5 ms·

One thing I've not seen mentioned in the coverage so far: how much payload is sacrificed by the need to keep fuel in reserve for the return to base? I'm guessi

by mrec 11y ago

One thing I've not seen mentioned in the coverage so far: how much payload is sacrificed by the need to keep fuel in reserve for the return to base? I'm guessing the sacrifice is roughly equal to the mass of unburnt fuel in the booster at the point of booster separation, but don't much trust my intuition on these things.

msandford 11y ago
You're right not to trust your intuition! Good call. It's actually going to be much, much less than that due to the nature of the rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
- msandford 11y ago
  Given that I'm not wrong as the above link shows (http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge...) would anyone care to explain to me for a second why I'm getting downvoted? I'm happy to be less stupid if that's the problem, but I don't know what thing it was that I did that was stupid.
rory096 11y ago
About 30% returning to landing site, 15% returning to a barge downrange. Note that the rocket's still perfectly capable of running an expendable mission if the payload requires it. The mass figure on their website[1] is for reusable. [1] http://www.spacex.com/falcon9 http://www.spacex.com/falcon9
mannykannot 11y ago
I don't think you could allocate all of the landing fuel load to payload as any addition to the payload also requires additional 2nd. stage fuel.
- mikeash 11y ago
  You can run the first stage longer without any extra second stage fuel. Adding performance to either one (within limits) increases the performance of the system. Using the landing fuel for a launch would allow for a heavier payload without changing the second stage. The first stage would do more of the total work.
  - tcheard 11y ago
    Except that the longer you run the first stage, the longer you have to carry the weight of the first stage. It eventually becomes much more efficient to jettison the first stage and use a slightly less powerful but much lighter weight second stage, because you no longer need as much power to get through the atmosphere. Which is the whole point behind staging (https://en.wikipedia.org/wiki/Multistage_rocket#Optimal_staging_and_restricted_staging https://en.wikipedia.org/wiki/Multistage_rocket#Optimal_stag...). It turns out when you add 10 lbs of weight to the first stage for reuse you lose approximately 1 lbs of weight from the max payload. (http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge...) On the second stage every 1 lb of reuse you lose 1 lb of payload, which is one of the big reasons why second stage reuse isn't really feasible on the F9.
    - mikeash 11y ago
      That makes no sense. It's never advantageous to ditch the first stage while it's still producing thrust. At some point when designing a rocket, it's better to add performance to the second stage rather than the first. But when presented with an existing rocket, you'll achieve maximum payload capacity by burning all fuel in the first stage before staging. Every second it burns is more delta-v imparted to the payload. The only reason you'd stage early is if you want to save fuel for e.g. a landing attempt, if the first stage is too powerful and the acceleration or speed would become too great, or if you simply don't need the extra performance to get the payload to where it's supposed to go.
  - mannykannot 11y ago
    Ah yes, good point - I was implicitly assuming the liftoff weight was maxed out, by thinking that any upper stage weight increase would have to come from the 1st stage fuel load.
johntb86 11y ago
According to http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge... every 10lbs of recovery hardware loses you 1lb of payload, approximately.
ChuckMcM 11y ago
> how much payload is sacrificed by the need to keep fuel > in reserve for the return to base? Simple answer: None. A more detailed answer is that building a system which can be reused is an economic proposition. So that the Falcon 9 can lift X Metric tons to orbit for $Y. The way in which they keep the value $Y low is by re-using the first stage. Every satellite project knows the throw weight of all the common launch vehicles and their cost per kilo. And that is how you plan you satellite design. Now at the moment SpaceX gets 9 merlin engines and the first stage booster back for "free" (which is to say that the cost paid assumed it would be consumed in the launch) but as they learn what they can do they will use that cost savings to offer cheaper launch services (more business) until they have a full launch schedule and then keep any excess value for re-investment. But an interesting question is this, given that they have a "used" first stage, who would be willing to launch on it? It has no track record and no reliability statistics other than it worked at least once before. To develop that information you need to re-launch them. And I'm hoping that SpaceX will make available some higher risk but lower cost "seats" on those test flights.
- greglindahl 11y ago
  SES already said that they're willing to be the first customer for a used booster[1]. Before they get that far, it'd be interesting to see if SpaceX flies a used engine, since they've got "one engine out" capability. Which has already been tested twice. 1: http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse-falcon-9-%C2%AD-for-the-right-price/ http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse...
  - ChuckMcM 11y ago
    That article raises an interesting point I really hadn't considered, who owns the first stage? If you pay "full price" for a launch and SpaceX recovers the first stage to re-use, did the person who paid to launch it own it or does SpaceX? It was presumed lost of course. No doubt there is language in the contracts about that. Would be interesting to see a launch contract.
    - 11y ago

by mrec 11y ago

msandford 11y ago

You're right not to trust your intuition! Good call. It's actually going to be much, much less than that due to the nature of the rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

msandford 11y ago
Given that I'm not wrong as the above link shows (http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge...) would anyone care to explain to me for a second why I'm getting downvoted? I'm happy to be less stupid if that's the problem, but I don't know what thing it was that I did that was stupid.

