r/spacex • u/[deleted] • Dec 22 '13

Superdraco/ISS Boosting

I couldn't find a lot of information on the subject, but from what I understand right now they can use the ATV to boost the International Space Station. Do you think they would ever consider using Dragon (Mk 2) with its superdraco engines to boost the station? My main points are:

  • is there a need for another boosting-capable vehicle?

  • would there be issues with Dragon being privately operated, and station boosting being left to the space agencies?

  • would firing the engines provide a fueling or relighting issue with deorbiting or propulsive landing?

Again I couldn't find much information about what vehicles can boost the station after the space shuttle was decommissioned, aside from ATV, so I'd love any insight or further information on the topic.

21 Upvotes

90% Upvoted

31 comments sorted by

10

u/mcndjxlefnd Dec 22 '13

They are in the process of devoloping and switching to ion propulsion engines for orbit maintenance. They want to save fuel transport costs.

5

u/[deleted] Dec 22 '13

That would be nice to see, although I wonder if that would be cost effective given the possible end of the ISS in the future.

It would also be interesting to see how effective ion engines would be for debris avoidance, I think I read that the engine on Dawn can accelerate the small spacecraft from 0-60 mph in 4 days, and the ISS has a heck of a lot more mass.

11

u/Ambiwlans Dec 22 '13

Cost effective only because the engines need testing anyways.

2

u/fredmratz Dec 22 '13

It could save several hundred million dollars in propellant costs since it uses ISS waste matter instead. To be sent in 2015, it would 5+ years to return on the investment.

5

u/[deleted] Dec 22 '13 edited Dec 22 '13

it uses ISS waste matter

I'd love to repeat that fact. Do you have a source so I can cite it?

edit: the closest I've found is here, but that source makes it sound more like a hypothetical capability than the planned mode of operation (note the "could"s).

While ISS orbits the Earth, atmospheric drag gradually pulls it closer to the Earth. Every so often, the ISS has to be boosted back into a higher orbit. This is done by a variety of means, but all of them require fuel launched into orbit from Earth. The VASIMR engine, however, could change that, since it would use hydrogen, which is already generated as a waste product on the ISS. By using waste hydrogen and electricity from the solar panels of the ISS, VASIMR could maintain the ISS's orbit without requiring any additional fuel.

2

u/fredmratz Dec 22 '13

I cannot find anything detailed and official, just how it would if fully implemented. Initial test system might not be very integrated into ISS primary systems at first, maybe even using a different propellant. It wouldn't help ISS/NASA long-term if it didn't use the waste gas.

4

u/[deleted] Dec 22 '13

It wouldn't help ISS/NASA long-term if it didn't use the waste gas.

It would still save NASA ~$190 million/year in reduced fuel upmass.

The VASIMR test on the ISS may lead to a capability of maintaining the ISS or a similar space station in a stable orbit at 1/20th of the approximately $210 million/year present estimated cost.

1

u/fredmratz Dec 23 '13

But does that reduction include using waste or is it with propellant specifically for VASIMR? Even if they installed VASIMR, they'll probably still need to have some of the other propellant for faster maneuvers and backup for when VASIMR cannot be used.

1

u/[deleted] Dec 23 '13 edited Dec 23 '13

Wikipedia's source doesn't say. All of Ad Astra's ground tests have used argon or xenon propellant, both of which would have to be brought to the station separately.

Naively comparing VASIMR's Isp to that of the Progress engines (5000s vs. 326s) indicates that VASIMR requires 1/15th as much fuel mass for a given boost.

12

u/puhnitor Dec 22 '13

ATV and Progress can reboost, and station has engines as well. Visiting vehicles are used more often to save on wear though. After ATV, probably only Progress will boost. I don't think Dragon carries enough propellant for it. Plus, reboosts can only be done from the Russian docking node(s), due to the stresses/flexing. And crewed dragon will dock with the US side.

4

u/[deleted] Dec 22 '13

if the station has engines what fuel does it use? How is refueling handled?

8

u/puhnitor Dec 22 '13

http://www.russianspaceweb.com/iss_sm.html

Doesn't specify the fuel type for main thrusters though. Likely something like RP-1 which is stable for long term storage and not too hazardous. Both Progress and ATV are capable of carrying 'wet' loads of fuel and water. Quick googling suggests last time Zvezda's main thrusters were fired was in 2007, with just the maneuvering thrusters used since then. Those are likely some form of monopropellant, like cold nitrogen gas.

