Not so very long ago, orbital rockets simply didn’t get reused. After their propellants were expended on the journey to orbit, they petered out and fell back down into the ocean where they were obliterated on impact. Rockets were disposable because, as far as anyone could tell, building another one was cheaper and easier than trying to reuse them. The Space Shuttle had proved that reuse of a spacecraft and its booster was possible, but the promised benefits of reduced cost and higher launch cadence never materialized. If anything, the Space Shuttle was often considered proof that reusability made more sense on paper than it did in the real-world.
But that was before SpaceX started routinely landing and reflying the first stage of their Falcon 9 booster. Nobody outside the company really knows how much money is being saved by reuse, but there’s no denying the turn-around time from landing to reflight is getting progressively shorter. Moreover, by performing up to three flights on the same booster, SpaceX is demonstrating a launch cadence that is simply unmatched in the industry.
So it should come as no surprise to find that other launch providers are feeling the pressure to develop their own reusability programs. The latest to announce their intent to recover and eventually refly their vehicle is Rocket Lab, despite CEO Peter Beck’s admission that he was originally against the idea. He’s certainly changed his tune. With data collected over the last several flights the company now believes they have a reusability plan that’s compatible with the unique limitations of their diminutive Electron launch vehicle.
According to Beck, the goal isn’t necessarily to save money. During his presentation at the Small Satellite Conference in Utah, he explained that what they’re really going after is an increase in flight frequency. Right now they can build and fly an Electron every month, and while they eventually hope to produce a rocket a week, even a single reuse per core would have a huge impact on their annual launch capability:
If we can get these systems up on orbit quickly and reliably and frequently, we can innovate a lot more and create a lot more opportunities. So launch frequency is really the main driver for why Electron is going reusable. In time, hopefully we can obviously reduce prices as well. But the fundamental reason we’re doing this is launch frequency. Even if I can get the stage back once, I’ve effectively doubled my production ratio.
But, there’s a catch. Electron is too small to support the addition of landing legs and doesn’t have the excess propellants to use its engines during descent. Put simply, the tiny rocket is incapable of landing itself. So Rocket Lab believes the only way to recover the Electron is by snatching it out of the air before it gets to the ground.
Lightweight Reusability
Reusability on SpaceX’s Falcon 9 comes at a considerable price. Between the additional hardware required and the propellants that need to be kept in reserve for the reentry and landing burns, the payload capacity of the rocket is reduced by as much as 40%. To accommodate this without compromising on the vehicle’s useful payload capacity, SpaceX has gradually been enlarging and upgrading the design. When it first flew in 2010, the Falcon 9 was 47.8 m (157 ft) tall and had a liftoff mass of 333,400 kg (735,000 lb); the version flying today has been stretched to 70 m (230 ft) in length, and tips the scales at 549,054 kg (1,210,457 lb).
For SpaceX, who have their eyes on the medium and heavy lift markets, this kind of vehicle expansion aligns well enough with their goals. But Rocket Lab isn’t looking to compete with vehicles of that scale. With a length of just 17 m (56 ft) and a maximum payload capacity of 225 kg (496 lb), their Electron rocket is of an entirely different class. The company is laser focused on providing bespoke launch capabilities for the so-called “smallsat” market. These smaller satellites would be considered the second or even third priority if they were launched on a larger rocket, but on Electron, they’re the primary mission.
Unfortunately, the reality of operating such a small rocket is that there’s precious little wiggle room for vehicle modifications. Every ounce of additional hardware they add to the rocket will reduce their already minuscule payload capacity. Even if the Electron could spare the propellants to perform a propulsive landing burn, the mass penalty for deployable landing legs would be unacceptable. If Rocket Lab can squeeze a bit more thrust out of their 3D printed Rutherford engines they’ll have some breathing room, but not much. Any modifications to the Electron for recovery purposes will therefore need to be exceptionally minimal.
This early in the program, Rocket Lab is reluctant to say what those modifications would entail. We can tell from the rendered video they’ve posted to their YouTube channel that the first phase will use a ballute decelerator to bring the Electron down to subsonic speeds, at which point a parafoil will be deployed to further slow the rocket. This is not entirely unlike SpaceX’s now defunct plans for recovering the second stage of the Falcon 9. But what we don’t see is what kind of thermal shielding will be required for Electron to survive the intense heat of reentry, or what method of stabilization and guidance it will use on its way back down; likely because Rocket Lab themselves don’t quite know yet.
Snagging a Falling Rocket
Even assuming Rocket Lab upgrades the Electron to the point it can survive reentry and deploy its deceleration devices, they still need to figure out a way to get it down on the ground in one piece. As shown in the rendered video, the plan right now is to snatch the Electron out of the air by flying a helicopter over the descending rocket and grappling it. This might sound like something out of a James Bond movie, but in reality it’s arguably the easiest aspect of Rocket Lab’s entire scheme.
