5.3). Drag is both good and bad. Drag provides the spacecraft with brakes, but it also produces the copious amounts of thermal energy that could destroy the craft. If the approach into the atmosphere is at too shallow an angle, drag will be too small, the air flow will provide too much lift and the craft will skip over the atmosphere instead of entering. If the angle is too steep, drag will be too large, producing excessive heat and deceleration g-force, which would destroy the craft and crew.
Deceleration
As at launch, astronauts’ seats during re-entry are oriented perpendicular to, and facing (‘eyeballs in’) the direction of acceleration, but this time acceleration is opposite to velocity, so they look backwards.
Traditional re-entry vehicles (such as were used in the 1960s and 1970s) were teardrop-shaped capsules with the blunt end pointing forward (Figure 2.5.4).
They allowed very little or no control once re-entry had begun and provided very little lift. Such a re-entry is called ballistic re-entry and requires larger re-entry angles. This kind of capsule subjected the astronauts to a maximum re-entry g-force of anywhere between 6 and 12. The Apollo re-entry angle was between 5.2° and 7.2°
The Space Shuttle introduced in 1981 has wings that provide lift and flight-control structures (such as elevons, a rudder/speed brake and a body flap) that allow considerable control over the descent, adjusting the vehicle’s aerodynamics to the changing density of the air, and making re-entry more gentle, with a maximum g-force of about 2–3. This degree of control also widens the safe re-entry corridor, allowing a gentle, low-g 1–2° re-entry. This is called glide re-entry.
To further decrease descent speed without excessive g-force, the Shuttle performs a series of S-shaped turns by rolling and banking, gently enhancing drag.
The Russian Soyuz capsule, in use continuously (in modified form) since the 1960s, is a more spherical variation of the traditional capsule shape but with attitude control thrusters, which provide some glide control during re-entry.
It usually yields a g-force of 4–5, but sometimes up to about 8 for a completely ballistic re-entry.
Ballistic re-entries are high acceleration but quick—between 10 and 15 minutes. A full glide re-entry is low acceleration but slow—Shuttle re-entry, for example, takes about 45 minutes. Soyuz is intermediate and takes about 30 minutes.
Heating
On re-entry, vehicles travel at well above the speed of sound. The speed of sound is sometimes called Mach 1 (after Ernst Mach (1838–1916) a physicist and philosopher who studied gas dynamics). Twice the speed of sound is called Mach 2 and so on. Supersonic means travelling faster than Mach 1. Hypersonic usually means faster than Mach 5 (oversimplifying somewhat).
Pressure builds up in front of projectiles. Sudden pressure changes normally propagate away as sound (at the speed of sound). In supersonic flight, however,
re-entry corridor insufficient d
rag
excessive drag
Figure 2.5.3 Thesafe re-entry corridor
this pressure wave is too slow to move out of the projectile’s way, so the pressure builds up to very high levels, forming a shock wave—the air equivalent of the bow wave in front of a speed boat.
The enormous mechanical energy of orbit must go somewhere. Drag converts it to thermal energy. Contrary to common sense, in hypersonic flight, a blunt projectile with more drag actually gets less hot than a more streamlined one. In the 1950s, Harvey Julian Allen proved this theoretically and explained why the sharp nose cones of intercontinental ballistic missiles were vaporising on re-entry. Hypersonic wind tunnel tests (see Figure 2.5.5) confirmed his theory.
In hypersonic flight, much of the heat generation takes place in the shock wave (the dark line wrapping around the front of both projectiles in Figure 2.5.5).
The shock wave does not touch the blunt projectile (Figure 2.5.5a) and so doesn’t transfer the heat efficiently to it, but it does touch the tip of the sharp projectile (Figure 2.5.5b), which gets much hotter. For this reason, re-entry vehicles (including the Space Shuttle) are blunt at the front, and hence the traditional teardrop shape of capsules.
The blunt front of the vehicle is also coated with a suitable heat shield with very high melting and vaporisation temperatures. It is also highly insulating to slow the rate of thermal conduction. Thermal insulating materials in most applications are almost always very porous because tiny pockets of gas are very poor thermal conductors. Some insulator materials are also designed to be highly light-absorbing (black) in the visible and near infra-red parts of the spectrum because such surfaces, when hot, also radiate thermal energy away more efficiently (radiative cooling).
Tiles on the Shuttle surface are made of 90% porous silica fibre, which is an excellent high melting point insulator, but it is brittle. The tiles on the hottest parts (the underside and leading edges) are also coated with a tough black glass to enhance radiation of thermal energy, but also to provide mechanical strength.
Broken tiles were believed to be responsible for the destruction during re-entry of the Shuttle Discovery in 2003.
