SpaceX’s Next Giant Leap: Second Stage Recovery

With the successful launch of the Bangabandhu-1 satellite on May 11th, the final version of the Falcon 9 rocket has finally become operational. Referred to as the “Block 5”, this version of the rocket is geared specifically towards reuse. The lessons learned from the recovery and reflight of earlier builds of the F9 have culminated into rocket that SpaceX hopes can go from recovery to its next flight in as few as 24 hours. If any rocket will make good on the dream of spaceflight becoming as routine as air travel, it’s going to be the Falcon 9 Block 5.

While there might still be minor tweaks and improvements made to Block 5 over the coming years, it’s safe to say that first stage recovery of the Falcon 9 has been all but perfected. What was once the fodder of campy science fiction, rockets propulsively lowering themselves down from the sky and coming to rest on spindly landing legs that popped out of the sides, is now a reality. More importantly, not only is SpaceX able to bring the towering first stage back from space reliably, they’re able to refuel it, inspect it, and send it back up without having to build a new one for each mission.

But as incredible a technical accomplishment as this is, SpaceX still isn’t recovering the entire Falcon 9 rocket. At best, they have accomplished the same type of partial reusability that the Space Shuttle demonstrated on its first flight all the way back in 1981. Granted they are doing it much faster and cheaper than it was done on the Shuttle, but it still goes against the classic airplane analogy: if you had to replace a huge chunk of the airliner every time it landed, commercial air travel would be completely impractical.

SpaceX has already started experimenting with recovering and reusing the payload fairings of the Falcon 9, and while they haven’t pulled it off yet, they’ll probably get there. That leaves only one piece of the Falcon 9 unaccounted for: the second stage. Bringing the second stage back to Earth in one piece might well be the most challenging aspect of developing the Falcon 9. But if SpaceX can do it, then they’ll have truly developed humanity’s first fully reusable rocket, capable of delivering payloads to space for little more than the cost of fuel.

Different Stages, Different Challenges

While the first stage is there to get the payload up, it could be said that the second stage is responsible for moving the payload sideways. The second stage absolutely pours on the speed: on the most recent launch it accelerated the payload from 8,019 km/h at stage separation to the 26,967 km/h required to maintain low Earth orbit in just a few minutes. Once the payload separates and continues on with its mission, the second stage is for all intents and purposes its own spacecraft moving at orbital speed and altitude.

Bringing it down to a gentle landing on Earth therefore has all the same challenges of landing any other spacecraft, except for the fact that the second stage has none of the hardware that would traditionally be necessary to pull off such a feat. It’s a bit like trying to land an airplane without landing gear. Or wings.

In early concept videos from SpaceX, the second stage was shown outfitted with a heat shield, landing legs, and even a retractable engine nozzle. All of these features would have worked together to make the stage capable of the same autonomous propulsive landings the first stage performs. But the problem with this “super” second stage is weight.

Every kilogram of recovery gear added to the second stage is one less kilogram of payload delivered to space. For a commercial launch provider like SpaceX, that is a problem. Fortunately, the Falcon 9 tends to be underutilized by most payloads, so there’s some wiggle room to play with. For example, the Bangabandhu-1 satellite weighed approximately 3,700 kg, which is less than half the Falcon’s capability for that particular launch profile: geostationary transfer orbit. So if the recovery hardware can be limited to less than 1,000 kg or so, it shouldn’t have an impact on the kinds of payloads the Falcon is likely to encounter.

An Unexpected Solution

Weight really adds up when building spacecraft. Consider that the landing legs on the Falcon 9 first stage weigh around 2,000 kg on their own. Any attempt at recovering the second stage needs to be done with the absolute minimum of additional hardware. A full heat shield like the Dragon capsule has would likely eat up too much of that mass budget, same with the “grid fins” used to stabilize the first stage as it falls back down to Earth.