In fact, it would appear that this part of the plan has already been completed. In 2017, Lockheed Martin (a strategic investor in Rocket Lab since 2015) partnered with a PDG Aviation Services to conduct a little-publicized mid-air helicopter recovery of a mock rocket stage, and the similarities are hard to ignore. The device used to snatch the leader cable behind the rocket’s parafoil shown in Rocket Lab’s rendering appears to be identical to the one seen in video of the Lockheed Martin test, which really makes the whole thing look like a low-key dress rehearsal.
One element of the recovery process that’s unclear after comparing both videos is how the Electron will safely be lowered to the ground. During the Lockheed Martin test the “rocket” was dropped rather unceremoniously on its faux engines, but that obviously won’t work with actual flight hardware. The fact that the official Rocket Lab video fades out before the Electron is actually brought back down to the waiting ship may mean the company still hasn’t sorted that part out yet. Though if you have the technical wherewithal to launch and recover an orbital rocket, figuring out how to softly lower it onto the deck of a ship shouldn’t pose much of a challenge.
Faster and Cheaper
The fundamental goal of any reusable launch system is for subsequent reflights to be more economical and less time consuming than simply building a new booster for each mission, and in such a demanding field, even relatively minor gains are generally considered a success. But ideally the less time and money spent between launches, the better. To that end, the unique nature of the Electron may make it particularly well suited for so-called “rapid reuse”: a concept wherein a rocket is reflown nearly as often as a commercial aircraft, needing only to be inspected and refueled before being sent on its next mission.
The Electron does away with the complex gas generator and turbine arrangement traditionally used in liquid rockets, and instead uses electrically-driven pumps for the fuel and oxidizer. Combined with its relatively simplistic 3D printed engines and how small the vehicle is, there’s simply not as much that needs to be checked out between flights. A smaller team can go over every inch of the Electron in a fraction of the time it would take a much larger crew to thoroughly examine the Falcon 9, for example.
SpaceX has so far demonstrated a turnaround time of a little over two months from landing to reflight, but Rocket Lab may be poised to reduce that time to a few weeks. Considering both companies have stated an ultimate goal of reflight within days of recovery, they’ve got their work cut out for them. But even if they never quite hit that lofty goal, it seems clear the days of crashing used rockets into the ocean are coming to a close.
Grabbing a falling thing coming back to earth isn’t a conceptually new thing. We did it in the 1960s with the film return capsules from the Corona spy satellite . Granted, these were probably smaller than the Electron rocket, but this seems a pretty solvable engineering problem.
These were always described as “film capsules” which conjures an image not much fancier than a film canister, but thinking of them as “space capsules”, with film inside, changed the way I imagined them. Not only do they need to survive reentry from orbit, they first need to orient and deorbit themselves!
And then on the way down, their location needs to be known precisely enough for the recovery plane to get a visual on the parachute. All with 1960s technology, pre-GPS and everything.
In the end, they weren’t all that hi-tech, and quite a lot of them simply got lost or got destroyed. The capsules had a salt plug that would melt if they landed at sea, so they’d go to the bottom, but some of the capsules hit land and at least one was found by Venezuelan farmers.
After that, they started adding “REWARD ON RETURN TO THE USA” texts in multiple languages on the capsules, so people would turn them in.
https://en.wikipedia.org/wiki/Corona_%28satellite%29
Quite a lot of them simply failed on launch, and the capsules were dropped all over the place with a poor recovery rate.
NASA seriously considered doing this with Saturn V first stages in the ’70s, using a helicopter custom-made by Hughes that would’ve dwarfed any helicopter ever built — think “too big to land in a football stadium (of either kind)” huge.
With a diameter of only 1.2m, it would seem pretty feasible to secure it horizontally under the helicopter with a second cable and winches so it could just be landed on the deck and lowered onto dollies–except 17m is an awfully long object to keep nice and level.
I think the most likely option is a big net to lay the rocket in. Simple and effective.
Though helicopters are in common use as cranes, so a net may actually be overkill.
A few years ago I would have said this was nothing but a PR stunt, but anymore it seems like anything is possible. Excited that we’re finally starting to see companies take the risks necessary to get us to the next level.
If memory serves it was just the film package and camera that re-entered, parachuted, then snatched from the air by a C130. It was very successful. Was also used with harness to grab a human off the ground. Skyhook.
This is a whole rocket body. No chopper is large and maneuverable enough to handle it plus there is the problem of main rotor diameter. The G forces of an airplane retrieval seem prohibitive. Would have to let it settle into the water and then retrieve it with a ship and handle salt water issues.
Empty mass is claimed to be 0.95 metric tons, which is *well* within the limits for even a light utility helicopter:
https://helicopterexpress.com/blog/how-much-weight-can-a-construction-helicopter-lift
Hiller had a plan to catch the Saturn V with a rocket tip helicopter. https://broadbrained.com/space-robotics/going-big-catching-a-saturn-v-first-stage-with-a-helicopter/ The museum has a nice models and it would have had a rotor speed of something like 10 rpm.
“The rotors would not have to turn very fast by helicopter standards, only about once per second.”
