With the upcoming attempts to simulate a landing over water with the Falcon IX first stage comes a possible opportunity to expedite actual re-usability.Â It just might be possible to grab a stage that has reached zero velocity over the water with a helicopter and return it to a barge or dry land for inspection and possible early reuse.
My understanding is that over the next several Falcon IX flights SpaceX is going to try to reenter the first stage and bring it to at least a momentary hover above the Atlantic before the stage goes swimming. Saltwater immersion is unlikely to result in a re-flyable stage even if it enters the water at near zero velocity. The recently fired hot nozzle will probably object among other things less dramatic.
Landing on a barge has been suggested several times and is even the target of a patent attempt by Blue Origin. In this early case, a barge may not be the best choice. The barge would either have to be exactly where the stage reaches the surface or the stage would need enough propellant to ensure reaching a possibly out of position vessel.Â The landing gear would have to be fully developed for the first landing (barging?) attempt, which is a mass and complexity issue. The tall and skinny visuals of the Grasshopper would concern me for a barge landing in anything other than a perfect sea state.
Starting with attempt one at reaching near zero velocity water relative with a Falcon IX, it might just be possible to rendezvous with a helicopter before the stage goes swimming. The factor of ten or so speed difference between a chopper and a barge might make the difference between possible and not for the vehicle meeting at the precise time of hover. The first rendezvous attempt might be no more than a tag-you’re-it with a fast maneuverable bird with a light line with fail-safe breaking strength. A momentary hook up with controlled line tension before the connection is released or breaks. It may not be possible to perform the rendezvous in this manner which would make a full blown recovery attempt questionable until a few answers are obtained.
Even before attempting a full stage rendezvous, it would make sense to try this with either the Grasshopper or a vehicle supplied by others like Armadillo or Masten. An over land demonstration with known vehicles would seem to be a low risk method of training pilots, rocket controls, and developing effective rendezvous techniques. The first trip several hundred miles off shore with few witnesses should not be the first try at a recovery.
If all the preliminaries point to a useful possibility of recovery, then it might make sense to go get a real one. I have no idea of the possible attach points on a Falcon IX first stage in simulated landing configuration. There might be something in the inter stage area that would be just fine for a simple hook from the chopper to snag, or there may not. If there is not, it might make sense to hang a cargo net under the chopper and simply net the stage. It seems possible that loads could be distributed in such a way as to eliminate damaging stresses on the stage so that it could be reused without rebuilding.
The hooked or netted stage could be gently lowered into a soft cradle specifically designed for the stage on a barge or land within range of the helicopter. With air refueling it might be possible to return to launch site for fast turnaround after some experience is gained.
It might just be possible to reuse the Falcon IX first stage earlier than commonly expected without even the mass and cost of landing gear. This would certainly be performance enhancing compared to a full return to launch site vehicle requirement.
These ideas are not all mine or all new. I just thought that it was a good time to mention them again.
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What about the rocket trailing a line attached to either a chute, balute, or balloon and then the helicopter snagging that line?
That would make it much easier to snag. I am under the possibly mistaken impression that they are not going to be using chutes on these hover tests. I was trying to visualize a method with no rocket mods at all, which seems a bit silly now considering the goal here.
There are two limiting factors here. One is the very tight time window for making the grab of a hovering rocket stage with a helicopter. This is not a trivial maneuver and it seems likely that a first-pass miss, or even just a slightly tardy arrival, would result in loss of the stage. There would be insufficient extra fuel on the F9 stage to allow extended hover in support of second tries unless one was willing to give up most or all of the claimed benefits of the scheme. If you carry enough extra fuel to hover for an extended period, you might as well just use it to do the much less risky and complex return to launch site recovery.
