7 ms·

Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle? I was under the impression that SpaceX was trying

• icarus_drowning 10y ago
  As I understand it, the main difference is that the Falcon rockets are (or are eventually meant to be) much less dependent on refurbishment between launches. The Space Shuttle required extensive and expensive work between launches to the extent that many critics[1] claimed that it wasn't truly "reusable". (IMHO, while the shuttle program definitely didn't achieve its goals, it seems like calling it "reusable" is fair). As for gliding back: the Falcon booster does not actually achieve orbit-- when the main engine cuts off, it is on a ballistic trajectory back to the ocean. While it may be possible to design some sort of gliding apparatus to "save" a booster on a sub-orbital trajectory, it is (again, as I understand it), much simpler to simply adjust that trajectory via a boostback burn that reverses or slows that trajectory, and to then perform a suicide burn[2] to recover the stage, either on a barge or (on lower orbit missions) back on a land-based pad. I am not a rocket scientist, nor do I have experience in the space industry-- these thoughts are just based on what I've read following the SpaceX reusability program as closely as I can for the past few years. I do play a lot of Kerbal Space Program, though. [1] https://en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle_program https://en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle... [2] http://space.stackexchange.com/questions/10307/what-is-a-suicide-burn http://space.stackexchange.com/questions/10307/what-is-a-sui...
  • SEJeff 10y ago
    They also like to call the landing a "hover slam" in that it burns at the very last second and slams down pretty hard.
    • ygra 10y ago
      Ideally they reach zero velocity the instant the legs touch the ground. There was only one hard landing (that didn't result in a fireball) so far, and the legs are designed to take the remaining speed (they are replaced anyway and not reused). They also have no other option. Even one engine at lowest throttle (70 %) is powerful enough to lift the almost empty booster again, so they cannot hover (which would make things a bit easier, at the expense of needing more fuel).
    • WalterBright 10y ago
      Yes, I learned that maneuver playing "Lunar Lander" in college. (The original BASIC version.) It was the only way to land without running out of fuel.
    • verytrivial 10y ago
      I can't find the reference, but I remember reading that the landing is complicated by the fact that even at minimum thrust, with the tanks nearly empty, the ship is so light that even with a single engine, it would accelerate back up after hitting zero velocity. So if the engines are restarted too early etc. it could "miss" the ground! (If someone can confirm/deny this? I'm seeing approx 28T dry mass, thrust per engine of 66T and min thrust of 70%, but that dated guess work from http://space.stackexchange.com/q/4466/ http://space.stackexchange.com/q/4466/ )
• walrus01 10y ago
  wings, retractable landing gear and aerodynamic control surfaces are WAY heavier and more complicated than 4 legs and a sensor/software based vertical balancing system that controls already-needed engine gimbals.
  • robotresearcher 10y ago
    And the fuel to slow down.
    • sqeaky 10y ago
      Thanks to the rocket formula We know that the fuel to slow down weighs the least. It doesn't have to lift any other fuel, so there can be less of it.
      • robotresearcher 10y ago
        That formula also tells us it took a lot of fuel to lift that slow-down fuel to the top of the ride...
        • gozur88 10y ago
          Not nearly as much as the wings, though, and with extra fuel you always have the option of lofting a heavier payload and throwing away the booster.
• malanj 10y ago
  Space shuttle's main engine was the "boosters". A huge liquid (very cold hydrogen and oxygen) tank which supplied the fuel to the actual shuttle during launch, and two smaller solid fuel boosters. The tank and boosters were all destroyed on launch. SpaceX rockets use a much simpler engines, that burn oxygen and kerosene. The main cost is in the first stage of the engine - think two rockets on top of each other, the first stage is the bottom rocket that first first. When the first rocket is done, it falls away. This first stage accounts for the majority of the cost of the launch. Think of it was being equivalent to the main tank and 2 boosters of the shuttle. What SpaceX manages, which no-one has done before, is to retrieve that big + expensive first stage for reuse.
  • bluehawk 10y ago
    The boosters were actually parachuted back down to earth and reused. Though the general consensus is that it didn't save any money, and may have been more expensive than if they had just been disposable.
