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Few questions. Is Grasshopper intended to be a vehicle that ships a payload into a low earth orbit and return itself back to the launch pad? With that, a quest

Few questions. Is Grasshopper intended to be a vehicle that ships a payload into a low earth orbit and return itself back to the launch pad? With that, a question of descent, once it's reached orbit and released its payload, would it re-enter with a short burn towards its earthly destination, decelerate with, say, parachutes, and then do its final descent onto the pad with the rocket? I can't see it doing its entire descent with a rocket, that would use so much fuel, and require so much extra weight.

ceejayoz 13y ago
I believe it's a technology demonstrator for the eventual version of the Dragon capsule that has powered landing at a spaceport rather than the current water splashdown.
- cecilpl 13y ago
  No, it's a demonstrator for a full reusable rocket that returns all stages to the launch pad autonomously.
jlgreco 13y ago
Grasshopper is a test vechicle, they plan on doing this with the F9R. My understanding is that the F9R will burn once to de-orbit (well, the first stage will be suborbital to begin with), aerobrake without a parachute for a short period, and then burn again for the final landing. I may be wrong about the aerobraking step, but they aren't planning on using a parachute regardless. IIRC they are expecting a 10% drop in total cargo to LEO when the F9 is used in the reusable configuration. It takes far less fuel to get down than it does to get up since they only have to decelerate the lower stage (not the upper, or the cargo) and since the lower stage will be mostly empty at that point (much lighter). Furthermore the initial decent burn will be at a much lower outside atmospheric pressure (near vacuum?) resulting in a higher efficiency.
- deleted 13y ago
  [deleted]
- puetzk 13y ago
  Also, the rocket will be falling at its terminal velocity, having lost the rest of its speed to air drag. Even without parachutes (and they don't intend to carry any), that's only ~120 m/s - the upper stage and payload are gone, the 1st stage has big tanks but only a little fuel left, so its density is low. The final burn for propulsive landing only has to cancel that last little bit.
  - jlgreco 13y ago
    Yup, air resistance works with you on the way down, not against you, and it is courteous enough to start small and work itself up when you are on the way down.
    - troels 13y ago
      Doesn't it result in a lit of heat from the friction? Or is that only a problem at much higher speeds?
      - jlgreco 13y ago
        There will be heat; even the suborbital Mercury-Redstone missions needed heat shields. I don't know how SpaceX is planning on handling that. Maybe the trajectory they put themselves on is going to be slow enough to survive reentry using just the main engine nozzles as make-shift heat shields?
        antubbs 13y ago
        Can't speak to the plan for the second stage (which will do a once-around and de-orbit if I understand correctly), but the first stage will not be following a ballistic trajectory and re-entry like a normal suborbital launch. Its goal is to return to the pad a few minutes after launch.
EthanHeilman 13y ago
Fuel is cheap/rockets are expensive, descent doesn't burn much fuel. This should allow mass to orbit at 1-10% of current cost. The big problem to be solve here is firing a rocket while traveling backward at high speed. If they can pull off the Phase 3 test (high speed descent burn), space access is revolutionized (a space business boom): * "transhab" space hotels (closer than you think, Bigelow already has two test stations in orbit), * lower cost human trips to mars, * massive increase in number, size, and speed of exploratory probes, * communications satellites for everything (Google in the sky). http://en.wikipedia.org/wiki/Grasshopper_(rocket)#Test_phases http://en.wikipedia.org/wiki/Grasshopper_(rocket)#Test_phase...
- svantana 13y ago
  >Fuel is cheap Not true considering you need to bring that extra fuel up in the first place, so you need a lot more fuel just to carry the weight of the descent fuel into space. It is a really tricky cost/benefit equation, but Musk seems to have figured that out.
  - jlgreco 13y ago
    The material cost of the fuel is something like $200k, which is peanuts for what launches as a whole usually cost. They cut into their payload to orbit budget to bring more fuel along to return with, but doing so allows them to reuse all the hardware which is the actually expensive part. Once you are able to re-use the hardware then the cost to orbit can be lowered, which means that the 10% payload cut you took earlier actually isn't all that expensive in the first place.
    - _delirium 13y ago
