Author Topic: SpaceX Dragon 2 Updates and Discussion - Thread 2  (Read 558702 times)

Offline skater

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1260 on: 05/25/2018 12:24 PM »
When they were planning propulsive landing, how did they plan to land for an abort?  Would aborts have left enough fuel for propulsive landing, or did they plan on carrying parachutes just for abort landings?

Offline clongton

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1261 on: 05/25/2018 12:37 PM »
When they were planning propulsive landing, how did they plan to land for an abort?  Would aborts have left enough fuel for propulsive landing, or did they plan on carrying parachutes just for abort landings?

An abort would result in a splashdown in the ocean under parachutes.
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Offline rpapo

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1262 on: 05/25/2018 02:30 PM »
Here's where I think we are: Berthing interface can take useful reboost-level loads but is not any better than the docking interface (and perhaps not as good). So - can SuperDracos throttle back enough to stay within ISS acceleration/jerk limits? You only need to fire an opposing pair, not all of them.
Another consideration: if the Dragon is docked in the normal place (PMA/IDA-2), then the SuperDracos will be pointed in the direction of orbit.  Firing them would slow down the ISS unless the entire ISS were first flipped from its normal orientation.  This is not a problem with the Russian Progress when they are docked at the opposite end of the station.

Similarly, if a Dragon 2 were docked at PMA/IDA-3, firing the SuperDracos in either of those orientations would result in the station spinning end over end.

And regular Dracos have the ability to fire in any direction, so the only remaining problem is impingement of the exhaust on parts of the ISS.  The Space Shuttle could avoid this, for the most part, because it extended so far above and below the docking port.  Again, none of this is an issue for a ship docked at the Russian end, whether it was a Progress or an ATV, since the exhaust fired away from the station.
« Last Edit: 05/25/2018 02:36 PM by rpapo »
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Online Lars-J

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1263 on: 05/25/2018 04:12 PM »
Here's where I think we are: Berthing interface can take useful reboost-level loads but is not any better than the docking interface (and perhaps not as good). So - can SuperDracos throttle back enough to stay within ISS acceleration/jerk limits? You only need to fire an opposing pair, not all of them.
Another consideration: if the Dragon is docked in the normal place (PMA/IDA-2), then the SuperDracos will be pointed in the direction of orbit.  Firing them would slow down the ISS unless the entire ISS were first flipped from its normal orientation.  This is not a problem with the Russian Progress when they are docked at the opposite end of the station.

Similarly, if a Dragon 2 were docked at PMA/IDA-3, firing the SuperDracos in either of those orientations would result in the station spinning end over end.

And regular Dracos have the ability to fire in any direction, so the only remaining problem is impingement of the exhaust on parts of the ISS.  The Space Shuttle could avoid this, for the most part, because it extended so far above and below the docking port.  Again, none of this is an issue for a ship docked at the Russian end, whether it was a Progress or an ATV, since the exhaust fired away from the station.

Turning the station around to allow reboost from the US segment side is not a big issue. It was done MANY times for Shuttle reboosts.

Offline Herb Schaltegger

Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1264 on: 05/25/2018 10:55 PM »
Here's where I think we are: Berthing interface can take useful reboost-level loads but is not any better than the docking interface (and perhaps not as good). So - can SuperDracos throttle back enough to stay within ISS acceleration/jerk limits? You only need to fire an opposing pair, not all of them.
Another consideration: if the Dragon is docked in the normal place (PMA/IDA-2), then the SuperDracos will be pointed in the direction of orbit.  Firing them would slow down the ISS unless the entire ISS were first flipped from its normal orientation.  This is not a problem with the Russian Progress when they are docked at the opposite end of the station.

Similarly, if a Dragon 2 were docked at PMA/IDA-3, firing the SuperDracos in either of those orientations would result in the station spinning end over end.

And regular Dracos have the ability to fire in any direction, so the only remaining problem is impingement of the exhaust on parts of the ISS.  The Space Shuttle could avoid this, for the most part, because it extended so far above and below the docking port.  Again, none of this is an issue for a ship docked at the Russian end, whether it was a Progress or an ATV, since the exhaust fired away from the station.

Turning the station around to allow reboost from the US segment side is not a big issue. It was done MANY times for Shuttle reboosts.