    • mikeyouse 10y ago
      Yeah they would parachute into the ocean, get filled with air, then literally dragged back to port: http://www.gettyimages.com/detail/news-photo/solid-rocket-booster-recovery-ship-liberty-star-tows-one-of-news-photo/51581818?esource=SEO_GIS_CDN_Redirect#solid-rocket-booster-recovery-ship-liberty-star-tows-one-of-the-space-picture-id51581818 http://www.gettyimages.com/detail/news-photo/solid-rocket-bo... All that stress + salt water meant a fairly extensive refurb process.
      • sqeaky 10y ago
        These seem like problems with that design, not something fundamental. Could one conceivably use different materials (that maybe didn't exist back then) resistant to aquatic stress and salt, to have made the old plan much more cost effective? Following this line of thought I think you wind up with something close to what SpaceX does. Because the next largest save-able cost would be the boat crew to go recover it. So attach a propeller to the rocket to have it come back on its own. Then you want to save that cost and complexity so have it come to your boat with no crew and touch down gently.
• pdelbarba 10y ago
  The engines have minor differences, namely that the space shuttle used fully closed cycle engines while the falcon 9 does not. All this means though is that the efficiency is slightly lower because some of the propellant is burned and the combustion product dumped overboard to power the turbo pumps that push the rest of the propellant into the main combustion chamber. While this reduces efficiency, this makes the engine somewhat easier to design and I believe the next gen spacex engines will have this feature. The vertical landing design is actually really smart because it allows a relatively un-aerodynamic object to get back to the ground without adding heavy wings and fairings. Rockets don't actually like having fins when they go up since the dynamic pressures get to be extremely high. Rather, they opt for gimbaled engines to steer (this also helps when there's no atmosphere to steer with). The only downside to this approach is that some of the fuel has to be retained for the return trip, though this is a relatively small fraction. The other huge issue you have to deal with when gliding back is having some sort of landing gear in addition to the control surfaces. You can put wheels on it but that's a lot of weight and if you land it horizontally, without the pressure of the propellant keeping the body rigid, it will most likely crumple.
  • greglindahl 10y ago
    F9 FT can launch ~ 5.3 metric tons to GTO with landing, or ~ 8 metric tons without. I'm not sure if I'd call that a relatively small penalty. It does allow SpaceX to launch most GTO satellites. I expect Block 5 will do a bit better.
    • itp 10y ago
      I think both views are essentially correct. It is a relatively small fraction of the fuel load, but thanks to the impact of the rocket equation, that relatively small fraction translates (as you are pointing out) to a sizable difference in payload to a given orbit.
  • WalterBright 10y ago
    The landing gear would also need to be retractable, a big increase in weight and complexity. The complexity will add a lot of failure-critical reliability problems.
• aninhumer 10y ago
  >why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home Basically, the physical and aerodynamic stresses of decelerating vertically are fairly similar to those of accelerating vertically. Whereas designing something which can work as a rocket and a plane would be a lot more difficult, and probably more expensive to produce. It's more difficult to control vertically, but if you can get it right you save a lot of complexity in the physical engineering.
• InclinedPlane 10y ago
  The Shuttle Orbiter (the reusable part) was not a complete stage as it used an expendable external tank. It did reuse the engines, which one would imagine would be a significant cost savings, but the Shuttle system was such a complex and compromised beast that it ended up being one of the most expensive launch vehicles of all time. Vertical landing for the Falcon 9 made sense because it meant they only had to add a small amount of extra weight and new systems (landing legs, grid fins, etc.) in order to achieve powered precision landings. In contrast, adding wings, landing gear, other aerodynamic components, and potentially other engines (in order to achieve powered landings) adds a lot of extra complexity and weight. Additionally, the stage is already designed to handle vertical flight and forces, making it strong enough to handle strong aerodynamic forces while flying horizontally is no small change. And vertical landing enables barge landings out at sea as well as dramatically reducing the size of the landing facilities (a small pad versus a landing strip). All of which saves costs, complexity, and enables a higher percentage of the vehicle's performance to be used by the customer versus for recovery.
