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I find this confusing. It's awesome, that's for sure. But it seems like lifting all the fuel needed to do a soft landing is inherently a tradeoff for less paylo

I find this confusing. It's awesome, that's for sure. But it seems like lifting all the fuel needed to do a soft landing is inherently a tradeoff for less payload capacity. What is the benefit, then? Is it just so much cheaper to be able to re-use the first two stages?

cdash 13y ago
There is an estimated 40% reduction in payload to become reusable but since the fuel is only less than 1% of the cost in launching the rocket it is pretty acceptable.
EthanHeilman 13y ago
Fuel is very inexpensive (~2% the cost of a rocket launch), rockets are very expensive. If you can trade off fuel for not having to build a new rocket you drop the cost to orbit by orders of magnitude. Additionally you can build more expensive rockets that are more efficient since rocket creation becomes a capital rather than a reoccurring cost.
- petenixey 13y ago
  But why not just use a parachute and airbags? They're going to be a vast amount lighter than the fuel you'd need for a rocket landing.
  - deletes 13y ago
    Parachute and/or airbags are not necessarily lighter than fuel on a rocket as big as falcon. Parachutes don't scale linearly with the weight of the cargo. Plus they introduce an additional complexity and a point of failure.
    - simonh 13y ago
      On large heavy objects they also impose significant stress on it's structure.
  - cryptoz 13y ago
    You cannot land a large payload on Mars with a parachute due to the very thin atmosphere.
    - EthanHeilman 13y ago
      When I first read this I thought you were missing the point, then I remembered Musk's ambition to live on mars and the planned landing sequence for the Red Dragon. Good point!
  - artursapek 13y ago
    I don't think a parachute would give them the amount of control they want over where it lands. Maybe the idea is to eventually have many of these things docking in the same area, like an airport. If you have a fleet of multi-million dollar rockets, you want to be in as much control of them as possible.
  - EthanHeilman 13y ago
    You can't direct where the rocket ends up, highly likely that the rocket will get damaged or much less likely that the rocket will damage something else (This is why NASA dumps it's rockets over the ocean). Then you have find a way to get the rocket home, which involves time and money. Why not just burn fuel (rocket fuel is only a few times more expensive that burning water) and bring the rocket home? If you need more payload build a bigger rocket. You could also add wings to the rocket and fly it home which has it's own set of trade-offs and benefits(see space shuttle).
    - TeMPOraL 13y ago
      > highly likely that the rocket will get damaged And because you don't know whether it is damaged or not (sometimes the damage may not be obvious), it's likely that the rocket will have to get a long post-flight inspection to check if it is suitable for another flight. It's something you can almost completely avoid if you land the rocket gently. I read somewhere, that the costs of recovering SRBs of Space Shuttle from the ocean and then inspecting and fixing them were many times greater than building another pair of boosters.
      - InclinedPlane 13y ago
        The SRB thing isn't true. The costs ended up being pretty much the same for new vs. refurbed. The thing is, with solid boosters the "rocket engine" tends to just be giant aluminum cylinders, almost all the complexity of the job is in casting the fuel and putting the segments together, which is completely orthogonal to the reusability aspects. Also, part of the allure of SRBs is that they are cheap to manufacture, comparatively (this is a false savings, due to increased operational complexity, but it's still very tempting), so even if a significant amount of money could be saved per SRB through reuse it wouldn't have affected the cost a launch much.
        bfe 13y ago
        The SRBs don't just impose higher operational complexity, but also far higher acoustic load that requires much higher structural strength and therefore weight on everything else.
  - InclinedPlane 13y ago
    Parachutes are a pain in the ass to replace. But more than that taking a dunk in the ocean isn't so good for rockets, and spending the better part of a day with a search and recovery team and all the relevant equipment necessary out looking for the rocket and hauling it back home isn't cheap, it also adds a lot of delay to the whole workflow. Compare that to a propulsive landing on the launch pad next to the assembly facilities. You don't have to have a whole special team of folks on hand. The first stage is back in your hands in a matter of hours, where it can be transported back to the assembly building using a simple crane or other specialized equipment. And it can be inserted into the processing workflow much faster than if it had been out in the ocean, and with less overhead of having to clean it, refurbish it, inspect it, etc.