In more traditional space capsule ballistic re-entry, drag is higher, so heat is generated more rapidly, and insulation and radiation alone are not enough. In these cases, the insulating heat shield is also designed to vaporise and erode (ablation). The hot, vaporised and ablated material carries thermal energy away rather than conduct it to the capsule, similar to the way in which evaporating sweat carries away excess heat from your skin. The pressure from this ablation also helps to push away the hot gas convecting from the shock wave. The shield must be thick enough to last the journey and provide sufficient insulation.
Two modern examples of ablating materials are phenolic impregnated carbon ablator (PICA) and silicone impregnated reusable ceramic ablator (SIRCA). In the Chinese space program, one of the ablation materials used is blocks of oak wood. It’s cheap and easy to work. As it chars, it forms charcoal, which is porous and almost pure carbon, making it an extraordinarily good thermal insulator with a very high melting point. Another advantage is that porous carbon is very black and radiates thermal energy efficiently. However, it is mechanically weaker than more ‘high-tech’ ablation materials.
During re-entry, superheated air surrounding the vehicle is ionised. The air becomes a plasma—a conductive soup of free positive and negative charges that, like the Earth’s ionosphere (see in2 Physics @ Preliminary pp 153–4), reflects
Figure 2.5.4 Re-entry vehicles: (a) Gemini 1964–1966 (b) Apollo 1966–1975 (c) Soyuz 1960–present (d) Space Shuttle 1981–present a
b
d c
radio waves, so the astronauts cannot communicate with the Earth for several minutes during re-entry. This problem has been solved for the Shuttle by communicating via a satellite above it, since only the bottom of the Shuttle has significant ionisation.
Landing
Drag depends on the projectile’s cross-sectional area and speed. Drag cannot stop a projectile completely because, during deceleration, drag decreases until it exactly cancels the weight of the projectile and deceleration stops—the projectile has reached terminal speed (see in2 Physics @ Preliminary p 45). The terminal speed of a capsule is too high for it to land safely. To slow the capsule further for the landing, drag is enhanced (and terminal speed decreased) by using parachutes to increase the effective area of the capsule.
The final ‘touchdown’ could be on land (typical of Russian missions) or a
‘splashdown’ in the water (typical of US missions pre-Shuttle). Russian Soyuz also has soft-landing engines that fire just before it touches the ground.
The Space Shuttle lands on a runway, much like an aeroplane (Figure 2.5.6) but it uses parachutes to help it brake. During landing, the Shuttle (which has been described as being ‘like flying a brick with wings’) is controlled entirely by computer.
Another issue is accurate targeting of the landing site. The steeper the re-entry angle, the smaller the horizontal component of motion (range) and so the more accurate the prediction of the final landing site. However, the Shuttle makes up for its shallow re-entry, because its aeroplane-like flight-control structures allow adjustment of the landing path. The shape of the landing path is also designed to be more forgiving. The Shuttle approaches the runway roughly opposite to the landing direction. Four minutes from touchdown, it does a
‘heading-alignment’ loop, to adjust precisely to the direction of the runway (Figure 2.5.6).
Figure 2.5.5 Hypersonic wind tunnel tests. (a) The crescent-shaped shock wave is detached from the blunt projectile, but (b) touches the tip of the sharp projectile.
CHECKPoInT 2.5
1 Define orbital decay and explain what causes it.
2 Because of drag, satellites at altitudes below ~1000 km can do nothing to combat orbital decay. True or False?
Explain.
3 What other astronomical body can affect the rate of orbital decay? Explain.
4 Discuss how drag is ‘good and bad’ for re-entry.
5 Outline what can happen if a spacecraft attempts re-entry with too shallow or too steep an angle.
6 Explain why astronauts face backwards during re-entry, unlike at launch.
7 Outline why occupants of the Space Shuttle experience lower g-force during re-entry than in the more traditional re-entry vehicles.
8 Define the terms supersonic and hypersonic.
9 What is a shock wave?
0 Outline why a pointy hypersonic projectile is more likely to melt than a blunt one.
1 Explain why a capsule with a parachute slows down more than without one.
1 1
Mojave
Edwards Airforce Base
Runway 23
Altitude 25 000 m
Figure 2.5.6 Scale drawing of the relatively gentle descent of the Space Shuttle. The Shuttle is drawn at 1 minute intervals to touchdown. The squares on the ground are 10 nautical miles (18.5 km) wide.
chapTER 2
This is a starting point to get you thinking about the mandatory practical experiences outlined in the syllabus. For detailed instructions and advice, use in2 Physics @ HSC Activity Manual.