The other limiting factor here is the helicopter you would need to implement this scheme. The empty weight of an F9 v1.1 first stage is, presumably, more than the 15 tons the v1.0 1st stage is said to weigh, but I can find no hard numbers on just how much more. The heaviest lift helicopter extant is the Russian Mi-26 with a lift capacity of 20 tonnes (44,000 lbs.). No other helicopter can handle more than 20,000 lbs. at a whack. Even the monster Mi-26 might be marginal for the job unless the F9 propellant tanks are run dry in hover. There is at least one Mi-26 in civilian livery operating in North America so something might, in principle, be arranged. To recover all three F9 cores from an FH launch, though, would need three Mi-26’s and I don’t know that this is feasible. There is also the problem that F9’s will shortly be launching from two places on opposite sides of the continent and from a third place (Brownsville, TX, most likely) in two or three years. FH’s would launch from two of these places (Vandenberg & Brownsville). Ferrying Mi-26’s frequently over long distances is not practical or economical. Nor is maintaining a fleet of at least seven Mi-26’s in three locations (one at Canaveral and three each at Vandenberg and Brownsville), plus at least one spare in each place to allow for malfunctions on launch days. That’s 10 Mi-26’s and three sets of ground support facilities, plus maintenance and flight crews. That’s a lot of extra overhead; way more than is justified by the marginal extra lift capacity obtained from a less strenuous recovery trajectory and no landing legs. I think SpaceX’s announced approach is the correct one if the alternative is mid-air helicopter recovery. I’ll admit it’s a neat idea, but I don’t think the business case closes.
If the F9 booster’s descent were slowed by a parachute, wouldn’t that allow a helicopter multiple chances to snag it?
I’m under the impression that launch from Texas is in part to facilitate a feet dry touchdown of the first stage in Florida.
My understanding is that at least the first two test launches of the F9 included 1st stage recovery parachute systems built by Irvin, the same contractor that did the recovery chutes for Apollo. They didn’t work because the stages tumbled and broke up from aerodynamic stresses before the parachutes could deploy. The F9 v1.1 1st stage has an active control system based, I believe, on cold gas thrusters to orient and control the attitude of the stage post-MECO. It’s also supposed to carry deployable landing legs at some point, though I don’t know if they will be installed on early serial numbers of the uprated F9. It is my understanding that a good chunk of what would otherwise be the weight penalty for these additional subsystems is made up for by deleting the mass of the erstwhile recovery parachute system.
Even assuming full positive aerodynamic control of a descending F9 1st stage and a resurrected recovery parachute system, helicopter recovery is still problematical. Hooking onto something that is hovering is tricky enough. Hooking onto something that is falling is much tougher. Also riskier. The rotor downdraft of an Mi-26 might well collapse a recovery parachute’s canopy, especially as the force would be asymmetrically applied as the helicopter approached from any given side. Matching the helicopter’s sink rate to that of the parachuting stage would also be extraordinarily difficult. The helicopter could easily spill the chute’s air by running into it from above. Worse, it could tangle itself in the chute and shroud lines and crash both itself and the stage.
I hope someone with actual helicopter piloting experience joins this discussion thread. I think the idea of mid-air helicopter recovery is at very best, unacceptably risky and expensive. I think the idea of mid-air recovery of a parachuting stage is (a) impossible, and (b) suicidal. But I’d like to hear from a veteran helo pilot on this one.
There are dozens of concepts for recovering payloads in the air including some that even I consider exotic. I would be surprised if SpaceX hasn’t looked several of them over to some degree. If a recovered stage is worth ten million or more, then I would expect a certain degree of enterprise in getting it home if there is any reasonable chance of success.
Helicopter is simplest and easy to explain. The various other aircraft recovery techniques would be considerably more controversial. Consider the one with the parachute on hundreds of feet of cable above the falling stage. An airplane with a grabber flies between them and snags the cable towing the whole assembly back to base. Now there you have something that is difficult to prove convincingly.
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What is old is new again. Below is link to a proposal by Hiller Aircraft in the 1960’s to recover Saturn V boosters using a massive helicopter.
If the stage can be guided to a pad then a barge should be a reasonable target…why not fit the barge with a tower and clamps that grab the stage? Save weight on the stage as no need for legs, if you have a catastrophic failure the barge is far enough away from stuff to preclude collateral damage. A barge can be maneuverable and adjust to grab the stage as well. Way cheaper than a helo and no need to rely on FAA certs.
Forget helicopters. Match velocities with a plane in dive. The vehicle does not have to ever hover in that case. Hovering is for the purpose of testing landing without a capture vehicle.
I’ll suggest adding a set of hinged covers for the engine area and just let it hover, shutdown the engines, slam the doors closed, and go bobbing in the ocean. A single rear airbag would be ideal but I can’t see how it would manage to seal the entire engine area.
Another option would be to seal the outside of the engines against seawater and maintain a large positive pressure on the inside so water won’t get up into the combustion chamber. If that idea is feasible, it might be simple enough to just extend a skirt around the base of the rocket and fill it with compressed air, as long as the center of mass of the stage stays far enough below the center of buoyancy that it doesn’t become tippy.