    - anonymfus 13y ago
      But why everybody compares landing solely on parachutes in ocean with landing solely on engines on land? For example, Soyuz use multiple parachutes for slowing down and engines for guiding and final soft landing on land.
      - JshWright 13y ago
        The Soyouz lands 'somewhere' in a fairly large target area. The recovery overhead is still non-trivial. Something as big and 'light' as the F9 first stage will be slowed down tremendously by the atmosphere. No need to add all the extra complexity and weight of parachutes.
      - InclinedPlane 13y ago
        The Soyuz does not "soft land" anywhere. It basically crash lands at a speed which doesn't cause any injury. For a first stage rocket the advantages of coming down on land via just parachutes are pretty much non-existent to negative. First off, if the goal is to save fuel from having to do a propulsive return to the launch site then that's not going to happen (except for the 2nd stage and capsule). The US has a lot of sparsely inhabited land but it doesn't have the same huge swathes of uncared for steppe that Russia/Kazakhstan have where they can just dump spent rocket stages everywhere with nary a care. There are range safety issues there that can't easily be avoided. Second, a giant rocket stage coming down on just parachutes is going to be damaged more on land than at sea. If you're trying to avoid the weight of landing gear you're just going to end up with the rocket engines crunching into the ground, which isn't going to be good at any speed. OK, so you can't save RTLS fuel, and you can't avoid having landing gear, at that point the only difference is a tiny little dribble of fuel to bring the stage in for a controlled powered landing. So you might as well just do that and be done with it.
  - rst 13y ago
    Parachutes were the first thing they tried, on the first stages of the first Falcon 9 launches. It didn't work; the stages broke up on re-entry before they slowed down enough to deploy the parachutes. The response was to try active control to keep them intact --- and I guess they figure they might as well keep it all the way down.
  - danielweber 13y ago
    Parachutes are really hard. Ask Carmack at Armadillo, but make sure he's in a good mood first. It's not because he's an idiot or anything. Plus, parachutes create a new failure mode: parachute deploying when it shouldn't.
    - schiffern 13y ago
      …or not deploying when it should.
  - VLM 13y ago
    Think like systems administrator. They've gotta have a rocket engine and a nav system or its not much of a 1st stage. However adding parachutes, airbags, etc, is new systems. Which means much lower reliability and higher expense.
  - HodCarrier 13y ago
    Elon wants rapid (think aircraft), turn around times. That's what's driving the decisions.
Lambdanaut 13y ago
Basically, yes. Fuel costs make up a marginal part of the rocket's cost. One reason the Space Shuttle was such a massive failure was because it was supposed to be "re-usable", yet it really wasn't. It threw away most of its volume every time it launched, yet it still had to carry more fuel into space for the soft landing. It had downsides from both sides of the equation.
stuff4ben 13y ago
I thought the same thing. One of the Youtube commentswas pretty insightful in that it mentioned that when you do a parachute landing into the ocean, it's landing in salt water. Therefore the craft has to be taken apart inspected, cleaned, and put back together. So I'm assuming they've done the cost benefits and figured it's cheaper this way. I wonder if a combination parachute/thruster landing would be more feasible though? Sort of like the Mars Science Lab without the sky-crane.
- ryanbrush 13y ago
  Not to mention the operational costs of a small navy to recover the rocket and transport it to a launch site.
  - mikeash 13y ago
    I believe the "small navy" was a political and technological requirement and not something you'd need today. A single ship ought to do now. Political, because it was the Space Race, and a lot of it was about shows of force. And hey, why not? You have a bunch of carrier battle groups just waiting around for a hot war, and you need to have them out training anyway, so why not use them to pick up spacecraft from time to time? Technological, because guidance wasn't necessarily very accurate. It's interesting to look at the miss distances here: http://en.wikipedia.org/wiki/Splashdown_(spacecraft_landing)#Manned_spacecraft http://en.wikipedia.org/wiki/Splashdown_(spacecraft_landing)... Some of these landed hundreds of miles from their target. However, by the time Apollo came around, they were all very close. You definitely want a big recovery fleet to cover a lot of area when you can't be sure it'll land on target, but that's not so much of an issue these days.
    - emp_zealoth 13y ago
