Why is Rocket Reusability Important?

Right now, we are entering an exciting new era of space travel and exploration. There have been many advancements to the technology used for space exploration, including the vehicles that enable us to reach space in the first place: rockets. However, rockets are still extremely expensive. The Delta IV Heavy, manufactured by the United Launch Alliance(a launch service created as a joint venture between Lockheed Martin and Boeing), costs approximately $350 million USD per launch. NASA originally intended for their space shuttles to solve this problem and ultimately bring down the cost of space travel The shuttles were capable, but they wound up being extremely expensive as well, averaging a cost of around $450 million dollars per launch. Why is it that rockets are so expensive? Well first off, rockets are made of expensive materials and require thousands of man-hours to manufacture. Also, the more important reason is because almost every single rocket that has ever existed was expendable. That means that, after launch, and successful payload deploy, every single part of the rocket is thrown away. Imagine having to buy a new car every time you need to drive somewhere, and then promptly smashing it to pieces afterward. That’s basically what rockets do, except on a much, much larger scale.

How Existing Rockets Deal with Reusability

One rocket has managed to break the mold of rockets being one-time use vehicles. The iconic Falcon 9 rocket, developed by a private aerospace company called SpaceX, has a reusable first stage booster. To explain what this means, you must first understand that rockets have different sections called stages. They often consist of a first stage, and a second stage on top of it. The first stage is generally the more powerful stage, lifting itself and everything else out of the thick parts of the atmosphere and to space. But that’s not all; in order to deliver the payload to a proper orbit and not just fall back down again, the spent first stage separates and falls back down to the earth(usually into the ocean) and the second stage takes over. The second stage is, basically, a smaller, new and unused rocket sitting on top of the first stage. This stage is usually the more efficient stage, with an engine optimized for the vacuum of space.

After taking over, the second stage accelerates itself and the payload to at least the speed of 7,800 m/s or about 17,500 mph, in order to reach orbital speed. After deploying the payload, the second stage usually then performs a retrograde(opposite to the direction of travel) burn in order to de-orbit itself, and burns up on reentry in the atmosphere. What SpaceX does to actually reuse their first stages is that they land their first stages using propulsive landing techniques they’ve nearly perfected over the years.

After stage separation, the first stage coasts up to its apogee, or the furthest point from the earth that the stage reaches, after which it will begin falling back down. It then uses tiny thrusters called RCS(reaction control system) thrusters that can change the pitch, yaw, and roll of a spacecraft to orientate it to the angle it will be at when it re-enters the earth’s atmosphere, engines first. It also deploys a set of grid-shaped fins. When it reaches the point where the atmosphere begins to thicken, it burns three of its nine engines for a "re-entry burn" in order to protect it from atmospheric entry heating and massively reduce its velocity in the process.

The rest of the way down, it uses the constantly thickening atmosphere to its advantage and gets rid of most of the rest of its velocity, all while steering it by adjusting the grid fins. Finally, it reaches the point where it begins the final landing procedure; this is the hardest part of the whole landing, and must be timed perfectly. It activates one of the nine engines in what’s sometimes referred to as a "suicide burn" or "hoverslam". This one rocket engine has to activate at the perfect time to slow down the stage to the perfect velocity at the perfect altitude; too early and the first stage starts ascending again, too late and the stage violently collides with the surface. SpaceX has managed to be able to do this consistently without numerous failures since 2015, when they first managed to land a first stage booster successfully.

After the landing, the boosters are taken to SpaceX’s facilities to be refurbished. "Refurbish" would be the slightly more accurate term here, as SpaceX still needs to do some substantial work to get the booster ready to re-fly. Their eventual goal is to be able to land the booster, have a period of 24 hours where they merely clean it and check it, and re-fly it, ten times in a row. This is something that can be achieved with even more experience and small tweaks to the design. SpaceX has achieved something that has never been done before, which was once thought impossible to do. Their first stages are reportedly substantially cheaper to refurbish than build new. However, there is one more step in reusability that they have not taken, which is to reuse the second stage as well. This seems like the natural next step, but for now, SpaceX decided to not pursue second stage reuse in order to focus their efforts on the creation of their next-generation rocket, the starship/super heavy rocket which is designed to take humans to mars. Second stage reuse is possible and viable, but no one has ever attempted it. So why haven’t they? What are the main challenges presented by second stage reuse?

The Challenges of Second Stage Reuse

Firstly, the main challenge of second stage reuse is the amount of heat an orbital spacecraft experiences during re-entry. This is the one of the biggest challenges in creating a spacecraft that needs to re-enter the atmosphere, such as NASA’s space shuttles. Re-entry heating happens because when something is entering the atmosphere, it encounters more and more air molecules as the atmosphere thickens closer to the earth.

Orbital spacecraft have so much velocity that the air molecules accumulate at the front of the craft. The air molecules essentially start to get more and more compressed. This concept can be explained with the law of conservation of energy, which states that energy cannot be created nor destroyed, but rather transferred or converted into a different form. The accumulation of air molecules begins to take away the kinetic energy from the craft in the form of drag, reducing its velocity, but it also becomes thermal energy.