• colordrops 10y ago
  You can't use wings to land on a planet without a significant atmosphere.
  • ygra 10y ago
    Context here for those who don't know: SpaceX' declared goal is to put humans on Mars. Mars has enough atmosphere to cause trouble (burning up in free fall), but too little for any kind of coming to a stop with parachutes. Cf. how Curiosity landed, which wouldn't have been necessary on Earth. In any case, SpaceX doesn't like to be constrained by things that only work on Earth, and while parachutes were considered for recovering the first stage they found it wouldn't really work (and dropping things into the ocean tends to not do them good). So propulsive landing (despite sounding more complicated) is actually a better option than others here.
  • gozur88 10y ago
    I've never found this point very compelling. It's not like the first stage of anything will ever be landing on Mars.
• willvarfar 10y ago
  I'm surprised there aren't parachutes still involved. Early on they tried parachutes but ditched them, iirc.
  • manicdee 10y ago
    It turned out that parachutes are great for soft uncontrolled landings, but if you want a soft controlled landing you need some form of propulsion. So if you want propulsion why not use the main engines, and since you are going to use those formthe soft controlled landing why not use them for the descent too. At this point you have traded several tons of parachutes and many more points of failure which could cause loss of mission (parachutes are complex mechanisms) for a smaller weight of fuel and exactly the same points of failure as a rocket always has. Then SpaceX added grid fins, since purely propulsive landing was not going to be anywhere near accurate enough. Classic example of working to your strengths, where the greatest strength of a rocket is acceleration along its primary axis :D
    • gozur88 10y ago
      >It turned out that parachutes are great for soft uncontrolled landings, but if you want a soft controlled landing you need some form of propulsion. They could do something like the Russians do with Soyuz - small solid rocket boosters that fire right at landing. But yeah, that's a lot more things that have to go right.
      • simonh 10y ago
        Also solid rocket motors are a lot less weight efficient than liquid fuelled rockets and are not throttleable and are awkward to gimble. Also where would you put them without increasing air resistance on the way up? Also even with the rocket motors Soyuz hits the ground pretty hard.
        • gozur88 10y ago
          You could put them around the rocket nozzles. They wouldn't have to be very big. And yes, Soyuz hits pretty hard. But Soyuz was developed over fifty years ago. I'll bet we could do better today.
• anovikov 10y ago
  Winged stage will not have enough kinetic energy at cutoff to allow it turn around and fly back to launch site, and landing it at the ocean takes building and operating a whole aircraft carrier - too expensive (and probably harder to safely automatically land than the way they do it now). So, that winged stage would have to redirect back to launch site the way SpaceX does it now - using fuel. And, it will have to always do it, not having the option of ASDS launch, severely cutting the payload on GTO and other high energy trajectory missions. For RTLS launches, it would do approximately as good as their current approach, the fuel for landing burn+legs will approximately weight as much as wings and wheels. But would definitely cost more to develop. A winged stage also having a jet engine, allowing it to fly back as a powered airplane, not a glider, would solve both issues, and will whole lot more efficient than current SpaceX approach for RTLS, and about as good as ASDS, but a lot, a lot more expensive to develop. They would have to become an aircraft company in addition to being a rocket company.
  • kirrent 10y ago
    Do you have any references at all for a winged stage being feasible in any way? Considering that there are plenty of rocket stages out there that can't even be moved horizontally unless they're pressurised I doubt anything vaguely resembling the Falcon 9 could ever be retrofitted in the manner you describe. What form would these, presumably retractable, transonic capable, necessarily light wings even take? How would they ever be as light as the remaining fuel load in an ASDS landing, especially when wheels are added?
    • anovikov 10y ago
      Russians played with exactly such a stage, that had wheels from Mig-29 and engines from Yak-40. It was called Baikal and was feasible, but eventually never flew.
      • anovikov 10y ago
        https://en.wikipedia.org/wiki/Baikal_(rocket_booster) https://en.wikipedia.org/wiki/Baikal_(rocket_booster) Here is a Wiki article on it. Reverse: it was supposed to use engine from Mig-29 and wheels from Yak-40 :) Anyway, it never went anywhere beyond mockup stage.