      I remember reading about the issue in a book called "This New Ocean" apparently by the time Apollo was on the go NASA had to ask the military to position the ships off to the side of the splashdown zone - the guidance improved so dramatically they were afraid of hitting the carrier directly
InclinedPlane 13y ago
Almost the entirety of the cost of an orbital launch is in manufacturing and operational costs. Cost of fuel is basically just noise. If you can reduce operational turnaround time and operational complexity, even at a cost of payload, then you'll save so much money it'll be worth it in the long term. SpaceX is aiming for a re-assemble, gas up and go workflow. If they can pull it off it might reduce their per flight launch costs by a factor of 10 in the short term and perhaps as much as 100 if they get really good at it (although that's likely several generations of hardware down the line, at best). With that sort of thing on offer a reduction in payload is easily justifiable. Imagine if somebody replaced your car with a version that had twice the carrying capacity, but would only run for one tank of gas. It wouldn't be a worthwhile trade would it?
- mikeash 13y ago
  To put it in perspective, the fuel used to launch a Space Shuttle cost in the neighborhood of $1 million, while the cost of the entire launch was somewhere around $500 million to $1 billion.
  - InclinedPlane 13y ago
    For the Falcon 9, it costs about $200k for propellant per launch (Kerosene is cheaper than LH2 and the F9 puts about 1/10th as much mass in orbit) while the price of the launch is about $50 million.
  - wiredfool 13y ago
    Another perspective is that the Falcon9 carries about as much kerosene as a 747.
    - mikeash 13y ago
      That's a good one, although a 747 gets its oxygen for free.
      - DavidSJ 13y ago
jordan0day 13y ago
I imagine there's a lot less fuel required to land than is required to take off. That is, on launch, the rocket is probably (eventually) reaching speeds of several thousand miles per hour. Upon landing, the atmosphere does most of the slowing-down for you, so you only need enough fuel to reduce the speed from terminal velocity (not sure what this would be for a rocket, probably several hundred miles an hour, at least) to 0. So, I'm sure it's not an insubstantial amount of fuel, but maybe less than you'd think?
- JshWright 13y ago
  Not to mention that the giant flying fuel tank (er... 'rocket') is _substantially_ lighter now that you've burned most of the fuel off.
  - InclinedPlane 13y ago
    And separated from the 2nd stage and the payload, of course.
- Laremere 13y ago
  Two other points to consider: Atmosphere is slowing you down as you go up as well, so lift off requires fighting that as well. Therefore the energy you've expended is not entirely stored in potential energy. (Fun fact: If it weren't for the atmosphere, rockets would actually take off almost horizontally. They go up at first to get out of the thickest air before going sideways.) The craft is much lighter, because it no longer has the payload, and also has used almost all of its fuel.
- Florin_Andrei 13y ago
  > I imagine there's a lot less fuel required to land than is required to take off. Right. At take off, the delta vee is the escape velocity PLUS all the losses due to friction, which are tremendous. At landing, the delta vee is only equal to terminal velocity. EDIT: Okay, plus some flying time on top of the landing point.
  - 13y ago
cbhl 13y ago
"With an advanced rocket you can do maybe two to three percent of your lift-off mass to orbit, typically. And then reusability subtracts two to three percent. So then you've got nothing toward or negative and that's also not helpful. So, the trick is to try to shift that from two to three percent in the expendable configuration, to make the rocket mass efficiency, engines efficiency, and so forth so much better so that it moves to around three and half to four percent in the expendable configuration and then try to get clever about the reusablility elements and try to drop that to around the the one and a half to two percent level so that you have a net payload of about two percent." http://youtu.be/vDwzmJpI4io?t=27m http://youtu.be/vDwzmJpI4io?t=27m (Watch from 27m for about 50 seconds.)
mtgx 13y ago
Fuel cost is only 2-3% of the mission's cost, according to Elon Musk.
Florin_Andrei 13y ago
As Elon Musk put it, it's the difference between filling up your car at the end of a trip, and then going on another trip immediately, versus having to build a whole new car almost from scratch at the end of each trip.
bdonlan 13y ago
According to http://www.popularmechanics.com/science/space/rockets/elon-musk-on-spacexs-reusable-rocket-plans-6653023 http://www.popularmechanics.com/science/space/rockets/elon-m... : > "The payload penalty for full and fast reusability versus an expendable version is roughly 40 percent," Musk says. "[But] propellant cost is less than 0.4 percent of the total flight cost. Even taking into account the payload reduction for reusability, the improvement is therefore theoretically over a hundred times."