rory096 11y ago

About 30% returning to landing site, 15% returning to a barge downrange. Note that the rocket's still perfectly capable of running an expendable mission if the payload requires it. The mass figure on their website[1] is for reusable. [1] http://www.spacex.com/falcon9 http://www.spacex.com/falcon9

mannykannot 11y ago

I don't think you could allocate all of the landing fuel load to payload as any addition to the payload also requires additional 2nd. stage fuel.

mikeash 11y ago
You can run the first stage longer without any extra second stage fuel. Adding performance to either one (within limits) increases the performance of the system. Using the landing fuel for a launch would allow for a heavier payload without changing the second stage. The first stage would do more of the total work.
- tcheard 11y ago
  Except that the longer you run the first stage, the longer you have to carry the weight of the first stage. It eventually becomes much more efficient to jettison the first stage and use a slightly less powerful but much lighter weight second stage, because you no longer need as much power to get through the atmosphere. Which is the whole point behind staging (https://en.wikipedia.org/wiki/Multistage_rocket#Optimal_staging_and_restricted_staging https://en.wikipedia.org/wiki/Multistage_rocket#Optimal_stag...). It turns out when you add 10 lbs of weight to the first stage for reuse you lose approximately 1 lbs of weight from the max payload. (http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge...) On the second stage every 1 lb of reuse you lose 1 lb of payload, which is one of the big reasons why second stage reuse isn't really feasible on the F9.
  - mikeash 11y ago
    That makes no sense. It's never advantageous to ditch the first stage while it's still producing thrust. At some point when designing a rocket, it's better to add performance to the second stage rather than the first. But when presented with an existing rocket, you'll achieve maximum payload capacity by burning all fuel in the first stage before staging. Every second it burns is more delta-v imparted to the payload. The only reason you'd stage early is if you want to save fuel for e.g. a landing attempt, if the first stage is too powerful and the acceleration or speed would become too great, or if you simply don't need the extra performance to get the payload to where it's supposed to go.
- mannykannot 11y ago
  Ah yes, good point - I was implicitly assuming the liftoff weight was maxed out, by thinking that any upper stage weight increase would have to come from the 1st stage fuel load.

johntb86 11y ago

According to http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge... every 10lbs of recovery hardware loses you 1lb of payload, approximately.

ChuckMcM 11y ago

> how much payload is sacrificed by the need to keep fuel > in reserve for the return to base? Simple answer: None. A more detailed answer is that building a system which can be reused is an economic proposition. So that the Falcon 9 can lift X Metric tons to orbit for $Y. The way in which they keep the value $Y low is by re-using the first stage. Every satellite project knows the throw weight of all the common launch vehicles and their cost per kilo. And that is how you plan you satellite design. Now at the moment SpaceX gets 9 merlin engines and the first stage booster back for "free" (which is to say that the cost paid assumed it would be consumed in the launch) but as they learn what they can do they will use that cost savings to offer cheaper launch services (more business) until they have a full launch schedule and then keep any excess value for re-investment. But an interesting question is this, given that they have a "used" first stage, who would be willing to launch on it? It has no track record and no reliability statistics other than it worked at least once before. To develop that information you need to re-launch them. And I'm hoping that SpaceX will make available some higher risk but lower cost "seats" on those test flights.

greglindahl 11y ago
SES already said that they're willing to be the first customer for a used booster[1]. Before they get that far, it'd be interesting to see if SpaceX flies a used engine, since they've got "one engine out" capability. Which has already been tested twice. 1: http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse-falcon-9-%C2%AD-for-the-right-price/ http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse...
- ChuckMcM 11y ago
  That article raises an interesting point I really hadn't considered, who owns the first stage? If you pay "full price" for a launch and SpaceX recovers the first stage to re-use, did the person who paid to launch it own it or does SpaceX? It was presumed lost of course. No doubt there is language in the contracts about that. Would be interesting to see a launch contract.
  - 11y ago