Keeping the ISS up there

[Obba]

Not sure if this is the right area to post this question…

 

But if the ISS is going to weigh 1,000,000 pounds when complete (500,000KG’s).

How do they intend to keep it up in up orbit?

 

From my understanding, every time the Shuttle leaves, it uses it’s boosters to ‘elevate’ the ISS.

So when, and if the ISS reaches this weight. How do they intend to move this mass upwards?

Or do they just intend to have extra fuel for the boosters for longer burn time?

 

On those lines I would say that they will have a ‘Booster’ craft solely designed to elevate the ISS – maybe launched every 6 months when the Shuttle retires in 2010.

[insane_alien]

the ISS is designed with thrusters. the fuel supply can be replenished by resupply craft. no specialized craft is needed.

[D H]

The Shuttle is not the only means by which the ISS maintains altitude. The Progress and ATV can also reboost the ISS while they are docked to the ISS. The ISS can also reboost itself using the SM engines on the Zvezda Service Module using fuel supplied by either a Progress or an ATV.

[Obba]

So these SM engines and the Zveda service module will have enough burn time (assuming that the thrusters stay the same size), to move this 1,000,000 pounds of mass appropiately?

[insane_alien]

seeing as they were designed to do that, yes, yes they do.

[Sisyphus]

I suspect there's a lot less thrust needed than you imagine. The only reason it's needed at all is friction with the upper atmosphere, which at that altitude is almost negligible. I think it's something like a quick burst every few months.

[insane_alien]

it actually raises the ISS by a few tens of kilometers. though this is surprisingly easy to do. and it can be done with a long slow burn. if you were on board you probably wouldn't notice.

 

also IIRC the time between burns is on the order of years not months.

[CaptainPanic]

Aren't ion-thrusters supposed to be the way to go in case of long and slow burn times?

[insane_alien]

yes, but they are impractical for the ISS as they are too weak. they would only serve to slow down the orbital decay but not prevent it. perhaps they will be retrofitted one day.

[CaptainPanic]

Yeah, ok... so the most (or: the few) ion-thrusters were designed for tiny satellites that were shot into the universe.

 

So, I do understand that a single ion thruster that was built for a 500 kg satellite will not do the job for a 500 ton station. But why not scale it up? Or even more simple: just use more than 1?

 

The whole trick of ion thrusters, as far as I know, is that they have a better power/weight ratio than conventional thrusters. In the end, you carry less stuff into orbit to achieve the same.

[insane_alien]

well, the ion thrusters for the sattelites don't have to deal with anything close to the friction found at such low altitudes (yes, i just called 200 odd km low).

 

multiple ones could be done but would be very expensive.

 

they have an absolutely crap power to weight ratio. the most powerful ones in exitence cannot even produce a single newton of thrust (getting close though). EDIT: actually there is one that can produce 2.9N highly experimental ones can produce more than 10N but start to lose that ultra high impulse. it also requires 75kW to run which would drain the ISS's power for the months it would require to get back up to altitude.[/EDIT]

the trick they have is that they can pruduce a much larger impulse than any other type for the same fuel mass. this is down to the extremely high exhaust velocity.

[D H]

The ISS falls about 100 meters per day. This number depends on many factors, including

• The ISS configuration. The ISS is turning into a big huge kite as we add solar arrays and other stuff,
  
• The ISS attitude. The kite's drag depends on the orientation,
  
• The ISS altitude. The atmosphere roughly thins out exponentially with altitude,
  
• The time in the solar cycle. Sunspots raise the upper atmosphere a lot,
  
• The season of the year. The solar arrays have to point at the Sun, and the solar beta angle has an annual period,
  
• The time of day. The Sun and Moon raise tides in the atmosphere as well as in the ocean, obviously affecting drag.
  
• The time of the month. Atmospheric tides pile up just like ocean tides (spring tides versus neap tides).
  

 

A plot of ISS altitude as a function of time:

 

 

Each of the jags upward represents an attitude maneuver. Sisyphus was closer to the mark than insane: a maneuver occurs every few months. Note that the ISS has been kept rather low for the past few years. Solar activity has been very low in the last few years, making a lower altitude practical. A lower altitude makes it easier for the Shuttle to get to the Station in terms of fuel and in terms of sensitivity. When the ISS is at higher altitudes the Shuttle launch window becomes very narrow.

Edited June 11, 2008 by D H

[Obba]

Great post there DH.

 

Thanks.

[CaptainPanic]

they have an absolutely crap power to weight ratio. the most powerful ones in exitence cannot even produce a single newton of thrust (getting close though).

the trick they have is that they can pruduce a much larger impulse than any other type for the same fuel mass. this is down to the extremely high exhaust velocity.

 

Ah, yes... I feel such an amateur now... I've expressed it better than me. Thanks.

 

Still, I fail to see the major downside of the ion thrusters... The only argument I understood is the costs. When you stay close to earth it's relatively cheap to get something up there... I can imagine that a kg of fuel is more expensive if you have to carry it beyong Mars before using it than when you're talking about merely 200 km. It's probably simply cheaper not to use ion thrusters.

 

Or would the friction on just the ion thruster itself already exert more force than it's thrust? That would be another argument.

 

cool post D H!

[insane_alien]

well, its not really the cost of the fuel. its the cost of expanding the power production/storage capabilities of the ISS, you need a LOT of power for ones with decent thrust and power isn't in such a big supply for the ISS its not as if it can just plug into the mains.

[D H]

It is a cool graph. You can see easily the reboosts. You can also see that solar activity makes the atmosphere expand. The ISS was a lot more aerodynamic in late 1990s than it is now (it was smaller then and had far fewer solar arrays). You would expect the orbital decay rate to increase with time as structure was added to the ISS. Instead, the exact opposite happens! At the start of the graph, the orbital decay rate at 400 km altitude is about the same as orbital decay rate at 350 km altitude at the end of the period. When the ISS dove to below 340 km in 2000, it was diving. 2000 was a solar max year. Six years later the sun is quiescent, which lets the atmosphere shrink, which in turn makes spacecraft in low Earth orbit suffer less orbital decay.

[Mr Skeptic]

What about the systems that use the earth's magnetic field to propel themselves? Would that also require too much power?

[insane_alien]

yes as the earths magnetic field is quite weak so you would need to generate a rather large magnetic field to get a significant amount of force.

[D H]

What about the systems that use the earth's magnetic field to propel themselves? Would that also require too much power?

The vehicles that do use the Earth's magnetic field do so to maintain attitude, not altitude. Maintaining attitude of a small unmanned Earth observation satellite doesn't require a whole lot of energy. Maintaining altitude is a horse of a different color.