So how do get the second stage through the atmosphere and stabilize it? The first hint at the answer comes from a recent Tweet by Elon Musk:

Party balloons and a bouncy house? If anyone else said something like that, we’d just assume it was kind of joke. But we thought it was a joke when he Tweeted about sending his Tesla Roadster to space, and we all know how that turned out. So what does it all mean?

Meet the Ballute

The idea of using an inflatable balloon to slow down high altitude supersonic vehicles was pioneered in 1958 by Goodyear. NASA demonstrated that the so-called ballute (as it’s both a balloon and parachute) concept could be used for spacecraft reentry when a small one was used to safely decelerate a test article from Mach 4.2 in 1968. Unfortunately tests with larger ballutes failed, and ultimately the concept was never used for the space program.

From Elon’s Tweets, it looks like SpaceX is looking to revisit the ballute concept, using it to ease the Falcon 9’s second stage journey through the atmosphere. After performing a de-orbit burn, the second stage could deploy a ballute to help slow and stabilize it as it comes back down from space. But that’s only half the problem. You still have to get it on the ground without damaging it.

Catch a Falling Star Stage

With the second stage at a low enough altitude and speed thanks to the ballute, it could then deploy either a traditional parachute of parafoil to make the final approach towards the recovery point. As the second stage would likely not have any landing gear or legs due to weight constraints, the landing area would apparently be an inflatable structure of some type that can catch the stage without damaging it. In principle this is very similar to the work currently being done to catch the Falcon 9’s fairings with a large ship-mounted net.

Again, this would borrow heavily from earlier NASA research. In 1963, experiments were performed to determine if the first stage of the Saturn rocket could be recovered using an inflatable wing structure.

Iterative Design

At the risk of trivializing the accomplishments of SpaceX, it’s fair to say that very little of their technology is actually new. Rather, they combine Silicon Valley style R&D and modern construction techniques with technology pioneered during the Space Race of the 1960’s to rapidly produce evolutionary improvements. This allowed them to get to orbit in a fraction of the time it would have taken had they started completely from scratch, and now it seems they’ll be turning their attention towards iterating through NASA recovery concepts from the Gemini and Apollo programs to help turn Falcon 9 into the world’s first truly reusable rocket.

That said, it wouldn’t be the first time SpaceX abruptly changed their approach. The final method for second stage recovery could be vastly different from what Elon has been hinting at. It’s also possible that they abandon it entirely. Even with only partial reuse of the Falcon 9, they’re by far the cheapest game in town.

The bottom line is, we just don’t know yet. It’s interesting to theorize like this, but until we watch a live YouTube stream of a Falcon 9 second stage riding down from space under a balloon, anything’s possible. One thing’s for sure though, no matter what their plans are, they’ve got the world’s attention.

60 thoughts on “SpaceX’s Next Giant Leap: Second Stage Recovery

  1. In the movie “2010”, they used a ballute (more like several of them) to slow the Russian spacecraft to orbit Jupiter.
    Whether it is really feasible, will be seen, but it was a cool scene in the movie.

  2. I think a more likely result here is a cost reduction of the second stage rather than a recovery…

    I haven’t done all the math, but it seems like a lot of technical challenges, and a lot of resources for recovery, and all it will do is save 10-20% of the launch cost of a F9, less for a FH… seems like reducing the cost to make a second stage is going to be the way to go when all the numbers are figured out…. make them cheap and drop them into the ocean when they are done.

      1. The first stage recovery already costs them a huge chunk of the potential payload (2/3 the kinetic energy for the upper stages), and the second stage mass increases diminishes that further. Soon they’re building rockets that are 3x the size necessary for the payload they can lift.

        One has to wonder whether it wouldn’t just be cheaper to build the smaller rocket on the cheap and use it once, because the rocket equation means the costs diminish rapidly when you scale down. Even the materials can be cheaper when you don’t have to support such a hulking tower of a rocket.

        1. This is one of those times you can be very sure that a large team of very smart people took a number of very long hard looks at the basic physics involved and how those do or don’t line up with the market for space launchers. This isn’t some back of the napkin plan.