It actually *can* be a big deal, depending on a lot of things. The most significant of this is probably PV orientation (especially during high-beta periods) and interference with micro-g experiments. These concerns can be worked around, but if they can be avoided, so much the better.
« Last Edit: 05/25/2018 10:57 PM by Herb Schaltegger »
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Offline Joffan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1265 on: 05/26/2018 01:45 AM »
Here's where I think we are: Berthing interface can take useful reboost-level loads but is not any better than the docking interface (and perhaps not as good). So - can SuperDracos throttle back enough to stay within ISS acceleration/jerk limits? You only need to fire an opposing pair, not all of them.
Another consideration: if the Dragon is docked in the normal place (PMA/IDA-2), then the SuperDracos will be pointed in the direction of orbit.  Firing them would slow down the ISS unless the entire ISS were first flipped from its normal orientation.  This is not a problem with the Russian Progress when they are docked at the opposite end of the station.

Similarly, if a Dragon 2 were docked at PMA/IDA-3, firing the SuperDracos in either of those orientations would result in the station spinning end over end.

And regular Dracos have the ability to fire in any direction, so the only remaining problem is impingement of the exhaust on parts of the ISS.  The Space Shuttle could avoid this, for the most part, because it extended so far above and below the docking port.  Again, none of this is an issue for a ship docked at the Russian end, whether it was a Progress or an ATV, since the exhaust fired away from the station.

I strongly believe that any reboost would always fire thrusters (almost) directly away from the station (as a net effect). The load would need to be transmitted straight through the interface, not exerted off-axis - I doubt the docking bolts are intended for such loads. Which means that only ports with a centreline aligned to the center of gravity of the station could be used for reboost, and ports at the US end would need the station to be turned.

Again this should be a matter of record, for past reboosts, and in fact a matter of engineering design of the station, if anyone has that.


Turning the station around to allow reboost from the US segment side is not a big issue. It was done MANY times for Shuttle reboosts.

It actually *can* be a big deal, depending on a lot of things. The most significant of this is probably PV orientation (especially during high-beta periods) and interference with micro-g experiments. These concerns can be worked around, but if they can be avoided, so much the better.

Presumably the US segment typically being prograde side means that every US reboost will require the station to be turned. So the Dragon is not special in this regard.

« Last Edit: 05/26/2018 01:47 AM by Joffan »
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Offline guckyfan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1266 on: 05/26/2018 11:49 AM »
If the US side needs to take over boosting is there any reason to not turn the station permanently? As long as the Russians can do reboost there is no reason the US would have to do it at all. The ability is enough.

Offline deruch

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1267 on: 05/26/2018 01:50 PM »
If the US side needs to take over boosting is there any reason to not turn the station permanently? As long as the Russians can do reboost there is no reason the US would have to do it at all. The ability is enough.

It could mess with some of the external experiments that are predicated on specific orientation.  They'd have to spin it around horizontally, keeping nadir/zenith the same, because otherwise all the Earth-facing instruments would be pointed in the wrong direction.  But, then things like materials science projects that want to be facing ram or wake would be facing the wrong direction. 
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Offline guckyfan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1268 on: 05/26/2018 05:48 PM »
If the US side needs to take over boosting is there any reason to not turn the station permanently? As long as the Russians can do reboost there is no reason the US would have to do it at all. The ability is enough.

It could mess with some of the external experiments that are predicated on specific orientation.  They'd have to spin it around horizontally, keeping nadir/zenith the same, because otherwise all the Earth-facing instruments would be pointed in the wrong direction.  But, then things like materials science projects that want to be facing ram or wake would be facing the wrong direction.

I see, yes. But there would be ways around it. All such experiments are time limited, I think. They could reorient some as well. All a matter of timing experiments and preparing for the switch.

Offline Comga

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1269 on: 05/26/2018 06:22 PM »
I strongly believe that any reboost would always fire thrusters (almost) directly away from the station (as a net effect). The load would need to be transmitted straight through the interface, not exerted off-axis - I doubt the docking bolts are intended for such loads. Which means that only ports with a centreline aligned to the center of gravity of the station could be used for reboost, and ports at the US end would need the station to be turned.

Again this should be a matter of record, for past reboosts, and in fact a matter of engineering design of the station, if anyone has that.


Turning the station around to allow reboost from the US segment side is not a big issue. It was done MANY times for Shuttle reboosts.

It actually *can* be a big deal, depending on a lot of things. The most significant of this is probably PV orientation (especially during high-beta periods) and interference with micro-g experiments. These concerns can be worked around, but if they can be avoided, so much the better.

Presumably the US segment typically being prograde side means that every US reboost will require the station to be turned. So the Dragon is not special in this regard.

Chris G's recent article on Cygnus says that the current flight will reboost the station.
I have asked if anyone knows the planned ISS orientation or other details but no one has yet replied.