• sandworm101 10y ago
  On reason, of many, is structural. A rocket's first stage is basically a giant coke can with a big rock (the engines) at one end. It cannot just lay down on the ground horizontally. That type of landing would require all sorts of internal structure, not to mention dedicated landing gear. It probably wouldn't be round any more. Warp a rocket by even a single degree and it will never be good to fly again. On the other hand, the big coke can is perfectly happy to stand vertically.
• grey-area 10y ago
  Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle? Shuttle boosters are solid-fuel, so they can't restart, can't land, and were ditched in the ocean and refurbed. In contrast the 1st stage of spacex rockets can land itself. I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title. For certain values of re-usable. It was pretty much stripped and refurbed for each mission, at enormous cost. The costs of spacex are orders of magnitude smaller. I'm also kindof curious why they decided to go with a vertical-landing-design, This design works everywhere, not just earth.
• JDulin 10y ago
  For a brilliant rant on the horrible, useless design of the Space Shuttle, it's hard to beat Idle Words: http://www.idlewords.com/2005/08/a_rocket_to_nowhere.htm http://www.idlewords.com/2005/08/a_rocket_to_nowhere.htm
  • mstade 10y ago
    This made me laugh: And of course, there was John Glenn, monitored inside and out, blood tested, urine sampled, entire organism analyzed for signs of accelerated aging. Close observation of the Senator suggested that there might not be any medical obstacles to launching the entire legislative branch into space, possibly the most encouraging scientific result of the mission.
    • 24gttghh 10y ago
      Yet you skipped this one? As tempting as it is to picture a blood-spattered Canadarm flinging goat carcasses into the void,...
      • mstade 10y ago
        Ha ha I wouldn't say skipped so much as cherry picked. There are plenty more golden nuggets in that article, the one I quoted just happened to be the one that had me laughing out loud.
        • sqeaky 10y ago
          There is so much to like about that essay, even the citations are good. Here is my favorite: > The Soviet Shuttle, the Buran (snowstorm) was an aerodynamic clone of the American orbiter, but incorporated many original features that had been considered and rejected for the American program, such as all-liquid rocket boosters, jet engines, ejection seats and an unmanned flight capability. You know you're in trouble when the Russians are adding safety features to your design.
• tankenmate 10y ago
  The large overriding reason the shuttle had wings is so that it could land "cross range", i.e. divert to land either left or right of the flight path. This way if there was a transatlantic abort landing (TAL) required the Space Shuttle could land in Spain, Portugal, France, Diego Garcia (and in varying years Gambia, Nigeria and a few others). This was a requirement if the shuttle was launching with a classified payload (i.e. NRO satellite).[0] [0] https://en.wikipedia.org/wiki/List_of_space_shuttle_landing_sites https://en.wikipedia.org/wiki/List_of_space_shuttle_landing_...
  • mikeash 10y ago
    The Air Force also wanted to be able to launch the Shuttle on a polar orbit out of Vandenberg, do a single orbit, and land back at Vandenberg on the next pass. The idea was that if there was a critical mission for the Shuttle (capturing or disabling a Soviet satellite, for example) and the Soviets were in a shootin' mood, this would minimize the window of risk to the mission. Of course, the Earth rotates underneath the Shuttle while it's in orbit, so when the Shuttle comes back around in a polar orbit it's quite some distance to the west of the launch site, so it needs to be able to make significant maneuvers for this to work. Vandenberg never ended up hosting Shuttle launches, of course. It was nearly ready, with mission planned and everything, when Challenger exploded. This caused a reevaluation of the whole idea, and it ended up being scrapped. But the design consequences remained part of the vehicle until the end.