          Besides, even if you were to have a third of the useful payload, it simply means you need to launch your rocket three times or more to break even. The ability to have three distinct orbits is something that’s quite desirable in the current market as well. Launch it three times and you just halved the cost.

          1. Never assume an entrepreneur like Elon Musk operates on thoroughly calculated and estimated predictions rather than drinking his own kool-aid over the hype. You see what goes on with his other companies, which regularily promise twice what they actually end up delivering.

            If this was NASA or ULA, I would say they have gone through the trouble, but since this is Musk, I’m willing to be they might have, but then Musk said “Let’s do it anyways, NASA is paying more for pre-orders if we pretend to be developing something cool”.

          2. “it simply means you need to launch your rocket three times or more to break even”

            Actually, even Musk himself has estimated it takes about 10-12 launches to recoup the additional cost and break even with just the re-usable first stage, and that alone loses the rocket 2/3 of its launch capacity because they need so much of the fuel to return.

            Now the problem becomes that there’s going to be a small probability of a launch failure. If we take the industry average 4% then the same rocket surviving up to 12 launches without complete rebuild is 61%. In other words, nearly 40% of the re-usable first stages are expected to be a loss. Add landing failures to the list, and the probability of making any profit is even lower.

            They haven’t actually re-used any of the rockets more than twice. This is still very much a concept in testing, and whatever SpaceX or Elon Musk says should be taken with a spoonful of salt.

          3. Say, you’re starting with 10 rockets, and each of them has to complete 12 flights to break even. Assume the cumulative failure rate from launch/landing is 4% per flight. When the probability of survival drops below 10% you can stay none of the rockets have survived. That’s your cut-off point.

            The probability of a rocket surviving n launches is therefore 0.96^n
            (to the power of n in case formatting eats the symbol)

            Given those probabilities, one rocket may survive up to 56 launches, two rockets to 38 launches, 3 to 28, 4 to 22, 5 to 16 etc.

            So it’s those lucky few that should make the profit. Four are guaranteed losses, the rest have a cumulative effect of 102 profitable launches, so 1 rocket is theoretically worth 10 rockets when it is re-usable in this manner – but – this assumes the probability of failure nor refurbishment cost increases over subsequent launches: that the refurbishment is 100% effective every time. That’s unlikely.

            Say for example, if we add 1% point increase in the failure rate over each re-use, none of the rockets survive over 12 launches. Even with a 0.1% point increase results in no rocket making more than 30 launches.

            The probability used is (0.96 – 0.001n)^n

            This is the big unknown that depends on how well SpaceX can pull off the whole process, and this is something you can’t estimate without actually trying it out.

          4. Btw. in the above example, the 0.1% increase in failure rates results in a 1:4 cost benefit ratio. One average rocket ends up doing the work of four single-use rockets. If the capacity is diminished by 2/3 then the ratio is 1:1.3 and the benefit over a single use rocket is roughly 25% reduction in launch costs.

            Which is easily attained by simply using the smaller rocket to match the payload.

          5. I’m pretty sure it was 12 total launches to recover the R&D, not 12 launches of a single rocket to recover the cost of the hardware.

            So arguably, if they’d stayed on block 4, and reused 12 boosters, just once each, that would’ve covered their R&D costs. I suspect that’s not entirely true, as block 4 were quite expensive to recondition. If block 5 live up to promises and require only minimal refurb, then I think their breakeven will arrive quite quickly.

          6. >”I’m pretty sure it was 12 total launches to recover the R&D, not 12 launches of a single rocket to recover the cost of the hardware.”

            Don’t forget that the cost of the launch isn’t just the rocket. It’s all the man-hours that go into preparation and the refurbishing process, safety checks, re-evaluations etc. etc. so the actual amount of money saved per rocket can easily be just 1/12th of the entire launch costs.

        2. 3x the size that can be use 5+ times is still a savings. Also “just building” more rocket sizes is an extraordinary money pit (as they found out with the heavy). Their model works great because they can launch really heavy things once or normal things many times all using the same design.