      That does assume that the hardware can be reused without significant refurbishing. The extensive maintenance needed due to damage incurred each flight was one reason the reusable shuttle ended up with a much higher per-launch cost than the expendable Soyuz. One can surely do much better than the shuttle, but cheap expendable systems are not that easy to beat with a reusable vehicle.
SuperChihuahua 13y ago
Here's a video explaining it all: http://youtu.be/abLC1l3loFA http://youtu.be/abLC1l3loFA
- jug6ernaut 13y ago
  Love how this video completely skips the most difficult aspects of the flight for the components. Going from inverted/horizontal entry flight to the vertical landing position.
  - jlgreco 13y ago
    If you look at the Falcon 9 renders on the spacex website, you'll see that the centermost engine sticks very slightly further out than the rest (also that video shows the old engine configuration, future Falcon 9's are going to have their engines arranged in a circle with one in the center.) That engine may be on a gimbal that allows a descent that would look something like the LEM's descent. Initial re-orientation in vacuum could be done with cold gas thrusters similar to what grasshopper currently has.
btilly 13y ago
To understand this project you have to remember that Elon Musk's goal is Mars colonization. The goal is a rapidly reusable launch system that can be deployed anywhere in the solar system. A parachute system would work well here on Earth, but the martian air is too thin for it to work there. Therefore they are aiming to make it work straight off of rockets. As for the fuel issue, if I remember Elon's figures right, SpaceX currently takes a $100 million rocket, fills it with $100k of fuel, launches it, and throws away the rocket. Hence the desire to reuse the rocket. He's far from the first to try this. But the potential failure mode that others encountered is along the lines of what you indicate. You wind up with a reusable rocket that gets to altitude and back, but which is unable to lift a useful payload on top of that. He thinks that he can avoid this, but has not proven it yet. However even if he needs 10x as much fuel per pound lifted into orbit, he's still saving money. (His actual goal is, I think, 10% of his current launch cost.)
danielweber 13y ago
Parachutes are really hard to get right. John Carmack knows a lot more about rocketry than (almost?) anyone on this board (definitely more than me), and he has had them fail multiple times.
- Gravityloss 13y ago
  To be precise, John wasn't involved in Armadillo much when they had the recent string of parachute failures.
chad_oliver 13y ago
If I remember correctly, if they used a maximum burn right before impact they would only need about 1 tonne of fuel to take the first stage from orbit to the ground. Of course, that would require some crazy control systems, but it's a useful lower bound.
jccooper 13y ago
Grasshopper is a test vehicle with the aim of making the first stage of the Falcon 9 reusable. Grasshopper, for example, has one motor, while an F9 first stage will have nine. (Thus the name.) The landing gear is also obviously not streamlined for rocket use. But the tankage and avionics and propulsion and such are all from an F9 first stage. So it's pretty close to what will really fly, but not quite. Though it may go into "space", a Falcon 9 first stage will not go to orbit; it is decidedly suborbital. This makes it much easier to recover, as it is not going that fast (for a rocket) at engine cut-off. (The second stage takes the payload the other 2/3 of the way to orbit. While they imagine recovering the second stage, that's a lot harder.) Being suborbital, the first stage probably will not do a retro-burn, as it will come back to Earth naturally. Entering the thicker part of the atmosphere it will reach its terminal velocity, possibly falling on its side to make that slower. They might use drogue chutes for this, or might not. Once it has fallen to some frighteningly small distance from the ground, it will right itself and use a small fraction of its launched fuel (probably close to what current F9 stages keep in reserve) to slow down to landing velocity from terminal, steering along the way to land exactly where it needs to go. I once did some rough numbers to guess that the landing propulsion will take 10-20s with 1-2 of the motors, using 5-10% of the fuel. (With only 5% fuel, a rocket stage is actually crazy light, which makes this work.) Hair-raising, but well within the capabilities of today's sensors, actuators, and computers. No one's actually done this yet, but it seems plausible. I worry mostly about the stage surviving the initial re-entry, because reports are that earlier F9 first stages, which were supposed to water-land on parachutes, were totally trashed, possibly during that event. SpaceX actually has that data, though, so that's either not the case or they have some plan to make it work.
- cdash 13y ago
  The F9R is planned to stage at a lower height to combat the problem you talked about of the first stage being trashed on their earlier attempts to recover it I do believe.

