Since the first V2 rocket sailed above the Kármán line back in 1944 and right up until the modern era, the trajectory of most space-bound rockets was more or less the same: after expending their propellants they would either crash into some desolate steppe or plunge into the ocean. In either event, the rocket was disposable. The important bit up top might go on to explore the stars or send a human crew off on their mission, but the booster rocket that lifted the spacecraft out of the atmosphere was always going to be sacrificed for the cause.
With the benefit of hindsight, we now know the Shuttle wasn’t quite the spaceflight revolution that NASA had hoped for. The age of reusable rockets didn’t truly begin until 2015, when SpaceX landed the first stage of their Falcon 9. To date they’ve repeated the feat nearly 600 times, all the while increasing the reliability and speed of their operations. Today the Falcon 9 is the most prolific launch vehicle in history, and nearly every other rocket in active development is being designed to include some element of reusability.
Most recently, China demonstrated that they could recover their Long March 10B rocket by gently bringing it down into what amounts to a giant butterfly net. While it might seem a bit quaint compared to rockets that land on their tails like something out of a 1950s sci-fi movie, the idea offers considerable promise.
There and Back Again
But why did it take 70+ years before we were able to regularly refly orbital-class rockets? It’s not that there’s anything inherently complex about reusing a spent rocket. Sure, there’s a case to be made that material science improvements have made the engines robust enough for repeated use. But even if you had to rebuild the engines after each flight it would still be better than slamming the whole vehicle into the ocean. Similarly, there’s nothing particularly unique about the structure of the Falcon 9 that enables it to fly multiple times — it’s a big metal tube with tanks inside of it, just like essentially every rocket that has flown before it.
The revolutionary technology demonstrated by SpaceX in 2015 didn’t have anything to do with making their rocket go up, it was that they were able to safely bring it back without damaging or physically altering it. The Falcon 9 first stage that came back to Earth was in the same condition it was when it left the launch pad eight minutes or so earlier, albeit with empty propellant tanks and a layer of soot on the outside.
As such, most of the variability we see when comparing the reuse of past, present, and future rockets comes not from how the vehicle ascends, but how it ultimately comes to rest back down on Earth.
Splashdown is Easy, But Rough
Without question, the easiest way to recover a rocket intact is to simply slow it down before it hits the surface of the ocean using parachutes This is how all American crewed capsules, and more applicably the Space Shuttle’s Solid Rocket Boosters (SRBs), have been recovered after their flights.
But even when descending under multiple huge parachutes, splashdown isn’t exactly a gentle event. It could probably best be described as “survivable”, in that the vehicle and crew will come through the experience in one piece, but neither is likely to be terribly happy about it.
The situation of course ends up being even worse for the rocket, as its structure is going to be subjected to the brunt of the impact force. Additionally, a complex aerospace vehicle getting partially submerged in salt water is a recipe for corrosion and electrical issues, to say nothing of the thermal shock the hot engines will experience when getting dunked.
One could argue that the only reason this method of recovery worked for the Shuttle SRBs is because of their relative simplicity when compared to a liquid-fueled rocket capable of independent flight. At the risk of oversimplifying the structure of the SRB, at splashdown it was effectively a hollow tube with minimal avionics and thrust vector control (TVC) hardware that could simply be replaced before the next flight.
Still, the NASA document Solid Rocket Booster (SRB) Refurbishment Practices goes over the considerable work required to bring each booster back to flight status after coming down in the ocean. Given the challenges of refurbishing the boosters, it’s perhaps unsurprising that NASA elected to forgo their reuse on the Space Launch System despite its SRBs being largely identical to their Shuttle predecessors.
Teaching Rockets New Tricks
In the very early days, while they were still trying to reach orbit with the Falcon 1, SpaceX had actually considered a Shuttle SRB-style recovery procedure. But in the end they decided to outfit the Falcon 9 with deployable landing legs and the rest, as they say, is history.
Landing legs allow a rocket to come down on effectively any flat surface, be it a concrete pad next to the launch facility or a floating platform. But there are some fairly serious drawbacks to this approach. For one thing, the requirement for precise terminal guidance means parachutes are out of the question. The rocket needs fins, attitude thrusters, or other control surfaces to come down on the center of the pad.
It also means the rocket needs to perform a propulsive landing. That is, use its own primary engines to bring its velocity on touchdown to as close to zero as possible. This in turn requires engines that can not only restart in flight — a capability that has not traditionally been required by first stage boosters — but are able to throttle down low enough to control the rocket’s descent without simply pushing it back upwards. It’s difficult to overstate how unnatural a state of operation this is for a rocket. Indeed, it’s the antithesis of how nearly every rocket has operated since the Song Dynasty started experimenting with gunpowder in the 10th century.
Even if you can accomplish all that, the true cost of landing a rocket is in the extra mass. Although the legs will be stowed away and unused for 99.8% of the rocket’s flight time, it still has to lug all that weight uphill. If that wasn’t bad enough, there’s also the extra weight of whatever control mechanism is in place to guide the rocket’s descent trajectory as well as the propellant that needs to be kept in reserve for the landing burn.
All told, landing a rocket on legs comes with a massive payload penalty. In the case of the Falcon 9, the rocket’s maximum capacity to Low Earth Orbit (LEO) in its expendable configuration is approximately 22,800 kg (50,300 lb). But when outfitted with the hardware necessary to land, that number is reduced by nearly 25% to 17,500 kg (38,600 lb).
Dropping the Dead Weight
There was a time, not so very long ago, when critics doubted the financial viability of recovering and reusing rockets like the Falcon 9. But today, reuse has gone from theoretical to standard operating procedure. Outside of a few Old Space holdouts, it’s top of mind for every launch provider and critical for remaining competitive in a fast-moving commercial market. In November, Blue Origin even managed to land their New Glenn heavy-lift rocket on only its second flight.
So at this point the question isn’t whether or not future rockets will be reusable, but rather, what is the most efficient way to achieve that reusability?
With that in mind, it’s easy to see the appeal of China’s net recovery. While the rocket must still perform a propulsive descent — although in theory the necessary positional accuracy, and therefore the technical challenge, is somewhat reduced — it doesn’t need to have landing legs installed. This mass savings increases the vehicle’s useful payload capacity, which in turn makes it more profitable to operate. Achieving the same end goal while being easier and cheaper is an improvement in anyone’s book.
Admittedly, having the rocket come down in a huge net adds a certain amount of whimsy to the whole endeavor, but the overall logic is sound enough. It should also be said that SpaceX, for all the success they’ve had with landing their Falcon 9 on a set of deployable legs, are themselves planning on catching both the first and second stages of their next-generation Starship vehicle. Instead of a net, their goal is to pluck the rocket out of the air with a huge robotic pincer mechanism.
One is reminded of the old joke about how the Americans and Russians approached the problem of writing in space: NASA spent millions of dollars developing a pen that would work in microgravity, while their Russian counterparts simply used pencils. If China can demonstrate the ability to reuse a rocket they snagged in their net, the more elaborate methods of recovery employed by American rockets may one day look like a similarly overengineered solution.
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