• hoorayimhelping 10y ago
  I'm a programmer and I love rockets. I don't work in Aerospace and all of this is from my own knowledge and learning. Take that as you will. The SSMEs (Space Shuttle Main Engines - Aerojet Rocketdyne RS-25), required an almost complete rebuild after every launch. It was nominally reusable in that the parts could be reused, but it took a long time to refurbish and re-test the engines. Also, the first stage of the Space shuttle included the solid rocket boosters and the hydrogen fuel tank. The boosters were absolutely reusable (I seem to remember reading that of all the SRBs, all but four were reused - two from the first launch were never recovered, and two from the Challenger disaster that couldn't be recovered). But the Space Shuttle wasn't fully reusable like the Falcon first stage. To me, the difference between the Space Shuttle and the Falcon 9 is kind of like the difference between a doctor of mechanical engineering and a mechanical engineer. The Space Shuttle was an amazing (and complex) piece of tech - the engines used liquid hydrogen and liquid oxygen, which is incredibly efficient but notoriously difficult to work with (cooling hydrogen to a liquid sate and keeping it cooled is not trivial). It was a long (~10 years) process to develop the Space Shuttle from the existing tech we already had. The result was a highly tuned and complex engine that was incredibly efficient. It makes you feel a bit warm and fuzzy at how efficient and well engineered the engines are. SpaceX went the other way - they said we'll use a slightly less efficient set up (RP-1 (refined kerosene) and liquid oxygen) as our fuel, but we'll make up for it by making each individual launch much much cheaper. So while they may not have the fuel efficiency of the Space Shuttle, for the cost of a single Space Shuttle launch (~$400M per launch according to wikipedia), SpaceX could launch 6 or 7 Falcon 9s (~$60M per launch). The SSMEs are very efficient engines. The Falcon 9 is a very efficient platform. It's kind of like a big picture vs micro optimization sort of thing. (There is no judgement here - the Space Shuttle was developed by a government with much different concerns and constraints than the private business developing the Falcon 9). Having a fixed wing aircraft come down from space really doesn't make that much sense when we get down to it. It's a really great and interesting concept, but the realities of it make it not worth the trouble. Sure the Space Shuttle glided, but even a brick going 17,000 miles an hour will glide for a long while. This video (https://www.youtube.com/watch?v=B3JZtY7bcbM https://www.youtube.com/watch?v=B3JZtY7bcbM) gives a great overview into the complexities of a plane-shaped spacecraft hitting the atmosphere at 7 km/s. When a capsule (like the Apollo Command Module) hits the atmosphere, you only need to protect one side of it, and you just let the thing fall like a stone then slow it down with parachutes. The Space Shuttle Orbiter had to have different materials with different heat shielding properties all over its body because it would glide through the air at different angles. The bottom of the orbiter had an enormous surface area, and it needed to be thermally insulated. But heat shields are very heavy, and that amount of weight would make the launches impractical and landing impossible. So NASA had to develop new, light materials that were incredibly strong and also heat resistant. The Falcon 9's first stage is not going nearly as fast as the Space Shuttle Orbiter when it hits the atmosphere, so it's not really appropriate to make a direct comparison about the complexities there. But this is to me another part of the brilliance of SpaceX looking at the big picture - it's much easier to reuse the first stage (which is typically the most expensive part of a launch vehicle) which doesn't reach orbital velocity and capture as big a chunk as the savings as possible rather than being totally 100% efficient. I would not be surprised if NASA was 50 or 60 years ahead of its time with the Space Shuttle. It seems like private industry is catching up to where NASA was with the Saturn V in the 60s just now (we're still not close to a rocket as powerful as the Saturn V). We may see that in 20 or 30 years, the industry uses the materials and knowledge gained from the Space Shuttle to develop new and novel spacecraft. Again, I'm just a programmer who loves rocketry and space - I apologize if there are any technically inaccurate bits in this post.
