SpaceX Dragon 2 Updates and Discussion - Thread 2

[Jim]

I would think it could get to ISS fast without the ISS having to maneuver.. would also use up some of those nasty hypergols.

Launch provides the fast ascent and not so much maneuvering propellant.

[dror]

My expectation is NASA will have SpaceX burn off the hypergol fuel before re-entry. It will be another event (like removing an LAS tower), but would be less risky then landing with the fuel as you say.

What's the best use of the fuel if not for landings? Can they decelerate enough to take stress off the heat shield?
Or should they reboost ISS before undocking? Can D2 do reboosts?

This may be a silli question,
Can it be designed to transfer the access fuel to the ISS?
I mean, apart from the effect on the Dragon's CoG

[starsilk]

I would think it could get to ISS fast without the ISS having to maneuver.. would also use up some of those nasty hypergols.

Launch provides the fast ascent and not so much maneuvering propellant.

seems like plentiful maneuvering propellant should make it possible to overcome an uncooperative target.

[tater]

I think I would have the crew spacecraft monitor parachute deployment, and if there were to be an unsurvivable parachute failure, then it propulsively lands in the ocean. The best use would be to protect the crew if at all possible, no?

(obviously it would have to be programmed for that as a possible contingency)

[Jim]

My expectation is NASA will have SpaceX burn off the hypergol fuel before re-entry. It will be another event (like removing an LAS tower), but would be less risky then landing with the fuel as you say.

What's the best use of the fuel if not for landings? Can they decelerate enough to take stress off the heat shield?
Or should they reboost ISS before undocking? Can D2 do reboosts?

This may be a silli question,
Can it be designed to transfer the access fuel to the ISS?
I mean, apart from the effect on the Dragon's CoG

no, it would have to dock to the russian segment for that

[Jim]

seems like plentiful maneuvering propellant should make it possible to overcome an uncooperative target.

It has nothing to do with uncooperative target.  And there is a limit to what propellant can do. 
Like I said, fast ascent is a launch targeting issue and not maneuvering propellant

[Norm38]

Jim,
It was mentioned earlier that ISS itself has to maneuver to support a fast rendezvous. Is that not true? If it is, isn't it more efficient for the smaller visiting vehicle to maneuver than the ISS?

[starsilk]

seems like plentiful maneuvering propellant should make it possible to overcome an uncooperative target.

It has nothing to do with uncooperative target.  And there is a limit to what propellant can do. 
Like I said, fast ascent is a launch targeting issue and not maneuvering propellant

Uncooperative = political games.

There's a limit to what propellant can do? That's an odd statement. A lot of propellant can do all sorts of things in orbit... And dragon 2 will have a LOT to burn off.

[Comga]

A question I could answer myself if I had enough time to program the equations:
If a Dragon in a circular orbit about the altitude of the ISS used all of its maneuvering fuel to enter an elliptical orbit, by how much could it increase its orbital period?  What's the max delta-V?
In reverse, this is the maximum phasing per orbit that could be utilized by a Dragon boosted by the second stage into an orbit with an apogee above the ISS. (Which is limited by the rocket performance.  Not sure how that would be calculated.)
This would be an inverted version of a normal launch to the ISS where the arriving ship "catches up" from below and behind" only it wouldn't be limited by the atmosphere.

[soltasto]

My expectation is NASA will have SpaceX burn off the hypergol fuel before re-entry. It will be another event (like removing an LAS tower), but would be less risky then landing with the fuel as you say.

What's the best use of the fuel if not for landings? Can they decelerate enough to take stress off the heat shield?
Or should they reboost ISS before undocking? Can D2 do reboosts?

This may be a silli question,
Can it be designed to transfer the access fuel to the ISS?
I mean, apart from the effect on the Dragon's CoG

no, it would have to dock to the russian segment for that

I theory a revision to the International docking standard and a subsequent upgrade to the ISS US segment docking adapters could allow for water, fuel, oxidizer and pressurant transfer.

This kind of transfer is currently "reserved" from sections 3.4.2 to 3.4.5 in the official document attached below

International Docking System Standard
(IDSS)
Interface Definition Document (IDD)
Revision E

[Jim]

There's a limit to what propellant can do? That's an odd statement. A lot of propellant can do all sorts of things in orbit... And dragon 2 will have a LOT to burn off.

 Not really "a lot" in the true scheme of thing. 

Not when the spacecraft has limited orbital life. 

Uncooperative = political games.

And wrong about that.  ISS maneuvering means more propellant has to be brought up.

[Jim]

revision to the International docking standard and a subsequent upgrade to the ISS US segment docking adapters could allow for water, fuel, oxidizer and pressurant transfer.

Not feasible to plumb the ISS US segment for el, oxidizer and pressurant transfer.  It would still have to go to the Russian segment.

[Jim]

Jeesh, doe this have to be explained again.  The two day transfer to the ISS allows for daily launch windows.  The fast transfer does not and the amount of spacecraft propellant has limited affect on this.

[starsilk]

There's a limit to what propellant can do? That's an odd statement. A lot of propellant can do all sorts of things in orbit... And dragon 2 will have a LOT to burn off.

 Not really "a lot" in the true scheme of thing. 

random internet speculators out there say 400-450 m/s. that seems like a lot.

Not when the spacecraft has limited orbital life. 

superdracos can apply all of that in < 10 seconds. not that they would want to, but there will be more than enough capability to apply the delta-v quickly.

Uncooperative = political games.

And wrong about that.  ISS maneuvering means more propellant has to be brought up.

which means money needs to be paid for fuel supply. which means political games.

far, far simpler for the spacecraft to do it than the space station.

ah well, instead they can take all that dangerous fuel to the ISS for 6 months. then do the world's largest re-entry burn instead.