    1. 10-20% of one launch is still way more than my retirement fund. If a second stage can be successfully reused for some fraction of the cost of throwing it away, that is still a significant revenue stream and a far less polluting process to boot. Even if Musk gets 10,000 people to climb into the BFR(s) and move to Mars, I appreciate that he is trying to avoid leaving too much of a mess back here on Earth for the rest of us to deal with.

      Besides, Spacex is the cheapest launch service but only for now. Musk has proven that partial recovery can be done economically. The rest of the launch services rested on their laurels for years but now ULA and the rest of them are making noise about reusability. Ultimately this will start nipping at Falcon 9 margins. It makes sense to keep tweaking the price points,

        1. That’s why the US is currently uninhabited, except for a few natives. The first people had a terrible time, so they just gave up and stayed in Europe. Humankind as a species hates overcoming obstacles and impossible odds. We just don’t do it.

          1. Our ancestors were FAR hardier than we are … Just turn off the internet for a few hours and everyone screams bloody murder nowadays.

            Even as recently as middle 19th century, US president was born and lived in a cabin with a dirt floor. And not for only a week.

          2. [Miroslav]
            “Just turn off the internet for a few hours and everyone screams bloody murder nowadays.”

            Turn off the sunshine in Colorado (with clouds) for a few days, and the same thing happens.

        2. Assuming humanity overcomes the many huge technological challenges involved in creating a self-sustaining Martian colony, even if the first colonists don’t experience misery, few will follow them simply due to the enormous cost.

          Musk’s (wildly?) optimistic figure of $200,000 represents 6-8 years of average wages; passage from Europe to the US during the mid-19th century cost roughly 1-2 months wages (based on my few minutes of internet research, anyway).

          The first colonists who make it beyond survival mode will lay claim to the most valuable and accessible resources. What return would later colonists get? It’s not like Mars has vast, fertile land that the colonial government wrested control of from the original Martians and would sell cheaply to new immigrants.

          1. “It’s not like Mars has vast, fertile land that the colonial government wrested control of from the original Martians and would sell cheaply to new immigrants.”
            Manifest Destiny, all over again…

      1. >>The rest of the launch services rested on their laurels for years but now ULA and the rest of them are making noise about reusability

        Reusability is far from the only concern. Reliability is a huge factor and the Falcon-9 certainly has a good record so far, it still falls behind the Soyuz-2 and Ariane-5 with slightly lower and slightly higher GTO payload ratings respectively. Once the Falcon-9 gets a longer track record then we’ll see how proven it is.

        1. I’m glad someone finally mentioned reliability. Frankly I wouldn’t be too surprised if the QC department and engineering are driving trying to get a few 2nd stages home. The first stage benefited massively from inspecting recovered hardware, why wouldn’t the 2nd stage?

          As to where the recovery hardware goes? I’ve always assumed it would go in a customized payload adapter. It’s already big, empty, and rated to transfer the needed loads.

          Finally, Tom Nardi is missing piece of the puzzle that’s been discussed on r/spacex. i.e. how re-entry heating drops with increasing frontal area. If you go stupendously big, maximum temperature gets low enough that even a mylar party balloon will survive. (though something 100’s of meters diameter is needed for a Falcon-9 second stage…) As an added bonus, re-entry decelerators this large have enough drag near the ground that a final stage parachute is not needed.

      2. Afaik the Indians are still launching cheaper than Musk. He’s only relatively cheap against the other US based companies, which have the additional burden of maintaining multiple redundant launch sites as per government demand.

        1. If you’re talking about the Indian Mars orbiter, remember that it weighted only 15kg. Not to minimize their achievement but yeah, it was much cheaper to launch.

          According to SpaceX’s website, a Falcon 9 can send 4020kg to Mars.

          1. dana,
            You’re either badly misinformed, or downright disingenuous. The Indian MOM carried 13-15 kg of *instruments*. The spacecraft carrying them had a mass of 1337 kg in Earth orbit, and still put a half tonne in orbit around Mars.