That said, there are more ways to do reboosts than simple, inline arrangements like with Progress.
There are probably several ways Dragon could do a reboost.
I once did a calculation that maintaining the ISS orbit would take tens of seconds per week of firing one pair SuperDracos if Dragon 2 were inline, either in the position of the Progress, (which would require some new adapter) or on  Node 2 Forward with the ISS swung around. 
The planning for this might be important in discussions of future participation with the Russians.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline deruch

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1270 on: 05/27/2018 07:59 AM »
I strongly believe that any reboost would always fire thrusters (almost) directly away from the station (as a net effect). The load would need to be transmitted straight through the interface, not exerted off-axis - I doubt the docking bolts are intended for such loads. Which means that only ports with a centreline aligned to the center of gravity of the station could be used for reboost, and ports at the US end would need the station to be turned.

Again this should be a matter of record, for past reboosts, and in fact a matter of engineering design of the station, if anyone has that.


Turning the station around to allow reboost from the US segment side is not a big issue. It was done MANY times for Shuttle reboosts.

It actually *can* be a big deal, depending on a lot of things. The most significant of this is probably PV orientation (especially during high-beta periods) and interference with micro-g experiments. These concerns can be worked around, but if they can be avoided, so much the better.

Presumably the US segment typically being prograde side means that every US reboost will require the station to be turned. So the Dragon is not special in this regard.

Chris G's recent article on Cygnus says that the current flight will reboost the station.
I have asked if anyone knows the planned ISS orientation or other details but no one has yet replied.

That said, there are more ways to do reboosts than simple, inline arrangements like with Progress.
There are probably several ways Dragon could do a reboost.
I once did a calculation that maintaining the ISS orbit would take tens of seconds per week of firing one pair SuperDracos if Dragon 2 were inline, either in the position of the Progress, (which would require some new adapter) or on  Node 2 Forward with the ISS swung around. 
The planning for this might be important in discussions of future participation with the Russians.

The most detailed response that I have seen on the plans to test reboost with the OA-9 Cygnus can be found in the Q&A portion of the post-launch status briefing for that mission.  ChrisB linked a youtube copy of the briefing in the final post of the Antares/OA-9 launch discussion thread:  https://forum.nasaspaceflight.com/index.php?topic=44199.msg1824952#msg1824952

The section discussing it starts ~16m:50s.  They didn't quite get into the specifics on station orientation.  But did say that the timing works out well because they were already going to be "pitching the station over for another reason anyways". 

By the way, the ISS program representative at that briefing, in response to a follow-up question, explicitly said that they don't currently have any plans to use Dragon 2 to do reboosts even though the docking location would actually be better. 
Shouldn't reality posts be in "Advanced concepts"?  --Nomadd

Offline Comga

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1271 on: 05/27/2018 05:28 PM »
Chris G's recent article on Cygnus says that the current flight will reboost the station.
I have asked if anyone knows the planned ISS orientation or other details but no one has yet replied.

That said, there are more ways to do reboosts than simple, inline arrangements like with Progress.
There are probably several ways Dragon could do a reboost.
I once did a calculation that maintaining the ISS orbit would take tens of seconds per week of firing one pair SuperDracos if Dragon 2 were inline, either in the position of the Progress, (which would require some new adapter) or on  Node 2 Forward with the ISS swung around. 
The planning for this might be important in discussions of future participation with the Russians.

The most detailed response that I have seen on the plans to test reboost with the OA-9 Cygnus can be found in the Q&A portion of the post-launch status briefing for that mission.  ChrisB linked a youtube copy of the briefing in the final post of the Antares/OA-9 launch discussion thread:  https://forum.nasaspaceflight.com/index.php?topic=44199.msg1824952#msg1824952

The section discussing it starts ~16m:50s.  They didn't quite get into the specifics on station orientation.  But did say that the timing works out well because they were already going to be "pitching the station over for another reason anyways". 

By the way, the ISS program representative at that briefing, in response to a follow-up question, explicitly said that they don't currently have any plans to use Dragon 2 to do reboosts even though the docking location would actually be better. 

That made NO sense.
The discussion centered on the availability of fuel, including how OATK can modify Cygnus to carry MORE fuel.
However, with NASA demanding that Dragon 2 lands under parachutes in the ocean the abort fuel is fully avaiilable for station reboost. After deorbit it itís probably more of a liability than a benefit. It would be better to burn it on orbit particularly with engines that face fully away from the ISS.
And the axial port is better for reboost as you said.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Joffan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1272 on: 05/27/2018 08:04 PM »
The most detailed response that I have seen on the plans to test reboost with the OA-9 Cygnus can be found in the Q&A portion of the post-launch status briefing for that mission.  ChrisB linked a youtube copy of the briefing in the final post of the Antares/OA-9 launch discussion thread:  https://forum.nasaspaceflight.com/index.php?topic=44199.msg1824952#msg1824952

The section discussing it starts ~16m:50s.  They didn't quite get into the specifics on station orientation.  But did say that the timing works out well because they were already going to be "pitching the station over for another reason anyways". 