cdash 13y ago

There is an estimated 40% reduction in payload to become reusable but since the fuel is only less than 1% of the cost in launching the rocket it is pretty acceptable.

EthanHeilman 13y ago

Fuel is very inexpensive (~2% the cost of a rocket launch), rockets are very expensive. If you can trade off fuel for not having to build a new rocket you drop the cost to orbit by orders of magnitude. Additionally you can build more expensive rockets that are more efficient since rocket creation becomes a capital rather than a reoccurring cost.

petenixey 13y ago
But why not just use a parachute and airbags? They're going to be a vast amount lighter than the fuel you'd need for a rocket landing.
- deletes 13y ago
  Parachute and/or airbags are not necessarily lighter than fuel on a rocket as big as falcon. Parachutes don't scale linearly with the weight of the cargo. Plus they introduce an additional complexity and a point of failure.
  - simonh 13y ago
    On large heavy objects they also impose significant stress on it's structure.
- cryptoz 13y ago
  You cannot land a large payload on Mars with a parachute due to the very thin atmosphere.
  - EthanHeilman 13y ago
    When I first read this I thought you were missing the point, then I remembered Musk's ambition to live on mars and the planned landing sequence for the Red Dragon. Good point!
- artursapek 13y ago
  I don't think a parachute would give them the amount of control they want over where it lands. Maybe the idea is to eventually have many of these things docking in the same area, like an airport. If you have a fleet of multi-million dollar rockets, you want to be in as much control of them as possible.
- EthanHeilman 13y ago
  You can't direct where the rocket ends up, highly likely that the rocket will get damaged or much less likely that the rocket will damage something else (This is why NASA dumps it's rockets over the ocean). Then you have find a way to get the rocket home, which involves time and money. Why not just burn fuel (rocket fuel is only a few times more expensive that burning water) and bring the rocket home? If you need more payload build a bigger rocket. You could also add wings to the rocket and fly it home which has it's own set of trade-offs and benefits(see space shuttle).
  - TeMPOraL 13y ago
    > highly likely that the rocket will get damaged And because you don't know whether it is damaged or not (sometimes the damage may not be obvious), it's likely that the rocket will have to get a long post-flight inspection to check if it is suitable for another flight. It's something you can almost completely avoid if you land the rocket gently. I read somewhere, that the costs of recovering SRBs of Space Shuttle from the ocean and then inspecting and fixing them were many times greater than building another pair of boosters.
    - InclinedPlane 13y ago
      The SRB thing isn't true. The costs ended up being pretty much the same for new vs. refurbed. The thing is, with solid boosters the "rocket engine" tends to just be giant aluminum cylinders, almost all the complexity of the job is in casting the fuel and putting the segments together, which is completely orthogonal to the reusability aspects. Also, part of the allure of SRBs is that they are cheap to manufacture, comparatively (this is a false savings, due to increased operational complexity, but it's still very tempting), so even if a significant amount of money could be saved per SRB through reuse it wouldn't have affected the cost a launch much.
      - bfe 13y ago
        The SRBs don't just impose higher operational complexity, but also far higher acoustic load that requires much higher structural strength and therefore weight on everything else.
- InclinedPlane 13y ago
  Parachutes are a pain in the ass to replace. But more than that taking a dunk in the ocean isn't so good for rockets, and spending the better part of a day with a search and recovery team and all the relevant equipment necessary out looking for the rocket and hauling it back home isn't cheap, it also adds a lot of delay to the whole workflow. Compare that to a propulsive landing on the launch pad next to the assembly facilities. You don't have to have a whole special team of folks on hand. The first stage is back in your hands in a matter of hours, where it can be transported back to the assembly building using a simple crane or other specialized equipment. And it can be inserted into the processing workflow much faster than if it had been out in the ocean, and with less overhead of having to clean it, refurbish it, inspect it, etc.
  - anonymfus 13y ago
    But why everybody compares landing solely on parachutes in ocean with landing solely on engines on land? For example, Soyuz use multiple parachutes for slowing down and engines for guiding and final soft landing on land.
    - JshWright 13y ago
      The Soyouz lands 'somewhere' in a fairly large target area. The recovery overhead is still non-trivial. Something as big and 'light' as the F9 first stage will be slowed down tremendously by the atmosphere. No need to add all the extra complexity and weight of parachutes.
    - InclinedPlane 13y ago
      The Soyuz does not "soft land" anywhere. It basically crash lands at a speed which doesn't cause any injury. For a first stage rocket the advantages of coming down on land via just parachutes are pretty much non-existent to negative. First off, if the goal is to save fuel from having to do a propulsive return to the launch site then that's not going to happen (except for the 2nd stage and capsule). The US has a lot of sparsely inhabited land but it doesn't have the same huge swathes of uncared for steppe that Russia/Kazakhstan have where they can just dump spent rocket stages everywhere with nary a care. There are range safety issues there that can't easily be avoided. Second, a giant rocket stage coming down on just parachutes is going to be damaged more on land than at sea. If you're trying to avoid the weight of landing gear you're just going to end up with the rocket engines crunching into the ground, which isn't going to be good at any speed. OK, so you can't save RTLS fuel, and you can't avoid having landing gear, at that point the only difference is a tiny little dribble of fuel to bring the stage in for a controlled powered landing. So you might as well just do that and be done with it.
- rst 13y ago
  Parachutes were the first thing they tried, on the first stages of the first Falcon 9 launches. It didn't work; the stages broke up on re-entry before they slowed down enough to deploy the parachutes. The response was to try active control to keep them intact --- and I guess they figure they might as well keep it all the way down.
- danielweber 13y ago
  Parachutes are really hard. Ask Carmack at Armadillo, but make sure he's in a good mood first. It's not because he's an idiot or anything. Plus, parachutes create a new failure mode: parachute deploying when it shouldn't.
  - schiffern 13y ago
    …or not deploying when it should.
- VLM 13y ago
  Think like systems administrator. They've gotta have a rocket engine and a nav system or its not much of a 1st stage. However adding parachutes, airbags, etc, is new systems. Which means much lower reliability and higher expense.
- HodCarrier 13y ago
  Elon wants rapid (think aircraft), turn around times. That's what's driving the decisions.