I think the helicopter idea is a non-starter, and especially so when they actually have a self-recovery mechanism coming to fruition, for which this is merely the next experiment.
However, I am posting because no one seems to have noted the aerial recovery of returning space objects is an old, tried and true mechanism that goes back to the dawn of the black space age. That’s how the spy sat worked. They ejected film cartridges which re-entered and floated under chutes that were captured in flight by cargo planes.
It took a few tries to successfully capture the falling Corona capsules. I would expect it to take a few tries to get the Falcon stage back by any of the methods suggested here and at transterrestrial.com. Still it remains that a recovery attempt on stages that were certainly lost otherwise could be very good business.
Keep in mind the vast difference in scale between spysat film capsules and a 15-20 ton rocket stage. I couldn’t find any data on the exact weight of the film capsules, but the single-capsule spysats they were part of weighed only a ton or so all up. The later two-capsule models weighed roughly double this. The detachable parts included a de-orbit retro motor and a heat shield, both discarded by the time the film capsule deployed its parachute for recovery. These capsules were small, spherical and may well have weighed as little as a hundred pounds or so in their recoverable state.
Even so, it took a C-119 or C-130 to snag and reel them in. What would it take to do likewise with something two orders of magnitude larger and heavier, not to mention over 100 feet long and cylindrical? A C-17? The cargo bay is only 88 feet long. More worrisome still, it’s only 12 feet 4 inches high. The F9 first stage is 12 feet in diameter. Anyone imagine it possible to winch in a 30-plus-ton object flapping in a 200 knot breeze when there’s a maximum vertical clearance of four inches to play with? I don’t. Either the stage or the aircraft would be destroyed. Likelier both.
Assuming one could somehow overcome this fiddling difficulty, there would still remain the little matter of landing with 20 or more feet of the stage hanging, unsupported, out the rear cargo door. It would be an interesting bet as to whether the thing was destroyed by dragging on landing or by being bent as the carrier plane touched down. Midair recovery by plane is such an obviously wacky idea it makes midair recovery by helicopter seem only moderately deranged by comparison.
Sorry. This dog won’t hunt.
I’m under the impression that the F9 first stage can’t even hover at its landing fuel load. The thrust of one Merlin 1D engine, even at minimum throttle, is greater than the weight of the first stage at the time of recovery.
It is my impression that sea water is a nasty beast to be avoided if possible when it comes to flight hardware. Though a Sea Dragon chamber did fire after immersion, it was designed for it in a way that Merlins probably aren’t.
Individual ideas, including all of mine of course, may not hunt. The odds are quite good that a determined effort could locate and train a reliable retriever. Throwing up the hands and yelling impossible prevents the attempt at retrieval and guarantees loss of the stage indefinitely. I would make a small wager that I could brainstorm with a few experienced people and find several possible solutions in a weekend, at least one of which could be affordable and relatively soon.
A moderate rate of climb during a recovery attempt would not be all that terrible an option.
Jeff’s point is one I hadn’t considered. The announced sea-level thrust of the Merlin 1-D is 147,000 lbs. The Merlin 1-D is supposed to be throttleable down to 70%. That would be 103,000 lbs. Assuming the dry mass of the F9 v1.1 1st stage is ca. 40,000 lbs., then hovering would seem to require at least a ca. 32 ton propellant load.
And yet there is Grasshopper. It is said to be powered by a Merlin 1-D. It ascends, it hovers and it descends. So either the Grasshopper test flights are packing much larger propellant loads than alleged, or the Merlin 1-D is capable of much deeper down-throttling than publicly disclosed to date.
I’m inclined to think it’s the latter. The Merlin engine is based on the same pintle injector technology used in the Apollo LEM descent engine. That engine was down-throttleable to 10% of maximum thrust. If the Merlin 1-D can manage down-throttling to 20% it would have no trouble soft landing an F9 v1.1 1st stage.
Grasshopper is ballasted by 1) the enormously overbuilt and heavy landing gear. (Flight-weight hardware on a Falcon 9 will not look like Grasshopper.) and 2) Excess propellant onboard. I believe this is explicitly mentioned in their FAA paperwork. While Grasshopper can hover, F9R stages won’t be able to. Hence all the work on “hoverslam” descents with T/W > 1.
“Throwing up the hands and yelling impossible prevents the attempt at retrieval and guarantees loss of the stage indefinitely.”