  3. SpaceX’s long term goal is to get a fully-reusable BFR/BFS system that is cheaper to fly than a F9 and can carry more cargo. Musk is expecting to start testing the BFS next year. He has stated that he is expecting the operational lifetime of the F9 Block 5 to be about 300 launches before it is replaced by the BFR/BFS.

    As such, getting a reusable F9 2nd stage seems like it’s a lot of work for a short-term solution. It’s good to prove the concept, and to push the envelope to show that it can be done, but is it going to pay for itself?

    Granted, Musk announces aggressive deadlines for doing the impossible, and misses them frequently, but he and SpaceX have a habit of completing the goals, even if not on time. So BFR/BFS may not be ready to retire the F9 system after only 300 launches of F9b5, and 2nd stage reuse might be worth it.

      1. I’m pretty sure the Falcon name there only came after several marketing meetings trying to confine Musk that the government would not be involved/appreciate something with “Fucking” in it’s name….

  4. I like the idea of bringing back the hardware for re-use, and I like the idea of removing superfluous hardware from LEO even more.

    But, heck, it’s already darned near in permanent low-earth orbit. I can’t help but wonder if it wouldn’t be more use just parking it for re-use later *there*. It’s proven flight hardware, and it represents a huge amount of cost savings if you can manage to refuel and re-use it there instead of lifting another up.

    Make a lunar cargo shuttle with it. Heck, a Mars cargo shuttle. Or chase down defunct hardware and push it out of orbit and clean up the LEO space (hehe, a *vacuum cleaner*. geddit?)

    Sure, the logistics will be a pain. Plane change maneuvers are costly in delta-V, so probably next to impossible. Capturing and refueling will present real engineering challenges. But it seems a waste to bring it back. It’s not like it’s going to save much money.

      1. Even if none of the returned 2nd Stage is re-usable, it will be in a location to recycle its components (as opposed to sitting on the ocean floor, or burning up (consuming oxygen) in the atmosphere.)

    1. The logistics of orbital rendezvous make it less reliable and therefore less enticing to attempt. You also need to make the 2nd stage more complex either though remote (latency anyone) or AI control. I’m not sure AI is quite there yet.
      An orbit that’s useful for LEO rendezvous from the various space ports is probably also a very common track which would make everyone grumpy that they now can’t use their favorite approach or can only use it during specific windows which greatly confounds the other restrictions on launch windows.

      That’s not to say it won’t work, just that it might be more trouble than it’s worth.

        1. If you can match the reliability of single use boosters it’s well worth it. Fuel is cheap, hundred million dollar rockets and multi-million dollar payloads are not.

          1. One can’t just assume it will be “well worth it”. What’s the marginal cost of a 2nd stage (in large quantites), compared to total recovery costs? Don’t forget to include the opportunity cost of tying up labor and capital in engineering that solution and in chasing down the incoming stages.

            As a first crack: SpaceX said once a F9 launch costs $65M. Assume half that goes to operations, fuel, pad rental, paying off the FAA etc. and (dare we say it) maybe profit. So the rocket might actually cost half that ($32.5M).

            The second stage is 15% the dry mass of the rocket, so say its total cost is 15% of $32.5M = $5M. Its actual marginal cost (the amount you would save if you could re-use 100% of it instead of just building another one) is probably less than half that.

            That means all your costs in making the second stage recoverable: recovery operations, additional hardware, labour, fuel, administration, lost capacity of payload due to the additional recovery hardware you have to carry up, costs for refurbishment & consumable replacement, amortized engineering cost, interest on bond debt, etc, etc: they all need to be recovered somehow. I very much doubt the savings by recovering a measly $2.5M worth of flown 2nd stage hardware would ever pay that back.

          2. Paul,
            maybe they are doing it for the learning experience so they can apply the solution on later designs, like the BFR?
            And it’s good to push the boundaries of what people feel is ‘feasible’, it’s not always about the money.

