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The Falcon 9 upper stage didn't run out of propellant! It's that their ISS safety window didn't permit refiring. Orbcomm said it in their own report: "The OG2

The Falcon 9 upper stage didn't run out of propellant! It's that their ISS safety window didn't permit refiring. Orbcomm said it in their own report: "The OG2 prototype satellite, flying as a secondary payload on this mission, was separated from the Falcon 9 launch vehicle at approximately 9:00 pm EST. However, due to an anomaly on one of the Falcon 9’s first stage engines, the rocket did not comply with a pre-planned International Space Station (ISS) safety gate to allow it to execute the second burn. For this reason, the OG2 prototype satellite was deployed into an orbit that was lower than intended. ORBCOMM and Sierra Nevada Corporation engineers have been in contact with the satellite and are working to determine if and the extent to which the orbit can be raised to an operational orbit using the satellite’s on-board propulsion system" http://www.orbcomm.com/Collateral/Documents/English-US/ORBCOMM%20Launches%20Prototype%20OG2%20Satellite%20FINAL.pdf http://www.orbcomm.com/Collateral/Documents/English-US/ORBCO...

smoyer 14y ago
I found the article misleading as well ... if an engine was shut down, you'd assume (with some margin of error) that the remaining 8 engines would fire 9/8th as long to provide the same total thrust and would therefore burn about as much propellant as the original 9 engines.
- smackfu 14y ago
  Depends on where in the flight the engine stopped providing thrust too.
- maaku 14y ago
  Actually, rocket math isn't quite as easy as that and they burn quite a bit more, but yes that's a simplistic explanation of what happened.
- brunosan 14y ago
  Longer thrust would not be equivalent to bigger thrusts for non vertical paths, neither the arrival time or vectors once there. That's probably related. I can also imagine that even the weight profile will change, thus leading to different paths...
- jccooper 14y ago
  Orbital rendezvous with the ISS has a very tight window (2.5-10 minutes), since on-orbit propellant is very limited. They could add more, and make the window a bit bigger, but that subtracts directly from cargo, which is already small: just 1000 lbs. The launcher has to drop Dragon basically right underneath the ISS; it approaches by raising altitude just several km. (In orbit, speeding up or slowing down means a change in altitude as well, so catching up or slowing down requires a lot more fuel.) Since the second stage drops off Dragon just below and a little behind the ISS, a second burn to raise the orbit would necessarily take it past the ISS. NASA is, understandably, very cautious around the ISS. I'm a bit surprised the even allowed a secondary payload boost. It was probably on very, very tight conditions. Making up for the lost engine probably got them to about the right place, but the different flight profile meant it wasn't the perfect place. Presumably it was inside Dragon's maneuver window, but outside the parameters NASA set for the secondary payload. This could happen if the second stage got too far "ahead" of the ISS due to a longer, lower burn, putting the second stage stack immediately below the ISS instead of below and behind it. Dragon, being in a lower (slower) orbit, can just wait a bit and get in the right position, but the booster could not proceed immediately as it would go too close to the ISS and could not wait as it is battery powered. Thus: secondary payload doesn't get its orbit. Missing the secondary's orbit is unfortunate, but that is the nature of the secondary payload. You take a gamble in return for a discounted ride. It's appropriate for a prototype, as this is, and they'll probably still get their primary mission done--testing that it functions properly in orbit. Probably won't get to use it in their constellation if it works, which would have been gravy.

smoyer 14y ago

I found the article misleading as well ... if an engine was shut down, you'd assume (with some margin of error) that the remaining 8 engines would fire 9/8th as long to provide the same total thrust and would therefore burn about as much propellant as the original 9 engines.

smackfu 14y ago
Depends on where in the flight the engine stopped providing thrust too.
maaku 14y ago
Actually, rocket math isn't quite as easy as that and they burn quite a bit more, but yes that's a simplistic explanation of what happened.
brunosan 14y ago
Longer thrust would not be equivalent to bigger thrusts for non vertical paths, neither the arrival time or vectors once there. That's probably related. I can also imagine that even the weight profile will change, thus leading to different paths...
jccooper 14y ago
Orbital rendezvous with the ISS has a very tight window (2.5-10 minutes), since on-orbit propellant is very limited. They could add more, and make the window a bit bigger, but that subtracts directly from cargo, which is already small: just 1000 lbs. The launcher has to drop Dragon basically right underneath the ISS; it approaches by raising altitude just several km. (In orbit, speeding up or slowing down means a change in altitude as well, so catching up or slowing down requires a lot more fuel.) Since the second stage drops off Dragon just below and a little behind the ISS, a second burn to raise the orbit would necessarily take it past the ISS. NASA is, understandably, very cautious around the ISS. I'm a bit surprised the even allowed a secondary payload boost. It was probably on very, very tight conditions. Making up for the lost engine probably got them to about the right place, but the different flight profile meant it wasn't the perfect place. Presumably it was inside Dragon's maneuver window, but outside the parameters NASA set for the secondary payload. This could happen if the second stage got too far "ahead" of the ISS due to a longer, lower burn, putting the second stage stack immediately below the ISS instead of below and behind it. Dragon, being in a lower (slower) orbit, can just wait a bit and get in the right position, but the booster could not proceed immediately as it would go too close to the ISS and could not wait as it is battery powered. Thus: secondary payload doesn't get its orbit. Missing the secondary's orbit is unfortunate, but that is the nature of the secondary payload. You take a gamble in return for a discounted ride. It's appropriate for a prototype, as this is, and they'll probably still get their primary mission done--testing that it functions properly in orbit. Probably won't get to use it in their constellation if it works, which would have been gravy.