I draw a distinction between a “retriever” in the sense of a manned vehicle intended to fly in close formation with a rocket stage and grapple it, and stage recovery in general. The obvious way to get recovery without a retrieval vehicle in the case of Falcon 9 is SpaceX’s stated intention of using boost-back. In that context the downrange soft landing exercises are just a way to get useful data out of a stage that was guaranteed to be lost anyway, while also avoiding risky test activities near people and launch facilities. The progression from Grasshopper to testing on flight stages after their primary mission through to full boost back makes more sense to me then a hypothetical progression from Grasshopper to manned mid-air rendezvous to boost-back.
Their are two issues: capture and landing.
Capture is better done by matching velocity at speed so you do not have an hover issues.
Once captured, you can’t just reel it in, but you can very easily transfer it to a large cargo helicopter in flight. You can even transfer the weight as slowly as you need.
FWIW, I don’t think any sort of helicopter or chase-plane arrangement will work with a many ton rocket ~140 feet tall and 12 feet wide, whether falling at terminal velocity, more slowly, or even hovering. SpaceX has explicitly stated that their current intention is to be able to conduct a fast landing with thrust > weight, and be able to return the rocket to the launch site. So far, they have made amazing progress towards that goal.
Landing on a barge is likely to result in the barge capsizing from the suddenly raised center of gravity, especially if done far out at sea with waves interfering with things.
I’d expect any barge used for landing to be at least an order of magnitude more massive than the rocket. Boat mass is dirt cheap compared to rocket mass. Adding the rocket mass won’t appreciably impact barge stability.
At the end of the day, one can propose all kinds of buildable alternatives to return-to-launch-site recovery, but, apart from any considerations of workability, such a solution still has to be cost-effective or it’s worthless. To be practical, any such solution needs to be appreciably cheaper to design, build and operate than the difference in value of the maximum on-orbit load deliverable by a given rocket in expendable trim versus the lower on-orbit load deliverable by the same rocket suitably modified, and accommodating the parasitic residual fuel load required to implement RTLS recovery multiplied by the number of launches it is likely to serve over its service life. That’s it. If your giant helicopter or giant stage-snatching airplane or your sea-going barge/aircraft carrier costs more than this, the case for building it doesn’t close.
Now I personally think the barge idea is a lot closer to being sensible than the helicopter or fixed-wing aircraft notions, but building a massive sea-going barge still isn’t a nickle-and-dime proposition. Nor are even quite massive seagoing vessels immune to significant “attitude excursions” in even moderately rough seas. In anything but a dead calm, any vessel will be undergoing movement on all three axes due to wave action. The real problem with this, from a 1st-stage recovery standpoint, isn’t even the landing, but the bending loads imposed on the landed stage after it touches down and is locked to the deck in an upright posture. The stage immediately becomes a 10-story-plus baton wobbling about like the mainmast of a tall ship in a comparable sea state. The only way I can see to largely avoid such potentially catastrophic bending moments on the post-landing 1st-stage hardware is to equip the barge with a large Stewart platform. This would be a scaled-up version of the hydraulically-driven platforms used in flight simulators. It would compensate in real time, and under the load of a landed 1st-stage, for the movements of the barge hull. Needless to say, this also wouldn’t come cheap, though sourcing it from the amusement park ride industry rather than the aerospace industry would likely minimize the still considerable expense.
For aero-capture, what about the Stratolaunch carrier aircraft?
By design it is capable of carrying more than the mass of the near-empty first stage, and it has a cradle large enough to hold the stage, not just tow it on a cable. Meaning it should be able to winch in the stage and land with it back at the launch/refurbishment site.
[That said, mid-air capture can’t possibly be cheaper than landing legs. The testing alone would cost vastly more. It’s not like you can test it incrementally. And every one foot miss is a lost stage. Every tangled cable a lost carrier.]
Interesting idea. As you point out, the Stratolaunch carrier plane at least has the saving grace of actually existing (once it’s built) and being designed to haul whole, fully-fueled vehicles, not just mostly-empty first stages. That potentially solves the very considerable load carrying capacity problem associated with mid-air capture/recovery.
The main remaining problem I see is attitude. A descending first stage, either unpowered under chutes or powered using residual propellant, will be vertically oriented. The Stratolaunch carrier plane is designed to do its carrying horizontally. There seems to be a need for a very dicey maneuver by either the falling stage, the carrier plane or both in order to mate up in mid-air. I doubt this is safely possible. The carrier plane is very long-winged and probably can’t dive safely at any very steep angle. A falling stage could render itself horizontal under a chute, but then the chute would be between the stage and the carrier aircraft coming in from above. I don’t see any stable safe way for a falling stage to both control its descent speed and assume a horizontal posture without interposing something, like a chute, between itself and the carrier plane.