So 60 RPM.
Still nuts. Thanks for the link!
The rocket is 12,500 kg, it’s the only number I have I don’t know if that includes fuel. A parafoil for that size is going to be quite large and heavy. Assuming you aren’t looking for a rate of decent suitable for touching down, I think you’d need around 120 kg for the parafoil and rigging. 3x that if you intend to not slam into the ground or need it slow enough for a helicopter to catch it. I’m basing this on some numbers for humvee parachuts (which are for a ~7000 kg vehicle), no idea if I’m correct but I think I’m in the ballpark.
Point is, if you give up 100 kg or more (500 kg?) to parachutes you have less space for payload and fuel. A re-usable rocket tends to be heavier and require more fuel, in addition to the inefficiency that recovery entails. If your payload doesn’t need to return to Earth, then a single use rocket can be as thin as a soda can.
That figure is the launch mass, so it includes payload, propellants, and the second and third stages. The dry mass of the Electron’s first stage (remember they won’t be recovering anything beyond that) is only around 900 kg.
“figuring out how to softly lower it onto the deck of a ship shouldn’t pose much of a challenge.”
why would it even occur to you think it would? helicopters land suspended payloads slow and careful all the time.
“During the Lockheed Martin test the “rocket” was dropped rather unceremoniously on its faux engines”
And because everything in space travel is a challenge.
Also, an empty rocket is about as sturdy as an empty soda can, some rockets couldn’t even hold their own weight when unpressurized.
Make the engines and fuel pumps quick and easy to remove and replace, then ones that have been checked over and repaired as needed can be swapped into the rocket. With more engines and pumps than rockets, a steady stream of ready to fly parts can be available to fit to the re-used rocket bodies.
It’d be like swapping the engine with a rebuilt one in a small plane after every flight, but better that than having an un-noticed problem causing the launch to fail.
The rocket body itself is a part that needs to be replaced, because aircraft aluminum develops stress cracks under the vibrations and forces of a rocket launch. The structure is optimized for minimum weight which means it’s almost to the point of breaking up on the first go. Aluminum structures simply can’t be engineered to handle “infinite” stress cycles – they will crack eventually – and with a highly optimized structure it’s simply a suicide pact to try and fly it again.
In airplanes, the frame is inspected periodically to see how far the cracks have advanced, and when they are large enough, they replace the whole section of the frame, or the entire plane is sent off to the boneyard. With rockets, the amount of cycling stress an airplane sees in a decade is exhausted in a couple minutes of full thrust, so you’d have to be pretty good at engineering to predict whether the microscopic cracks from the previous flight won’t grow large enough to become macroscopic explosions in the next.
This is also the reason why SpaceX is trying to build the newest rocket out of stainless steel – steel is more ductile and can handle infinite stress cycles depending on the stress amplitude and load condition, and it can be healed by local heat treatment, so it’s theoretically possible to build a lightweight rocket frame that can be used more than once.
https://en.wikipedia.org/wiki/Fatigue_limit
The difficulty of predicting a fatigue failure in metals is that it’s very time consuming to subject a piece of metal to a bending test that repeats one trillion times to see if it actually does break eventually, and the results may or may not apply to the real article that you’re building.
All the mathematical models for fatigue failure are based on empirical observations, because there’s no universal theory for what causes fatigue failure in various materials, and the growth of cracks is highly dependent on external factors such as whether your weld seams have trapped hydrogen in them, or whether you’re in a corrosive environment, hot, cold… and the fact that predicting the stress cycles themselves is almost impossible – so designing and optimizing structures against the fatigue life becomes a matter of how well you want to sleep at night. It’s always a bit of a gamble.
That was a problem with the Shuttle at first the as the block I SSME was more rebuildable vs reusable and had to be removed and inspected every flight until the block II version came along which needed less disassembly during inspection and only needed removed ever five flights.
You can see some of the mindset of making inspection easier in Spacex’s Raptor in the integrated power head which makes it possible to inspect everything exposed to hot O2 just by removing the LOX turbo pump.
Catching the thing will be easy. Making it survive re-entry will be a challenge. Those carbon fiber tube-bodies don’t sound all that re-entry-heat resistant. Then there’s the “plasma-knife” at the fore-front of the shockwave as it re-enters. There might not be much left to catch, but the proof in the pudding is in the eating and we will see. :-)
It’s doable SNC is making a carbon fiber orbital vehicle and the Shuttle OMS pods were composite.
The heat seen on a sub orbital reentry is probably less than even what the OMS pods saw.
Surprising how long these “Skyhook,” mid-air retrieval systems have been around.
https://en.wikipedia.org/wiki/Mid-air_retrieval
From the ground: https://en.wikipedia.org/wiki/Fulton_surface-to-air_recovery_system
Air to Air which might be feasible also: https://en.wikipedia.org/wiki/Parasite_aircraft
I read ticket fuel is cheap. So if the rocket is reusable, and fuel is cheap, why not add weight and landing gear?
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