ceejayoz 13y ago

I believe it's a technology demonstrator for the eventual version of the Dragon capsule that has powered landing at a spaceport rather than the current water splashdown.

cecilpl 13y ago
No, it's a demonstrator for a full reusable rocket that returns all stages to the launch pad autonomously.

jlgreco 13y ago

Grasshopper is a test vechicle, they plan on doing this with the F9R. My understanding is that the F9R will burn once to de-orbit (well, the first stage will be suborbital to begin with), aerobrake without a parachute for a short period, and then burn again for the final landing. I may be wrong about the aerobraking step, but they aren't planning on using a parachute regardless. IIRC they are expecting a 10% drop in total cargo to LEO when the F9 is used in the reusable configuration. It takes far less fuel to get down than it does to get up since they only have to decelerate the lower stage (not the upper, or the cargo) and since the lower stage will be mostly empty at that point (much lighter). Furthermore the initial decent burn will be at a much lower outside atmospheric pressure (near vacuum?) resulting in a higher efficiency.

deleted 13y ago
[deleted]
puetzk 13y ago
Also, the rocket will be falling at its terminal velocity, having lost the rest of its speed to air drag. Even without parachutes (and they don't intend to carry any), that's only ~120 m/s - the upper stage and payload are gone, the 1st stage has big tanks but only a little fuel left, so its density is low. The final burn for propulsive landing only has to cancel that last little bit.
- jlgreco 13y ago
  Yup, air resistance works with you on the way down, not against you, and it is courteous enough to start small and work itself up when you are on the way down.
  - troels 13y ago
    Doesn't it result in a lit of heat from the friction? Or is that only a problem at much higher speeds?
    - jlgreco 13y ago
      There will be heat; even the suborbital Mercury-Redstone missions needed heat shields. I don't know how SpaceX is planning on handling that. Maybe the trajectory they put themselves on is going to be slow enough to survive reentry using just the main engine nozzles as make-shift heat shields?
      - antubbs 13y ago
        Can't speak to the plan for the second stage (which will do a once-around and de-orbit if I understand correctly), but the first stage will not be following a ballistic trajectory and re-entry like a normal suborbital launch. Its goal is to return to the pad a few minutes after launch.

EthanHeilman 13y ago

Fuel is cheap/rockets are expensive, descent doesn't burn much fuel. This should allow mass to orbit at 1-10% of current cost. The big problem to be solve here is firing a rocket while traveling backward at high speed. If they can pull off the Phase 3 test (high speed descent burn), space access is revolutionized (a space business boom): * "transhab" space hotels (closer than you think, Bigelow already has two test stations in orbit), * lower cost human trips to mars, * massive increase in number, size, and speed of exploratory probes, * communications satellites for everything (Google in the sky). http://en.wikipedia.org/wiki/Grasshopper_(rocket)#Test_phases http://en.wikipedia.org/wiki/Grasshopper_(rocket)#Test_phase...

svantana 13y ago
>Fuel is cheap Not true considering you need to bring that extra fuel up in the first place, so you need a lot more fuel just to carry the weight of the descent fuel into space. It is a really tricky cost/benefit equation, but Musk seems to have figured that out.
- jlgreco 13y ago
  The material cost of the fuel is something like $200k, which is peanuts for what launches as a whole usually cost. They cut into their payload to orbit budget to bring more fuel along to return with, but doing so allows them to reuse all the hardware which is the actually expensive part. Once you are able to re-use the hardware then the cost to orbit can be lowered, which means that the 10% payload cut you took earlier actually isn't all that expensive in the first place.
  - _delirium 13y ago
    That does assume that the hardware can be reused without significant refurbishing. The extensive maintenance needed due to damage incurred each flight was one reason the reusable shuttle ended up with a much higher per-launch cost than the expendable Soyuz. One can surely do much better than the shuttle, but cheap expendable systems are not that easy to beat with a reusable vehicle.

