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The Race for the 'Holy Grail' of Rocketry
How private companies are making space affordable
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The long-term survival of our species necessitates that we become multi-planetary. But the first step on that journey requires affordable space access; rockets that function like airliners to the heavens. We have already worked out partial rocket reuse, now the race is on for full and rapid reuse. Here, two private companies are leading the charge with radically different approaches to the same problem.
The Ultimate Challenge
As I have noted, space access has historically been limited by the extreme cost of expending expensive rockets after use. It’s akin to throwing away the 737 you took on your last vacation…obviously absent the reuse of that aircraft, ticket prices would be unaffordable for most.
But unlike aircraft that are singular vehicles, rockets typically require two stages to reach orbit. The first stage is optimized for high thrust, vaulting the craft out of the atmosphere as quickly as possible, before detaching and plunging into the ocean. The second stage is optimized for use above the atmosphere, bringing the payload to a speed of ~Mach 25, the minimum needed for a stable orbit, before it too drops off and burns up in the atmosphere.
Since the first stage detaches at a much lower altitude and velocity, reuse attempts have thus far focused on recovering this hardware. But first-stage reuse was a mystery until 2015, when SpaceX showed the world that it was possible to use a rocket’s own engines, combined with grid fins for control, to guide it to a soft landing back here on Earth.
Now, companies from China to Europe are working on similar concepts, and although these approaches are not identical, there is a clear pathway toward improving upon SpaceX’s work and establishing a truly rapidly reusable first stage. In a sense, first-stage reuse is now a (mostly) solved problem.
The second stage, however, flies much higher and faster, making reuse an immensely more challenging problem. The second stage needs to bleed a lot more energy to return itself to Earth; it needs to survive a fiery reentry. The solution here is not yet obvious, but two companies have come up with possible pathways to solving this problem.
SpaceX’s “Belly Flop”
Again, SpaceX is leading the way with its Starship rocket. The Starship's upper stage attempts to return to Earth by reentering on its side, literally doing a “belly flop” back into the atmosphere. To maintain stability, Starship will use electrically driven fins that function like the limbs of a skydiver.
To prevent the ship’s stainless steel hull from melting in the extreme heat, Starship uses some 18,000 ceramic tiles for protection, similar to the Space Shuttle. Once near the ground, Starship fires up its engines, gimbling them at a high angle to flip the rocket from its side and back to a verticle orientation. It then uses those same engines to guide the craft to a soft landing.
There are certainly a lot of unknowns here. SpaceX has demonstrated the belly flop, the flip, and landing in prototype vehicles, but they have yet to prove out the heat shield. Experience from the Space Shuttle tells us that ceramic tiles can be fragile and difficult to maintain. Further, the fins will require complex seals and actuators that need to perform perfectly on every flight. There are many failure points that could result in the loss of the vehicle.
Stoke Space’s “Aerospike”
Stoke Space is a new and relatively unknown upstart in the space industry but is emerging as one of the most innovative players in the arena. Stoke’s approach is very different from SpaceX. Instead of the second stage bellyflopping back to Earth, it will return “bottom” first, much as a capsule would.
The advantages of this approach are obvious. There is no need for fins, actuators, or to perform the “flip” maneuver; the stage is always orientated in the direction needed for landing. The downside is that craft will return to the Earth a lot faster and hotter. Stoke engineers have an ingenious way to handle the heat: the engine doubles as a heat shield.
Modern rocket engine nozzles are “regeneratively cooled.” Put very simply, before that fuel is burned in the combustion chamber, the engine pumps the cold fuel around the nozzle to absorb heat and prevent the nozzle from melting. Effectively, engineers make dual use of the fuel: for cooling and to be burned as the propellant.
Stoke takes this a step further. Unlike traditional rocket engines that are fully exposed and attached to the bottom of the fuel tanks, Stoke buries the engine hardware behind a heat shield. The chambers/nozzles are set inside the rocket around the perimeter, protecting them from the heat of reentry. Although it may look like Stoke’s design has many engines, this is actually one single engine feeding 30 separate chambers. During ascent, these chambers ignite, lifting the stage into orbit just like any other engine would. Due to their shape, the chambers collectively function as a kind of “aerospike” engine. The merits of aerospike engines are beyond the scope of this essay.
Notably, the fuel circulates under the heatshield “nozzle” first, before being routed to the chambers for combustion. This engine operates in an “expander cycle;” the heat from chamber ignition is used to warm the hydrogen fuel, causing it to expand. The high-pressure fuel is then fed into a turbine that drives the pump, pumping more fuel into the system. Expander cycle engines are very efficient because they use their own heat to sustain themselves.
Here is the interesting part: During descent, the valves to the chambers are switched off, but the fuel is still allowed to circulate through the heatshield “nozzle.” As the heatshield warms, it heats the hydrogen fuel inside, using it to drive the same turbines/pumps it did during ascent, pumping more fuel through the system for cooling.
The design is beautifully self-regulating; the hotter the heatshield becomes, the faster the turbines pump more fuel, thus providing more cooling. Once fully circulated, the fuel is vented out the center of the heat shield, providing yet additional cooling.
For landing, the chambers are switched back on, providing the thrust necessary for a soft touchdown. With Stoke’s approach, there are no fragile tiles to worry about, no separate cooling system, just one self-regulating system that performs the same function on both ascent and descent.
Same Problem, Two Approaches
So who has the better approach, SpaceX or Stoke? The answer probably depends on the scale of the vehicle. SpaceX’s approach is likely better suited to large rockets that have enough mass margin to handle the mass of the fins and actuators. Meanwhile, Stoke’s approach might be limited to smaller launch vehicles due to the scaling limitations of expander cycle engines.
Ultimately, neither solution is yet proven, and it will be some years before we know if either concept pans out. Solving upper-stage reuse is crucial to lowering the cost of space access and making humanity multi-planetary. By lowering that cost, we also open the door to more rapid innovation and human progress here on Earth.
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