Lambdanaut 13y ago

Basically, yes. Fuel costs make up a marginal part of the rocket's cost. One reason the Space Shuttle was such a massive failure was because it was supposed to be "re-usable", yet it really wasn't. It threw away most of its volume every time it launched, yet it still had to carry more fuel into space for the soft landing. It had downsides from both sides of the equation.

stuff4ben 13y ago

I thought the same thing. One of the Youtube commentswas pretty insightful in that it mentioned that when you do a parachute landing into the ocean, it's landing in salt water. Therefore the craft has to be taken apart inspected, cleaned, and put back together. So I'm assuming they've done the cost benefits and figured it's cheaper this way. I wonder if a combination parachute/thruster landing would be more feasible though? Sort of like the Mars Science Lab without the sky-crane.

ryanbrush 13y ago
Not to mention the operational costs of a small navy to recover the rocket and transport it to a launch site.
- mikeash 13y ago
  I believe the "small navy" was a political and technological requirement and not something you'd need today. A single ship ought to do now. Political, because it was the Space Race, and a lot of it was about shows of force. And hey, why not? You have a bunch of carrier battle groups just waiting around for a hot war, and you need to have them out training anyway, so why not use them to pick up spacecraft from time to time? Technological, because guidance wasn't necessarily very accurate. It's interesting to look at the miss distances here: http://en.wikipedia.org/wiki/Splashdown_(spacecraft_landing)#Manned_spacecraft http://en.wikipedia.org/wiki/Splashdown_(spacecraft_landing)... Some of these landed hundreds of miles from their target. However, by the time Apollo came around, they were all very close. You definitely want a big recovery fleet to cover a lot of area when you can't be sure it'll land on target, but that's not so much of an issue these days.
  - emp_zealoth 13y ago
    I remember reading about the issue in a book called "This New Ocean" apparently by the time Apollo was on the go NASA had to ask the military to position the ships off to the side of the splashdown zone - the guidance improved so dramatically they were afraid of hitting the carrier directly