By the way, the ISS program representative at that briefing, in response to a follow-up question, explicitly said that they don't currently have any plans to use Dragon 2 to do reboosts even though the docking location would actually be better. 

That made NO sense.
The discussion centered on the availability of fuel, including how OATK can modify Cygnus to carry MORE fuel.
However, with NASA demanding that Dragon 2 lands under parachutes in the ocean the abort fuel is fully available for station reboost. After deorbit it itís probably more of a liability than a benefit. It would be better to burn it on orbit particularly with engines that face fully away from the ISS.
And the axial port is better for reboost as you said.

One thing hinted at there is that Dragon's value in carrying downmass means that, before undertaking the same kind of testing process as Cygnus, they would have to satisfy themselves that using contingency fuel for reboost didn't impact the likelihood of success in that return phase. I didn't hear any other major objections - just that there is no plan in hand, which is as I would expect with Dragon 2 never having flown to ISS yet.
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Offline M.E.T.

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1273 on: 05/28/2018 10:25 AM »
If it was the landing legs poking through the heatshield that created the unacceptable risk for NASA, why could the ability to propulsively land on the heatshield not have been retained as a backup landing mechanism in case of chute failure?

That would really have been a nice differentiator, made possible by hardware already built into the design.

Online speedevil

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1274 on: 05/28/2018 10:39 AM »
If it was the landing legs poking through the heatshield that created the unacceptable risk for NASA, why could the ability to propulsively land on the heatshield not have been retained as a backup landing mechanism in case of chute failure?
This makes sense if you use logic, but less so if to do so may complicate and make more expensive the testing - for example NASA may require considerable advanced testing of such a contingency measure, and it may be both expensive and delaying to do.

Offline Johnnyhinbos

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1275 on: 05/28/2018 11:41 AM »
If it was the landing legs poking through the heatshield that created the unacceptable risk for NASA, why could the ability to propulsively land on the heatshield not have been retained as a backup landing mechanism in case of chute failure?

That would really have been a nice differentiator, made possible by hardware already built into the design.
Perhaps because keeping all that hypergolic fuel just for a backup presents a greater risk than the risk it mitigates...
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Offline clongton

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1276 on: 05/28/2018 11:57 AM »
If it was the landing legs poking through the heatshield that created the unacceptable risk for NASA, why could the ability to propulsively land on the heatshield not have been retained as a backup landing mechanism in case of chute failure?

That would really have been a nice differentiator, made possible by hardware already built into the design.
Perhaps because keeping all that hypergolic fuel just for a backup presents a greater risk than the risk it mitigates...

To date I have seen no mention of hypergolic venting prior to splashdown.
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Offline Elmar Moelzer

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1277 on: 05/28/2018 03:20 PM »
If it was the landing legs poking through the heatshield that created the unacceptable risk for NASA, why could the ability to propulsively land on the heatshield not have been retained as a backup landing mechanism in case of chute failure?

That would really have been a nice differentiator, made possible by hardware already built into the design.
I don't quite get that problem. The Dragon already has attachment points that stick through the heat shield and event he shuttle had things penetrating the TPS in various places.

Offline Nomadd

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1278 on: 05/28/2018 03:27 PM »


That made NO sense.
The discussion centered on the availability of fuel, including how OATK can modify Cygnus to carry MORE fuel.
However, with NASA demanding that Dragon 2 lands under parachutes in the ocean the abort fuel is fully avaiilable for station reboost. After deorbit it itís probably more of a liability than a benefit. It would be better to burn it on orbit particularly with engines that face fully away from the ISS.
And the axial port is better for reboost as you said.
Abort fuel wouldn't be "fully" available. It would be used for regular orbital maneuvering if not used for abort. I don't know how much extra they might have.

Offline envy887

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #1279 on: 05/28/2018 03:43 PM »


That made NO sense.
The discussion centered on the availability of fuel, including how OATK can modify Cygnus to carry MORE fuel.
However, with NASA demanding that Dragon 2 lands under parachutes in the ocean the abort fuel is fully avaiilable for station reboost. After deorbit it itís probably more of a liability than a benefit. It would be better to burn it on orbit particularly with engines that face fully away from the ISS.
And the axial port is better for reboost as you said.
Abort fuel wouldn't be "fully" available. It would be used for regular orbital maneuvering if not used for abort. I don't know how much extra they might have.

There should be enough for propulsive landing (plus margin) and also orbital maneuvering (plus margin), since the nominal mission profile included both orbital maneuvering and propulsive landing.

Propulsive landing probably required over 500 m/s, and now that should be fully available on a nominal mission.

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