I think you pretty much nailed the essence of things in your parenthetical parting shot.
I’m more of an enthusiast than engineer, but as they will already have a recovery ship in the area why not use an enclosed floating boom and pump some form of foam/chips into it to act as a floating barrier. You could fit an inward curving top to the boom to minimise the loss of material from the rocket blast, add some floatation devices to the stage that activate upon landing and you may get a relatively intact stage. It may not be reusable but it might be less damaged with more information gleaned from it.
Boom and foam chips would likely introduce more problems than they would solve. SpaceX will already be getting a lot of data from on-board sensor telemetry. Flotation devices capable of keeping an entire first stage from sinking are not simple or especially lightweight. The additional parasitic weight and expense of engineering and fabricating such a subsystem that would only be useful – if at all – for one or a very few flights does not seem likely to be justifiable based on incremental gains in knowledge from a recovered stage.
Keeping an empty stage afloat shouldn’t be difficult as long as the tanks and plumbing are intact. The tanks should displace several times the stage mass in water.
Keeping systems from corroding from immersion in sea water threatens to be much harder.
Net strung between 5-6 drone helicopters like the drone K-max used recently in afghanistan resupply that can lift 3 tonnes payload. have enough helicopters that one can fail and mission still be successful (ie redundancy)
These drone helicopters fly in a wide star formation to intercept the falling rocket. Have them descending rapidly at the point of catching the falling stage. This will then operate as a very gentle catch.
System has redundancy, lots of capacity, no human lives on the line and can probably cover a 10-20km diameter area. K-max cost about 5million each, though they would only need to be hired for a few days at a time, and much of necessary software is already developed for their use as drone resupply.
Easy to test on dummy payloads dropped by hercules.
Once developed it could also be used for other RLV’s to save on wings and landing systems.
“Keeping systems from corroding from immersion in sea water threatens to be much harder.”
Hybrid approach: Recovery ship creates a pool of fresh water (like a water-bag, open-top, pontoons holding it open. A small fresh water lake to land in, in the middle of the open ocean.
Although it’s a Hollywood and Pop.Sci favourite, helicopter really don’t like being tied together. The resulting force vectors are ugly.
The big flotation problem for recovered boosters is the transition from vertical descent attitude to horizontal floating attitude without wrecking the structure. The Shuttle SRB’s were open at both ends so they entered the water vertically, deployed flotation collars at their upper ends, then waited for recovery divers to attach flotation bags to their tail ends 100 feet down and be brought very gradually to horizontal attitude for towing back to the Cape.
F9R first stages, not being gigantic roman candles, will be closed at both ends. As Peterh correctly points out, they will float. That’s actually a bug, not a feature. F9R first stages are as tall as 10-story buildings. If one enters the water vertically, it won’t be stable in that attitude with all of that tall, skinny empty tankage inside. It’ll pop back up out of the water and fall over on its side like a felled tree. You don’t simply tip over a 10-story building, even in water, and expect to escape damage. And booster stages are built a lot less robustly than actual buildings. A booster stage braced sufficiently to stand that kind of lateral punishment would be too heavy to lift much, if anything, in the first place.
You could – maybe -avoid the post-water-entry “bounce” if you could flood the lower propellant tank quickly enough. Using the extant engine feed plumbing, plus a relief valve at the top of the lower tank wouldn’t get the job done fast enough. You’d have to, in essence, put inward-opening hatches in the lower propellant tank. More added complexity, weight and failure modes.
As for the drone helicopter swarm idea, there appears to be an insuperable scale problem. If it takes 5 or 6 drone helos to lift 3 tonnes, it’ll take 50 or 60 to catch/lift 30 tonnes. You can’t get that many robo-helos into a sufficiently small volume of airspace to do the job without unacceptable risk of fratricide. I’m afraid that’s another non-starter.
For those worrying about the 1st stage sinking, don’t. This issue has been discussed on several forums and the fact of the matter is that the stage has tremendous amount of positive boyancy. The engines are down about half to a metre but that’s it assuming the stage is fully intact which is the aim of a) the thrusters at the top of the stage, b) deep thottleable Merlin 1D, c) water landing first up.
You can do the calc’s yourself if you don’t wish to take my word for it.