Why aerodynamics?

There isn’t any air in space for the most part, but air is still relevant when you’re trying to get something into space because this pesky thing we humans call the atmosphere likes to get in the way. During launch, a rocket has to deal with the crushing pressure from high speed flight, torque on its body trying to pull it off course and heating from friction and other sources melting exposed parts. These are just some of the problems that have to be dealt both in the design and flight planning stages, and each is a major problem in its own right.

Air can crush you

Although rockets do climb really fast, the high speeds that they achieve cause extreme loads on the rocket. These loads appear mainly in the form of dynamic pressure, which is a function of the density of the air and the velocity of the rocket at any given time. This factor is especially relevant for rockets that have a high thrust to weight ratio, since their higher acceleration means they suffer higher dynamic pressures.

To protect the payload from these stresses, all cargo rockets have something called fairings. Fairings are the large solid parts on the tip of the rocket that encase the payload. They protect the often delicate cargo from both dynamic pressure as well as atmospheric heating that occurs at hypersonic velocities. As they are pretty heavy and completely useless once the rocket has left the atmosphere, fairings are usually detached and allowed to fall back to Earth as soon as doing so is safe in order to conserve fuel.

The fairings of a SpaceX Falcon Heavy (with Elon Musk’s now spaceborne Tesla Roadster inside)

Even with fairings, dynamic pressure can be a serious problem if not accounted for in flight planning. During some parts of the flight, forces can reach levels that can cause the fairings to crumple from the stress, which as you can imagine is a pretty bad thing. The part of the flight where the rocket experiences the maximum amount of aerodynamic stress is called max q. Most rockets have to throttle back their engines in order to ensure that aerodynamic loads remain under critical values during this period. After this point, dynamic pressure starts tapering off very quickly due to the rapidly thinning atmosphere.

Rockets also experience aerodynamic drag while in flight, which is the force that opposes the rocket’s movement through the atmosphere. Although drag is a factor that needs to be considered in flight planning, its overall influence on the path of the rocket is relatively low. This is because rockets are by nature long, narrow and heavy objects, which makes them exceptionally aerodynamic. Additionally, fuel losses due to gravity are much greater than those due to even the highest levels of drag reached during ascent, and so following a steeper trajectory to reduce drag will actually cause more fuel usage over the course of the launch.

Flying straight isn’t easy

In addition to applying force on the rocket against the direction of travel, the air can also push against the rocket laterally, generating torque. This can either be good or bad, depending on a couple of key factors. Before diving into how they are applied, let’s briefly go over them. As you might have learnt in school, the center of mass of a body is the point where the entire mass of the body can be concentrated. It is the point about which the body would rotate under an unbalanced torque. Along the same lines, a point known as the center of pressure can be defined, which is the point at which the all of the aerodynamic forces acting on the body can will be concentrated.

The location of the center of mass of a rocket depends on the distribution of mass along its length, but is generally low down at launch since the first stage contains much more propellant than successive stages. On the other hand, the center of pressure depends on the geometry of the vehicle, which is to say that things like fairings and fins that would appear distinct on a cross-section of the rocket will be the main contributors to the location of the center of pressure. In fact, the main purpose of fins on rockets is to tune the location of the center of pressure, as the relative location of the center of mass and of pressure has severe consequences on the stability of the rocket.

To visualize this, loosely hold a pencil by its midpoint and let it hang upside down. Now, point the index finger of your other hand directly up, and keeping it in that direction, try to rotate the pencil by pushing on it near its tip. You will find that it is quite easy for you to turn the pencil even while keeping your finger pointed straight up, to the point where you can even rotate it more than 90 degrees and flip it over.

Now, try to do the same thing but by pushing near the tail of the pencil. Here you will run into a problem. It is impossible to rotate the pencil by pushing on its tail while keeping your finger pointed up, since any force you apply on this part of the pencil will just move it towards its stable position of pointing straight down. In fact, even if some other force moves the pencil off center, your finger pressing at the back will ensure that it always keeps pointing straight down.

This little experiment is representative of the actual forces that act on a rocket in flight. Gravity simulates the acceleration that would be provided by the engines, and your finger simulates the force the ambient air is applying on the rocket. The point at which your finger touched the pencil represents the center of pressure of the rocket, while the point from where you were holding it represents its center of mass. Clearly, aerodynamic forces can be both beneficial as well as harmful in allowing you to get to orbit safely and efficiently. So, where do you think you should place the center of pressure of a rocket you are designing relative to its center of mass?

If you answered behind the center of mass, then you are absolutely correct. Having the center of pressure below the center of mass ensures that the rocket is in a stable equilibrium in flight. Any deviations from the velocity vector will be cancelled out by the aerodynamic forces, and the rocket will keep flying in a straight line even if no attempt is made to control it. Conversely, having the center of pressure in front of the center of mass would mean that the rocket is in an unstable equilibrium during straight flight. Here, any deviations would be amplified by aerodynamic forces, which means that the vehicle will have to constantly exercise active attitude control to stop itself from flipping over.