InclinedPlane 13y ago

Almost the entirety of the cost of an orbital launch is in manufacturing and operational costs. Cost of fuel is basically just noise. If you can reduce operational turnaround time and operational complexity, even at a cost of payload, then you'll save so much money it'll be worth it in the long term. SpaceX is aiming for a re-assemble, gas up and go workflow. If they can pull it off it might reduce their per flight launch costs by a factor of 10 in the short term and perhaps as much as 100 if they get really good at it (although that's likely several generations of hardware down the line, at best). With that sort of thing on offer a reduction in payload is easily justifiable. Imagine if somebody replaced your car with a version that had twice the carrying capacity, but would only run for one tank of gas. It wouldn't be a worthwhile trade would it?

mikeash 13y ago
To put it in perspective, the fuel used to launch a Space Shuttle cost in the neighborhood of $1 million, while the cost of the entire launch was somewhere around $500 million to $1 billion.
- InclinedPlane 13y ago
  For the Falcon 9, it costs about $200k for propellant per launch (Kerosene is cheaper than LH2 and the F9 puts about 1/10th as much mass in orbit) while the price of the launch is about $50 million.
- wiredfool 13y ago
  Another perspective is that the Falcon9 carries about as much kerosene as a 747.
  - mikeash 13y ago
    That's a good one, although a 747 gets its oxygen for free.
    - DavidSJ 13y ago

jordan0day 13y ago

I imagine there's a lot less fuel required to land than is required to take off. That is, on launch, the rocket is probably (eventually) reaching speeds of several thousand miles per hour. Upon landing, the atmosphere does most of the slowing-down for you, so you only need enough fuel to reduce the speed from terminal velocity (not sure what this would be for a rocket, probably several hundred miles an hour, at least) to 0. So, I'm sure it's not an insubstantial amount of fuel, but maybe less than you'd think?

JshWright 13y ago
Not to mention that the giant flying fuel tank (er... 'rocket') is _substantially_ lighter now that you've burned most of the fuel off.
- InclinedPlane 13y ago
  And separated from the 2nd stage and the payload, of course.
Laremere 13y ago
Two other points to consider: Atmosphere is slowing you down as you go up as well, so lift off requires fighting that as well. Therefore the energy you've expended is not entirely stored in potential energy. (Fun fact: If it weren't for the atmosphere, rockets would actually take off almost horizontally. They go up at first to get out of the thickest air before going sideways.) The craft is much lighter, because it no longer has the payload, and also has used almost all of its fuel.
Florin_Andrei 13y ago
> I imagine there's a lot less fuel required to land than is required to take off. Right. At take off, the delta vee is the escape velocity PLUS all the losses due to friction, which are tremendous. At landing, the delta vee is only equal to terminal velocity. EDIT: Okay, plus some flying time on top of the landing point.
- 13y ago

cbhl 13y ago

"With an advanced rocket you can do maybe two to three percent of your lift-off mass to orbit, typically. And then reusability subtracts two to three percent. So then you've got nothing toward or negative and that's also not helpful. So, the trick is to try to shift that from two to three percent in the expendable configuration, to make the rocket mass efficiency, engines efficiency, and so forth so much better so that it moves to around three and half to four percent in the expendable configuration and then try to get clever about the reusablility elements and try to drop that to around the the one and a half to two percent level so that you have a net payload of about two percent." http://youtu.be/vDwzmJpI4io?t=27m http://youtu.be/vDwzmJpI4io?t=27m (Watch from 27m for about 50 seconds.)

mtgx 13y ago

Fuel cost is only 2-3% of the mission's cost, according to Elon Musk.

Florin_Andrei 13y ago

As Elon Musk put it, it's the difference between filling up your car at the end of a trip, and then going on another trip immediately, versus having to build a whole new car almost from scratch at the end of each trip.

bdonlan 13y ago

According to http://www.popularmechanics.com/science/space/rockets/elon-musk-on-spacexs-reusable-rocket-plans-6653023 http://www.popularmechanics.com/science/space/rockets/elon-m... : > "The payload penalty for full and fast reusability versus an expendable version is roughly 40 percent," Musk says. "[But] propellant cost is less than 0.4 percent of the total flight cost. Even taking into account the payload reduction for reusability, the improvement is therefore theoretically over a hundred times."