The air heats up enough to actually start glowing and also creates a layer of plasma which envelopes the spacecraft. This is enough to rip apart spacecraft into fiery shards upon re-entry. To get over the problem of re-entry heating, spacecraft that need to survive re-entry will have to employ the use of a heat shield. Heat shields are made with many different materials, and there are different types of heat shields used for re-entry. The most common of these, however, is the ablative heat shield. Ablative heat shields are specially designed to slowly burn off, in the process creating a gas which takes the heat energy away by convection. This sounds easy enough to solve if you’re designing a reusable second stage; just put a heat shield in front of the second stage, and it’ll survive re-entry. However, the fact that the ablative heat shield would often have to be replaced between flights may or may not be desirable, since the point of a fully reusable rocket is for it to be fully reusable. Ideally, in between flights you’d not want to have to swap out anything, and instead just have to clean and refuel your reusable rocket stages. There are other types of heat shielding available however, such the reusable thermal protection system on the space shuttle. Instead of using ablative heat shields, the NASA space shuttles used many different materials to provide a robust and reusable heat shield. The space shuttle used a composite reinforced carbon-carbon material as a coating on its nose, wing leading edges and other areas where the heat was extreme, thousands of high and low temperature surface insulation tiles mostly on the underside of the shuttle, and many other types of heat shielding. These systems would be able to be reused, which was the main goal of the space shuttle: to be an economical, reusable, capable launch system. However, the heat shielding on the space shuttle was easy to break, which led to the Columbia disaster, when a piece of insulation foam hit the heat shielding during launch and broke a part of the heat shield significant enough that the shuttle burned up in the atmosphere on re-entry, killing all seven crew members on board. Also, the tiles on the space shuttle had to be meticulously checked between flights, and massively contributed to the shuttle’s long refurbishment time and large cost. Thus, a shielding system similar to the space shuttle could work on the second stage of a rocket, but maybe some other heat shielding system could be more efficient and less costly. Well that was pretty simple, just put a heat shielding system on the front of your spacecraft and you’re ready to go. However that’s not the only challenge of creating a fully reusable rocket second stage.

All the dangers of re-entry could theoretically be solved with one system or the next, but the one thing that adding protection systems can’t solve is the decreasing amount of payload for each piece of hardware added. Basically, for each kilogram or pound of weight you add to the second stage in reusability hardware, you have to take away a kilogram/pound from the payload capacity. This means that maintaining a similar payload capacity as expendable rockets will be a much bigger engineering challenge. An economical near-fully reusable rocket would have to be slightly more powerful than an expendable rocket of the same size, while also being more fuel-efficient.

Another slightly smaller but still very important issue is due to basic physics. This issue due to the fact that you’d have to put a heat shield on the opposite end to the engines. The stage would need to remain with the heat shield pointed prograde(in the direction of travel) in order to prevent the risk of other sensitive parts of the rocket overheating. However, any falling object would naturally, because of basic physics, want its heaviest point facing forward in its trajectory. For an empty/nearly empty rocket second stage, this would be near the engines. Thus, a second stage trying to make it safely through the atmosphere would naturally try to flip over to its engine pointing forward, which is absolutely not ideal. This would most likely cause the entire stage to overheat and burn up in the atmosphere. To solve this, the spacecraft would have to have something that keeps this oriented through the atmosphere. The tiny RCS(or Reaction Control System) gas thrusters typically used on spacecraft would be sufficient in the vacuum of space, but not once the stage enters the atmosphere. The craft would have to employ the use of some kind of aerodynamic stabilization mechanism, an example being the titanium grid fins used on the Falcon 9. This would have to be all carefully thought out as the heat from the re-entry of an orbital velocity craft would melt many metals, running the risk of destroying the fins.

The last issue to solve is the issue of landing the stage. So your second stage has survived atmospheric heating. Great! Now you have to figure out a method of landing it. You cannot simply flip your stage around and use the vacuum optimized engine for propulsive landing like a Falcon 9 first stage. Firstly, the amount of fuel left over might not be enough, and also, vacuum-optimized engines typically can’t operate safely at sea level. The better option here would be to use the more conventional parachute system.

We might soon see someone decide to attempt second stage reusability, and be able to perform it consistently and economically. For now, SpaceX isn’t pursuing second stage reuse on the Falcon 9 in order to focus on the development of their highly ambitious next generation Starship/Superheavy fully reusable mars rocket. But who knows, maybe one day they’ll come back to it. Maybe some other space agency/aerospace company will do it first. A practical medium lift rocket capable of full reuse would be a truly revolutionary development in the space industry, and could be used to help bring down the cost of resupply missions to the international space station, for one. Reusability is really the technology that will bring space travel to the next level. For now though, we’ll just have to wait and see what exciting new designs are developed.