• bmcusick 10y ago
  > Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle? It's sort of apples vs oranges to compare the Shuttle main engines vs the Falcon 9 engines, because the Shuttle main engines were on the the crewed upper stage of the Shuttle stack and the Falcon 9 booster is the boost stage of the Falcon 9 stack. The direct comparison is comparing the Falcon 9 booster (first stage) with the solid rocket boosters of the Shuttle stack, and comparing the Shuttle with the Dragon. Dragon v1 has a heat shield (like Shuttle) and then parachutes (unlike Shuttle). Dragon 2 will have reusable engines to land propulsively, which means it will land on land like the Shuttle. The Falcon 9 booster is much, much more sophisticated than the Shuttle boosters. The Shuttle boosters were solid fueled; once lit they could not be turned off or throttled. They just burned at maximum thrust until they ran out of fuel, which is why they needed parachutes to land in the ocean. Unfortunately NASA learned that salt water is terrible for engines, even if they land gently, and the cost of refurbishing the boosters after each launch (and the amount of time it took) was much higher than anticipated. Falcon 9's Block 5 booster should (hopefully) reach a point where it can land, get refueled, and immediately take off again, like a 747. > I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title. For the reasons I have above, you were right the first time. SpaceX is trying to make the first reusable booster that achieves orbit. Both Shuttle and Blue Origin's New Shepard are reusable, but neither of them can reach orbit by themselves. > I'm also kindof curious why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home like the shuttle did. Scaling laws. Vertical take-off, vertical landing (VTVL) rockets scale to any size. Wings don't. The Falcon 9, the Falcon Heavy, and future mega-rockets will all be able to use the same basic technology. Wings don't scale well at all.
  • bluGill 10y ago
    > Vertical take-off, vertical landing (VTVL) rockets scale to any size. Wings don't. The Falcon 9, the Falcon Heavy, and future mega-rockets will all be able to use the same basic technology. Wings don't scale well at all. You are correct about the disadvantages of wings, but wrong about why SpaceX isn't using them. SpaceX has always had the goal of landing on Mars and returning home. All their rockets have been developed with this in mind. They tested their first rocket, and got it to work. They sold it to Nasa and the like (and even developed it with some of their requirements in mind), but the goal was always to learn how to make bigger rockets that can get to Mars, that Nasa is willing to pay for the smaller ones is a side benefit. Mars does not have enough atmosphere for wings to work, therefore SpaceX is not interested in Wings. Even if reuse of the rocket was clearly more expensive that building a new one each time (it isn't) SpaceX would be working on reusable rockets now because when they get to mars they need to land the rocket and latter reuse it to return back to earth.
• neaanopri 10y ago
  I think that the space shuttle is really a re-usable second stage, albeit one which is firing its engines for the entire ascent. The aerodynamic demands on a reusable second stage are much greater, so comparing the space shuttle to the Falcon 1st stage isn't exactly fair. The Falcon 1st stage is more analogous to the Solid Rocket Boosters, in that it's a very high-thrust, high-density launch vehicle. The Solid Rocket Boosters are strictly speaking doing exactly what the Falcon 9 booster is doing: being recoverable. However, the Falcon 9 boosters have much greater capability to be re-used without refurbishment, since they don't encounter salt water.
• creshal 10y ago
  > Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle? The Space Shuttle's engines were extremely complex beasts that begat the most advanced alloys at the time – very much bleeding edge, with the tightest tolerances possible, to squeeze out performance that wasn't thought possible before, using highly aggressive propellants (hydrogen embrittlement was a huge problem). As a result, they only had a tenth of their planned life expectancy (average of 5-6 instead of 55 flights), and needed an 80% refurbishment between every flight to replace cracked turbine blades etc. pp. Falcon 9's Merlin engines, meanwhile, are best described as "boring": They're firmly a 60s design made cheaper than dirt thanks to modern manufacturing technology, and highly reliable thanks to a simple design and safe fuels (kerosene). SpaceX already demonstrated on the ground that Merlins can perform multiple missions without needing refurbishment. Or, in car analogies: The Space Shuttle is a Jaguar, Falcon 9 is a Ford. While you can, on the paper, drive a Jaguar twice without seeing a workshop, nobody managed it yet. > I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title. Kinda, sorta, in theory: While the orbiters did refly a lot of times, the engines (the most expensive part of the Shuttle) barely did, only in the "Ship of Theseus" sense. Same for boosters (fishing steel tubes out of the ocean and hammering out the kinks was a pure work creation measure), and the huge external tanks were made anew each time. > I'm also kindof curious why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home like the shuttle did. Is that a weight-concern? Precision and weight: Wings are dead weight for a vertically launched rocket, and increase drag (especially supersonic) further than folded-up landing legs do. Hoverslam point landings need very little added mass on the rocket, and you expose a far smaller surface to re-entry heating etc.