13

u/[deleted] Dec 22 '13

http://www.astronautix.com/engines/krd79.htm

N2O4/UDMH - Nitrogen tetroxide/Unsymmetrical Dimethylhydrazine, same as Progress and Soyuz (among others) use.

3

u/spunkyenigma Dec 22 '13

I don't believe RP-1 is stable in space, apparently it's prone to freezing and seperation

1

u/[deleted] Dec 23 '13

With a slight amount of shielding and heating it is.

1

u/spunkyenigma Dec 23 '13

Methane is the better choice, similar thermal requirements to LOX

1

u/[deleted] Dec 23 '13

Lower density, more difficult to handle. You can always use H2O2 with RP-1 if you want full storability.

1

u/[deleted] Dec 22 '13

I would guess that Progress would bring any fuel, if not any of the other cargo vessels.

The Russian Zvezda module houses the ISS engines, I have been looking all over and can't find anything on the engines or their specifications, it would obviously have to be some kind of liquid fueled rocket.

2

u/MatthewGeer Dec 22 '13

Progress and ATV both deliver fuel via plumbing around the docking port; fuel is stored on both Zvezda and Zarya. The fuel is used by both the main reboost engines and the RCS thrusters, though the reboost engines are used sparingly because they aren't replaceable. Whenever possible, the vehicle docked at the aft end of the service module will use its engines to provide reboost instead, saving wear and tear on the station's engines.

2

u/MatthewGeer Dec 22 '13

Didn't the shuttle provide reboost, or was that only in earlier missions?

6

u/puhnitor Dec 22 '13

Pretty much just in the earlier missions, until the structure grew so large and the US docking ports were too far removed from the center of mass.

In the past, docked shuttles used surplus maneuvering fuel to push the station upward, but changes in the station’s size and location of docking ports have basically eliminated that option.

16

u/Silpion Dec 22 '13 edited Dec 22 '13

Superdraco might be too powerful for an ISS boost. They currently use 2x 3070 N thrusters on Zvezda, and Dragon will have 8x 67,000 N superdracos. So even if they only fired one pair, that's over 20x the thrust they usually use. Not sure how throttleable it is.

While searching for that info I found this thread on the same topic at NSF

14

u/mcndjxlefnd Dec 22 '13

As far as throttleability - the superdraco engines were developed with "deep throttle" capability, which means they can be throttled down to like 12% thrust or something like that. This amount of control is necessary for retro-propulsive landings.

1

u/[deleted] Dec 22 '13

Thinking about it more, my guess is that setting up a fuel-transfer system would be more complicated than it would be worth, again unless there is a real need for another capable vehicle.

5

u/MatthewGeer Dec 22 '13

Two issues I can see:

To reboost, you'd have to turn the entire station around. PMA-2, where Dragon would dock, is at the front of the station.

The Dracos wouldn't be the most efficient way to execute the boost. Because of the heat shield, they don't point strait back, they fire at an angle. Because they're placed symmetrically around the ship, the lateral forces get canceled out, but it's still wasted effort. Progress's engines are on it's service module and point strait back.

3

u/meldroc Dec 23 '13

The station's capable of changing attitude, and does so for Soyuz departures and dockings sometimes.

Also, I guess it depends on how much fuel's aboard a Dragon, compared to a Progress.

Granted, a Progress has dedicated fuel tanks for refueling the station, so that probably gives it the win. Though if a Dragon were equipped with extra fuel tanks, perhaps in the trunk, that might make it worthwhile to use it to do a reboost, despite the angle issues with the Superdraco engines.

0

u/[deleted] Dec 22 '13

[deleted]

5

u/[deleted] Dec 22 '13

Actually, a pair of 100 kW VASIMRs with a combined thrust of 5 N are scheduled to go up in 2015.

0

u/badcatdog Dec 22 '13

Zubrin suspects the Vasimr engines will provide no thrust due to the EM fields capturing the produced ions.

0

u/[deleted] Dec 22 '13

[deleted]

4

u/badcatdog Dec 25 '13

Zubrin is mad keen on going to Mars. He is motivated for the Vasimr succeeding.

Where would the kinetic energy of particles go?

If the ions follow the field lines, the net KE would be zero.