It is important to note here that it will neither be possible nor necessary to actually have the center of pressure behind the center of mass of the rocket in every case and at all times. Flying along its velocity vector, the rocket is in an (although unstable) equilibrium, and so as long as its control systems can produce enough torque to counteract the force pulling it around at small angles of attack, it will be quite safe to have the center of pressure in front of the center of mass. In fact, it is often not worth it to add fins because of the added mass and drag, and so most modern rockets forgo fins for more advanced control systems that ensure that above problem never actually shows up.

Ah that’s hot

Although the atmosphere is quite prohibitive when it comes to launching spacecraft, the opposite is true for returning spacecraft to the Earth. The air provides a nice, squishy medium for the vehicle to decelerate in without requiring fuel, and it allows the use of parachutes for landing, which again greatly simplifies the process and saves on mass. This however isn’t as easy as it sounds; an orbiting spacecraft has six times more energy as compared to an equivalent mass of TNT, all of which needs to be shed away in order to land safely. And since you can only convert the energy into another form (in this case into thermal energy), the heat generated during re-entry is enough to turn a solid block of steel into molten lava several times over.

Diagram showing the separation between the skin of the spacecraft and the superheated shock front

Contrary to popular belief, heating during re-entry is not actually caused by friction. In fact, the spacecraft is travelling at such high velocities during re-entry that air molecules simply don’t have enough time to get out of the way. This causes a shock wave to form in front of the spacecraft, where air molecules are being rapidly accelerated to match the velocity of the spacecraft and being compressed to extremely high pressures, and by extension temperatures. Much of the heating that the spacecraft experiences is from this shock front, but as it isn’t in contact with the skin, only a fraction of the generated heat is transferred to the spacecraft. However, enough heat is still absorbed to raise the temperature of the skin to thousands of degrees Celsius, hot enough to melt even the most heat resistant materials available to us.

Furthermore, convective heat transfer isn’t the only way the surface of the spacecraft is heated. The temperatures in the shock front are high enough to cause air molecules to break apart and dissociate into their component atoms, creating a stream of superheated plasma that emits copious amounts of energy in the form of thermal radiation. This is for example the reason why the top of the Space Shuttle was white; that side of the spacecraft would only be receiving heat in the form of radiation from the plasma flowing past it, and so it made the most sense for it to reflect as much energy as possible. At higher re-entry velocities like those achieved on returning from the moon and beyond, this radiative transfer actually becomes more effective in transferring heat to the spacecraft, so both mechanisms need to be considered when designing a heat shield.

The spent heat shield of a manned capsule

Speaking of designing a heat shield, how does one do that? The simplest type of heat shield is the kind used on the Space Shuttle, whose thermal protection system or TPS consisted of a network of heat resistant foam tiles that have extremely low heat conductivity. These tiles could withstand temperatures of up to 1200 degrees Celsius, and more importantly their low conductivity stopped the heat from transferring  inward towards the weaker aluminium airframe below them. However, these tiles were not sufficient to defend against the temperatures generated at the leading edges of the Shuttle, which could get as hot as 1500 degrees Celsius. Here, the Shuttle used a material called reinforced carbon-carbon, which was able to hold its strength at those temperatures.

Although the Shuttle TPS was effective, due to the requirement of being reusable it was extremely heavy and thus greatly reduced the Shuttle’s payload capacity. On the other hand, spacecraft that do not have this restriction employ a technique called ablative shielding. Instead of using highly heat resistant materials, the heat shield is made out of compounds that are meant to boil off or ablate away as the spacecraft decelerates. The conversion of the material into gas takes heat from the surroundings, which contributes to keeping the heat shield cooler. In addition, the gas physically carries away the heat as it sheds from the spacecraft, and in doing so also creates an insulating layer between the skin and the shock front, further inhibiting heat transfer. All of this results in a heat shield that is relatively very light yet effective.

In conclusion, dynamic pressure, drag, aerodynamic stability and atmospheric heating are just some of the many factors that need to be considered when designing a rocket and planning a mission. Thus, aerodynamics plays quite a significant role in aerospace, even if there is no air in space.

Assignments on aerodynamics in aerospace

  1. You may have noticed that rockets carrying manned spacecraft don’t have fairings, instead opting for an entirely different arrangement for their noses. Explain the reason why fairings aren’t used and the function of the systems that are used instead of them.
  2. SpaceX are trying to land the first stage boosters of their Falcon 9 and Falcon Heavy rockets in order to reuse them. This presents some special challenges when it comes to designing a booster which is aerodynamically stable both while flying straight with a payload on its top as well as when descending engine-first through the atmosphere with no payload. State these challenges and the systems or techniques employed to overcome them.
  3. Why are the noses and leading edges of most spacecraft extremely blunt as compared to supersonic aircraft? Also explain how the reusable boosters referenced above deal with atmospheric heating when coming in to land.

Further reading