          3. @Paul
            The cost analysis has been done before: http://spacenews.com/spacexs-reusable-falcon-9-what-are-the-real-cost-savings-for-customers/
            The actual per-launch costs are around $36.7M including wages with the other $25M taken as profit and the rocket its self costing about $28M. A second hand Falcon-9, as it were, would go for between $37-$48M depending on how generous SpaceX are with their profits.

            >>Don’t forget to include the opportunity cost of tying up labor and capital in engineering that solution
            I’m not sure you want to go down that route given the cold war military funding that went into the Soyuz, Atlas, and Delta projects. Especially since much of the systems that went into SpaceX designs were learned from the Delta, and Delta Clipper projects. Gimballed engines and mid-stage thrusters were all developed in government projects.

          4. The cost analysis is only part of it. And it’s only a minor part of it.

            You’re forgetting that all of these things – the first stage, the second stage, the payload fairing – they’re not just expensive. They’re also huge. And complicated. It takes *time* to build them. And that’s time that you can’t spend *launching stuff*.

            To quote Jim Cantrell, on Quora: “Secondly — and I think that this is the dominant answer — reusability allows a marked increase in flight rates. Reverse engineered financial models of SpaceX show that to reach a good strong positive cash flow, they need more than the traditional 10–12 launches per year that sized rocket has demonstrated. Reusability should easily double the amount of flights possible from a mere production and logistics standpoint.”

            The cost savings is only part of it. In 2017, SpaceX had 18 launches, more than double that of the previous year, and a large number of them were reused. It’s not just about cost savings. It’s about turnaround time on the flight. It takes less money to refurbish the things, yes, but it also takes less *time*, and that’s the key. The company can accept less profit per launch with more launches.

  5. >>At the risk of trivializing the accomplishments of SpaceX, it’s fair to say that very little of their technology is actually new.
    Every idea Musks various companies have had is one that has at the least had a proof of concept built before. That seems to be their / Elons business model: find old patents, ideas and proof of concept designs rebrand them with a well paid marketing team and some cgi. Then wait for the media accolades.
    Despite acquiring the Tesla name he’s more of a Westinghouse or Edison. The latter pair got more done than the former so in the end it’s a positive.

    1. Doing something in rocketry is hard. Doing it reliably and usefully is almost impossible and that’s where SpaceX shines. Rocket science is extremely unforgiving and needing to get everything right at the same time is what makes it so hard. Yeah, somebody landed a rocket upright before, but not with any useful payload and it blew itself to bits after a few tiny hops. Everything that came after Grasshopper can be attributed to SpaceX. The dynamic computer modelling required to land a re-entering rocket successfully simply wasn’t possible until very recently. Nobody knew how to do this until SpaceX blew up enough rockets to figure it out.

      1. “…simply wasn’t possible until very recently.”
        Those who do not remember their history are doomed to repeat it…
        DCX successfully did vertical takeoffs and landings a quarter century ago.
        True, it was only a 1/3 scale model, and didn’t go to orbit, but it did work.
        20 frickin’ tons of SSTO aspiration.
        https://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X

        What I find really interesting about it was that it used commodity PC-104 x86 computers for real-time flight control.

        1. And some hinges from K-Mart for some parts. They met the need so there was no reason to specially build something fancy, much more expensive *yet no better for the need*. The DC-X was all about building it on the cheap while making it reliable. So despite having a working DC-X demonstrator, NASA chose the X-33 that had nothing but a lot of talk and some artist’s drawings.

          After the millions of $ were spent, X-33 showed that the technology of the time *couldn’t* build conformal shaped cryogeninc tanks, and part of an aerospike engine was built.

          Had NASA chosen the right project, we’d be popping payloads up to *and down from* orbit every day by now.

  6. Maybe a tailhook design couple with the ballute. Obviously SpaceX is planning for one or more landing pads depending on how well the reentry path can be standardized. Maybe guiding it between uprights with actively positioned arresting cables is an option.

    They do seem to like finding precision solutions to challenges.

