4 ms·

Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch? Also, there is something about using the same engine on every stage to gat

Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch? Also, there is something about using the same engine on every stage to gather more data per launch, but iirc different engine profiles are used for different velocities and air densities long the launch. I wonder what's the efficiency loss of using a single engine profile for the whole launch.

mbell 12y ago
> Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch? No, it reduces payload to orbit by ~30% or so. Fuel is insanely cheap in comparison to the cost of building the 1st stage.
- knappador 12y ago
  The separation velocity they chose for the first stage is very promising. Given the cheap cost of fuel and difficulty of making the 2nd stage body able to survive the much-higher re-entry velocity, I wonder about separating just the engine package and essentially hiding it in a typical capsule shaped cover. In any case, staging based on reducing overall cost and difficulty of recovery are pretty cool. I'm willing to bet money on ceramic-ceramic composites adding enough re-entry options (in addition to lighter nozzles) that the staging is changed yet again.
wolf550e 12y ago
Elon Musk said the fuel costs $300,000. The first stage and 9 Merlin engines cost ~$40,000,000. The Merlin 1-D Vacuum used on the upper stage does have a wide vacuum optimized nozzle, but the other parts are almost the same, to save costs during manufacturing and testing. The article is ok, but says nothing new to people who have followed SpaceX for years. There is a lot of info online about the company and its products.
jccooper 12y ago
Probably not. You only need a few seconds of propellant for landing: a mostly-empty stage weighs much less than a full one, and even one engine is enough to slow it down real fast. I've done some really rough guess-numbers before, landing on 5-10% of a full propellant load needed for landing. It's probably not even as bad as that, since they certainly fly with some performance reserve already, at least several percent, and most payloads are probably not max-weight. The landing propellant in many cases may be entirely free, mass-budget-wise. They do add some additional mass for the legs and fins, in addition to any extra propellant they need, so it's not free, but the first stage is not as performance-sensitive as higher stages (see: rocket equation), so they can get away with it. I don't see 'em getting second stages back any time soon, though, and I think they've given up on that, at least for F9. WRT engines, the efficiency based on air density is mostly a function of the nozzle, and the upper-stage Merlins have a nozzle extension to deal with this. Higher-efficiency fuels (i.e LH2) are historically preferred for upper stages, since they are more performance-sensitive, so SpaceX is losing some performance there. But LH2 is super-cryogenic and a beast to work with, and an entirely different engine. SpaceX's thing is that they are willing to sacrifice maximum efficiency for reasonable cost, so they decided to make that tradeoff. It seems like a good plan so far.
- slashnull 12y ago
  The weight of the empty stage definitely is something I overlooked. Thanks for the answer!
- TylerE 12y ago
  Also, much of the power of stage 1 is spent moving air out of the way. That helps rather than hinders on the way down.
ceejayoz 12y ago
> Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch? It does reduce payload capacity (30-40%, IIRC) but the expected upside is drastic cost reductions. If I can launch several payloads for the price of one as a result, it becomes well worth it, and if I've got a large payload I can either use the upcoming Falcon Heavy or we'll start doing more in-orbit assembly.
InclinedPlane 12y ago
It's funny seeing people complain about throwing away fuel when we're already throwing away rocket engines now. Spoiler: the engines are more expensive.
Symmetry 12y ago
Thanks to the wonders of the rocket equation mass that is only present on the first stage is much cheaper than mass that has to be boosted through the second stage as well. And air resistance is a big deal so it helps a lot that it's now working for you rather than against you.

mbell 12y ago

> Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch? No, it reduces payload to orbit by ~30% or so. Fuel is insanely cheap in comparison to the cost of building the 1st stage.

knappador 12y ago
The separation velocity they chose for the first stage is very promising. Given the cheap cost of fuel and difficulty of making the 2nd stage body able to survive the much-higher re-entry velocity, I wonder about separating just the engine package and essentially hiding it in a typical capsule shaped cover. In any case, staging based on reducing overall cost and difficulty of recovery are pretty cool. I'm willing to bet money on ceramic-ceramic composites adding enough re-entry options (in addition to lighter nozzles) that the staging is changed yet again.

wolf550e 12y ago

Elon Musk said the fuel costs $300,000. The first stage and 9 Merlin engines cost ~$40,000,000. The Merlin 1-D Vacuum used on the upper stage does have a wide vacuum optimized nozzle, but the other parts are almost the same, to save costs during manufacturing and testing. The article is ok, but says nothing new to people who have followed SpaceX for years. There is a lot of info online about the company and its products.

jccooper 12y ago

Probably not. You only need a few seconds of propellant for landing: a mostly-empty stage weighs much less than a full one, and even one engine is enough to slow it down real fast. I've done some really rough guess-numbers before, landing on 5-10% of a full propellant load needed for landing. It's probably not even as bad as that, since they certainly fly with some performance reserve already, at least several percent, and most payloads are probably not max-weight. The landing propellant in many cases may be entirely free, mass-budget-wise. They do add some additional mass for the legs and fins, in addition to any extra propellant they need, so it's not free, but the first stage is not as performance-sensitive as higher stages (see: rocket equation), so they can get away with it. I don't see 'em getting second stages back any time soon, though, and I think they've given up on that, at least for F9. WRT engines, the efficiency based on air density is mostly a function of the nozzle, and the upper-stage Merlins have a nozzle extension to deal with this. Higher-efficiency fuels (i.e LH2) are historically preferred for upper stages, since they are more performance-sensitive, so SpaceX is losing some performance there. But LH2 is super-cryogenic and a beast to work with, and an entirely different engine. SpaceX's thing is that they are willing to sacrifice maximum efficiency for reasonable cost, so they decided to make that tradeoff. It seems like a good plan so far.

slashnull 12y ago
The weight of the empty stage definitely is something I overlooked. Thanks for the answer!
TylerE 12y ago
Also, much of the power of stage 1 is spent moving air out of the way. That helps rather than hinders on the way down.

ceejayoz 12y ago

> Okay, but, doesn't it, like, at least double the amount of fuel needed for a launch? It does reduce payload capacity (30-40%, IIRC) but the expected upside is drastic cost reductions. If I can launch several payloads for the price of one as a result, it becomes well worth it, and if I've got a large payload I can either use the upcoming Falcon Heavy or we'll start doing more in-orbit assembly.

InclinedPlane 12y ago

It's funny seeing people complain about throwing away fuel when we're already throwing away rocket engines now. Spoiler: the engines are more expensive.

Symmetry 12y ago

Thanks to the wonders of the rocket equation mass that is only present on the first stage is much cheaper than mass that has to be boosted through the second stage as well. And air resistance is a big deal so it helps a lot that it's now working for you rather than against you.