[JasonAW3]

revision to the International docking standard and a subsequent upgrade to the ISS US segment docking adapters could allow for water, fuel, oxidizer and pressurant transfer.

Not feasible to plumb the ISS US segment for el, oxidizer and pressurant transfer.  It would still have to go to the Russian segment.

Jim, wasn't there some discussion about an interim propulsion module a couple of years back, when Russia was considering pulling their segment of the ISS out to form their own space station complex?

[Lars-J]

revision to the International docking standard and a subsequent upgrade to the ISS US segment docking adapters could allow for water, fuel, oxidizer and pressurant transfer.

Not feasible to plumb the ISS US segment for el, oxidizer and pressurant transfer.  It would still have to go to the Russian segment.

Jim, wasn't there some discussion about an interim propulsion module a couple of years back, when Russia was considering pulling their segment of the ISS out to form their own space station complex?

There were several variants proposed: https://en.wikipedia.org/wiki/ISS_Propulsion_Module

But none of them would use propellant transfer. It would have been an expendable stop gap module, and it was more of a back-up plan if Zvezda failed to launch.

[Jim]

random internet speculators out there say 400-450 m/s. that seems like a lot.

superdracos can apply all of that in < 10 seconds. not that they would want to, but there will be more than enough capability to apply the delta-v quickly.

Neither are going to make rendezvous quicker.

If the Dragon is not launched at the proper time, it will take many more days for it to catch up with the ISS. 

2 day rendezvous provides for launch opportunities every 24 hours - 23 minutes.

Fast transfer does not allow for 24hr scrub turnaround, the next opportunity is too soon and a day will have to be skipped.

The amount of Dragon propellant does not change this.

[Basto]

random internet speculators out there say 400-450 m/s. that seems like a lot.

superdracos can apply all of that in < 10 seconds. not that they would want to, but there will be more than enough capability to apply the delta-v quickly.

Neither are going to make rendezvous quicker.

If the Dragon is not launched at the proper time, it will take many more days for it to catch up with the ISS. 

2 day rendezvous provides for launch opportunities every 24 hours - 23 minutes.

Fast transfer does not allow for 24hr scrub turnaround, the next opportunity is too soon and a day will have to be skipped.

The amount of Dragon propellant does not change this.

I believe the disconnect here is people are misunderstanding what a "rapid" ISS rendezvous actually entails. It does not mean that the vehicle is actually traveling toward the ISS at a higher velocity. It just means you launch when the launch site is in the same plane AND ISS is at a specific angular distance to the launch site. Which means fewer launch windows.

I am pretty sure we do not want objects approaching the ISS for rendezvous with a velocity differential of 400+ M/S.

[LouScheffer]

random internet speculators out there say 400-450 m/s. that seems like a lot.

superdracos can apply all of that in < 10 seconds. not that they would want to, but there will be more than enough capability to apply the delta-v quickly.

Neither are going to make rendezvous quicker.

If the Dragon is not launched at the proper time, it will take many more days for it to catch up with the ISS. 

2 day rendezvous provides for launch opportunities every 24 hours - 23 minutes.

Fast transfer does not allow for 24hr scrub turnaround, the next opportunity is too soon and a day will have to be skipped.

The amount of Dragon propellant does not change this.

This is true with the current method of fast rendezvous, where you adjust the phase of ISS so it's in the right place when you launch in-plane.

But a different approach is certainly possible.  You launch when the ISS *phase* is right, then you do a plane correction to get into the ISS plane.   This is the approach used on Gemini 11, which achieved a 1.5 hour rendezvous.  Compared to the existing approach, this needs additional delta-V, to be provided by some combination of excess booster performance and payload maneuvering.  With this approach you get two opportunities each day.

Falcon 9 easily has enough raw performance to do this, if run expendable.  But if the booster does RTLS, then the margin is less and the booster may not be able to do all of the plane change needed.  In this case the amount of Dragon propellant may determine if this approach is possible.

This seems pretty unlikely, though. It's a  very different procedure, SpaceX does not currently offer  yaw steering, two day rendezvous for cargo seems fine, phasing for crewed missions only is not too much of  burden, and so on.

[starsilk]

random internet speculators out there say 400-450 m/s. that seems like a lot.

superdracos can apply all of that in < 10 seconds. not that they would want to, but there will be more than enough capability to apply the delta-v quickly.

Neither are going to make rendezvous quicker.

If the Dragon is not launched at the proper time, it will take many more days for it to catch up with the ISS. 

2 day rendezvous provides for launch opportunities every 24 hours - 23 minutes.

Fast transfer does not allow for 24hr scrub turnaround, the next opportunity is too soon and a day will have to be skipped.

The amount of Dragon propellant does not change this.

This is true with the current method of fast rendezvous, where you adjust the phase of ISS so it's in the right place when you launch in-plane.

But a different approach is certainly possible.  You launch when the ISS *phase* is right, then you do a plane correction to get into the ISS plane.   This is the approach used on Gemini 11, which achieved a 1.5 hour rendezvous.  Compared to the existing approach, this needs additional delta-V, to be provided by some combination of excess booster performance and payload maneuvering.  With this approach you get two opportunities each day.

Falcon 9 easily has enough raw performance to do this, if run expendable.  But if the booster does RTLS, then the margin is less and the booster may not be able to do all of the plane change needed.  In this case the amount of Dragon propellant may determine if this approach is possible.

This seems pretty unlikely, though. It's a  very different procedure, SpaceX does not currently offer  yaw steering, two day rendezvous for cargo seems fine, phasing for crewed missions only is not too much of  burden, and so on.

seems like a useful technology to (re) develop.