SuperChihuahua 13y ago

Here's a video explaining it all: http://youtu.be/abLC1l3loFA http://youtu.be/abLC1l3loFA

jug6ernaut 13y ago
Love how this video completely skips the most difficult aspects of the flight for the components. Going from inverted/horizontal entry flight to the vertical landing position.
- jlgreco 13y ago
  If you look at the Falcon 9 renders on the spacex website, you'll see that the centermost engine sticks very slightly further out than the rest (also that video shows the old engine configuration, future Falcon 9's are going to have their engines arranged in a circle with one in the center.) That engine may be on a gimbal that allows a descent that would look something like the LEM's descent. Initial re-orientation in vacuum could be done with cold gas thrusters similar to what grasshopper currently has.

btilly 13y ago

To understand this project you have to remember that Elon Musk's goal is Mars colonization. The goal is a rapidly reusable launch system that can be deployed anywhere in the solar system. A parachute system would work well here on Earth, but the martian air is too thin for it to work there. Therefore they are aiming to make it work straight off of rockets. As for the fuel issue, if I remember Elon's figures right, SpaceX currently takes a $100 million rocket, fills it with $100k of fuel, launches it, and throws away the rocket. Hence the desire to reuse the rocket. He's far from the first to try this. But the potential failure mode that others encountered is along the lines of what you indicate. You wind up with a reusable rocket that gets to altitude and back, but which is unable to lift a useful payload on top of that. He thinks that he can avoid this, but has not proven it yet. However even if he needs 10x as much fuel per pound lifted into orbit, he's still saving money. (His actual goal is, I think, 10% of his current launch cost.)

danielweber 13y ago

Parachutes are really hard to get right. John Carmack knows a lot more about rocketry than (almost?) anyone on this board (definitely more than me), and he has had them fail multiple times.

Gravityloss 13y ago
To be precise, John wasn't involved in Armadillo much when they had the recent string of parachute failures.

chad_oliver 13y ago

If I remember correctly, if they used a maximum burn right before impact they would only need about 1 tonne of fuel to take the first stage from orbit to the ground. Of course, that would require some crazy control systems, but it's a useful lower bound.

jccooper 13y ago

Grasshopper is a test vehicle with the aim of making the first stage of the Falcon 9 reusable. Grasshopper, for example, has one motor, while an F9 first stage will have nine. (Thus the name.) The landing gear is also obviously not streamlined for rocket use. But the tankage and avionics and propulsion and such are all from an F9 first stage. So it's pretty close to what will really fly, but not quite. Though it may go into "space", a Falcon 9 first stage will not go to orbit; it is decidedly suborbital. This makes it much easier to recover, as it is not going that fast (for a rocket) at engine cut-off. (The second stage takes the payload the other 2/3 of the way to orbit. While they imagine recovering the second stage, that's a lot harder.) Being suborbital, the first stage probably will not do a retro-burn, as it will come back to Earth naturally. Entering the thicker part of the atmosphere it will reach its terminal velocity, possibly falling on its side to make that slower. They might use drogue chutes for this, or might not. Once it has fallen to some frighteningly small distance from the ground, it will right itself and use a small fraction of its launched fuel (probably close to what current F9 stages keep in reserve) to slow down to landing velocity from terminal, steering along the way to land exactly where it needs to go. I once did some rough numbers to guess that the landing propulsion will take 10-20s with 1-2 of the motors, using 5-10% of the fuel. (With only 5% fuel, a rocket stage is actually crazy light, which makes this work.) Hair-raising, but well within the capabilities of today's sensors, actuators, and computers. No one's actually done this yet, but it seems plausible. I worry mostly about the stage surviving the initial re-entry, because reports are that earlier F9 first stages, which were supposed to water-land on parachutes, were totally trashed, possibly during that event. SpaceX actually has that data, though, so that's either not the case or they have some plan to make it work.

cdash 13y ago
The F9R is planned to stage at a lower height to combat the problem you talked about of the first stage being trashed on their earlier attempts to recover it I do believe.