  7. The Space Shuttle wasn’t remotely reusable. Every part in the lauch stages that flew again was stripped and essentially rebuilt like new, including the very expensive qualification testing. It was essentially building a new craft, except they did it the hard way and used scrap from previous missions to build it.

    Only the orbiter could be considered somewhat reusable, though it should be noted that that too was extensively rebuilt between flights. Just not to the degree that it was essentially a new vehicle.

    Of course, something like 70% of the cost a launch is in the first stage, so Space X got it right.

    1. There’s a documentary from 2003 titled (IIRC) Space Shuttle Garage. It was shown *one time only* on either Discovery Channel or The History Channel. (I’m leaning more toward it being on Discovery.) It showed the process the Shuttles went through between launches.

      Why it never got shown again is not long after was the loss of Columbia, and some things in the documentary weren’t all unicorns and roses and cotton candy. It showed some of the problems with rehabbing the Shuttles between flights.

      It appears to have been very effectively ‘memory holed’. After Columbia’s failure, I looked on the channel’s website to see if it was for sale or for any info on the show. Nope! None! As though it never existed.

    2. Most of the trouble was the TPS followed by the SSMEs that needed disassembly and inspection every flight.
      Some systems such as the OMS pods needed very little refurbishment work between flights.
      The block II SSME required much less maintenance than the previous versions and finally realized the goal of a high performance reusable engine.

  8. I’m thinking giant airbag on the water deployed from barge. With the giant airbag sitting on the water it could probably be smaller since the water would help cushion the impact. I suppose you could just drop it in the water like NASA has done for years with a flotation bag but I think the salt water landing would lead to longer time and more work before re use.

    1. The thing is coming in *from orbit*. They could plop it down wherever they want. Vandenberg, Canaveral, Hawthorne. Even Musk’s swimming pool if you really want to do a fresh water landing.

  9. You are speaking my language. A vacuum rated engine that can be refueled is much more valuable in orbit than if it is returned to earth. If the second stage can be converted to Methalox or Hydrolox it would be even more useful than it is now as a reusable second stage burning kerosene. Keeping Rocket grade Kerosene for long-term use and relighting is going to be a pain in the ars. But Methalox is much easier and can be made out of thin air and water on Mars, plus if you equip the tanks with doors, the empty tanks would be convertible into living space when they are empty. Both Methane and Oxygen would leave the storage tanks pristine and not stink like kerosene. Plus if you want to do retropropulsive landings on the moon or mars the vacuum rated motors on the second stage are perfect. Imagine building a rotating ring with about a 750 foot radius turning at 2 RPM with 300 of those second stages, fueled up and equipped with retractable landing legs. They could push a massive amount of mass into lunar or martian orbit and be used (and maybe reused) like landing craft to put away missions on the planet below. Face it, when it comes to landing on Mars, its probably going to take multiple attempts to learn how to do it right. A massive ship, with hundreds or thousands of souls living on board putting people down to explore and bring back geologic information and making decisions about where and if we should decide to invade Mars and infect it with our version of life. What an exciting future for those of you young enough to see it happen. I am too old to experience it but it seems like a viable future for you younger folks. I enjoy thinking about it.

  10. I think they should keep them up in space and group them together and reuse them to send on planetary missions.
    When the time is right send up a tanker to refill them in space and put on the other payload. And off they go.
    With half of the rocket up in orbit already that should save a bundle.

  11. One trick they could do is use a very cheap third stage for GTO missions as even something like a Star-48 would greatly reduce the amount of inert mass that needs to end up in the same orbit as it’s empty mass is a few hundred to a 1000kg vs 4,500kg for the mass of the F9 second stage.
    They could in theory bring back the kestrel engine for this vs buying upper stages from Orbital-ATK.
    Each kg of mass that doesn’t need to go into a high energy orbit could go to payload or recovery systems for the second stage.

  12. Its not about cost reduction. It’s about development of technologies to be used on the BFR. From Mars, it will be a single stage to orbit rocket containing second-stage technology. Recovering, examining, relying, and rerecovering Falcon second d stages teaches then how to do that with BFR.

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