Author Topic: Time to Raise the ISS Orbit?  (Read 25065 times)

Offline Danderman

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Time to Raise the ISS Orbit?
« on: 12/13/2014 10:31 pm »
One of the great cost drivers of the ISS program is the provision of propulsion to maintain the nominal ISS orbit. Right now, that nominal orbit is about 400 kilometers, and the Russian Progress vehicle supplies tons of propellant every year that is used to maintain the orbit.

One of the constraints on the altitude is that Progress and Soyuz cannot easily operate above 400 kilometers altitude; however, the introduction of Soyuz-2 as a launcher for these vehicles would probably resolve this issue - unless there are avionics issues that preclude operation of the vehicle at higher altitudes.  There is also an issue that Soyuz would have a slightly steeper re-entry, from, say, 500 kilometers altitude.

The use of Soyuz-2 would allow Progress to carry as much cargo to 500 kilometers as it currently does to 400 kilometers on Soyuz-U; the cost in prop for Progress to raise its altitude by 100 km is about 100 - 200 kg; however, the launch vehicle could inject Progress to an initially higher altitude with no loss of cargo mass.


There is probably a formula that could tell us the cost of raising the ISS orbit from 400 kilometers to 500 kilometers vs the savings in propellant at the higher altitude.

There are also some additional benefits in operating at a higher altitude, including more time in sunlight, less orbital debris, less damage from debris due to slightly lower velocities at altitude, shorter rendezvous times, greater field of view for photography, less blockage by the Earth for science experiments, etc.  Ground stations might gain an extra minute of  visibility, which is important to the Russians.

Gaining altitude does not have to happen all at once - the station could be raised to 425 kilometers to facilitate the current Soyuz and Progress operating regimes, and then the apogee could be raised to 500 kilometers, followed by the perigee, after the transition to the "MS" variants of these vehicles and Soyuz-2.

Another issue is that the orbital inclination could be slightly reduced, since the station would be able to "see" farther north at a higher altitude.

« Last Edit: 12/13/2014 10:37 pm by Danderman »

Offline Robotbeat

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Re: Time to Raise the ISS Orbit?
« Reply #1 on: 12/13/2014 10:36 pm »
More radiation at higher altitude since you're dipping more into the Van Allen Belts (specifically the South Atlantic Anomaly), but yeah, there probably is some merit to raising the orbit. It also improves the business case for launching cubesats. It does, however, lower the resolution of optical instruments on Station (while increasing their field of view).
« Last Edit: 12/13/2014 10:37 pm by Robotbeat »
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Offline Danderman

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Re: Time to Raise the ISS Orbit?
« Reply #2 on: 12/13/2014 10:38 pm »
More radiation at higher altitude since you're dipping more into the Van Allen Belts (specifically the South Atlantic Anomaly), but yeah, there probably is some merit to raising the orbit.

The Van Allen belts do not get down to 500 km, but the SAA might be slightly more intense at the higher altitude, don't know for sure.

Remember that HST is at 600 km, and our astronauts were fine there.

Offline Comga

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Re: Time to Raise the ISS Orbit?
« Reply #3 on: 12/13/2014 11:28 pm »
As a rule of thumb endoatmospheric drag decreases by a factor of two with each approximately 20 km of altitude.  Going up five "scale heights" would be an extreme change.   Plus the residual drag serves to reduce the debris density, preferentially scrubbing the environs of small untrackable debris, which have low ballistic coefficients.
The height of the ISS was already raised from about 360 km to 400-410 km after the retirement of the Shuttle, which suffered significant payload loss hauling its own mass to high altitudes (but could as shown by the aforementioned Hubble flights.)  NASA has ready found a new optimum. None of us has anything to add to their operational decision making.
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Offline Danderman

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Re: Time to Raise the ISS Orbit?
« Reply #4 on: 12/14/2014 12:29 am »
NASA has ready found a new optimum. None of us has anything to add to their operational decision making.

Given the new threat that Progress flights may be reduced or eliminated due to political and economic factors, I would suggest that this new optimum be revisited.

Having the ISS at 500 km and then losing Progress is very different from losing Progress at 400 km.

Offline Robotbeat

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Re: Time to Raise the ISS Orbit?
« Reply #5 on: 12/14/2014 12:32 am »
More radiation at higher altitude since you're dipping more into the Van Allen Belts (specifically the South Atlantic Anomaly), but yeah, there probably is some merit to raising the orbit.

The Van Allen belts do not get down to 500 km, but the SAA might be slightly more intense at the higher altitude, don't know for sure.

Remember that HST is at 600 km, and our astronauts were fine there.
Van Allen belts get more intense the higher you are. And yeah, the SAA is significantly more intense higher up. Astronauts were fine at HST at 600km, but they were there for a short period of time.
« Last Edit: 12/14/2014 12:33 am by Robotbeat »
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Offline Burninate

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Re: Time to Raise the ISS Orbit?
« Reply #6 on: 12/14/2014 05:05 am »
Yes, there is slightly more radiation at higher altitude, but the more pressing concern is that we can expect much higher densities of orbital debris, not all of which are trackable.

The Van Allen belts are particle clouds with smooth gradients of density, not distinct edges, and they shift in position with geomagnetic and solar wind shifts.
« Last Edit: 12/14/2014 05:08 am by Burninate »

Offline Danderman

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Re: Time to Raise the ISS Orbit?
« Reply #7 on: 12/14/2014 06:14 am »
Yes, there is slightly more radiation at higher altitude, but the more pressing concern is that we can expect much higher densities of orbital debris, not all of which are trackable.



Doesn't all debris at 500 km eventually make it to 400 km?


Offline Burninate

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Re: Time to Raise the ISS Orbit?
« Reply #8 on: 12/14/2014 06:42 am »
Yes, there is slightly more radiation at higher altitude, but the more pressing concern is that we can expect much higher densities of orbital debris, not all of which are trackable.



Doesn't all debris at 500 km eventually make it to 400 km?
Yes, but they spend much more time at 500-600km than they spend at 300-400km.

A highway has 600 cars per hour entering a 10 mile long section of 30mph speed limit without congestion, then it proceeds to a 10 mile long section of 60mph speed limit.  Each car spends 20 minutes on the 30mph zone, and 10 minutes on the 60mph zone.  Of the total cars on the highway, which is 20 miles long total, we can expect twice the density of cars on the slower section - 200 cars driving 30mph, and only 100 cars driving 60mph.

At 300-400km, debris decays very fast.  At 500-600km, it decays very slowly.  So there's more of it stuck up there at higher altitude, even assuming the number of debris bodies per day passing the 600km mark and the 300km mark (or the number of cars entering and leaving the highway) is exactly the same.

I am deliberately assuming there are no temporal effects (eg, all debris rates are constant) and there are no secondary collisions between 300km and 600km, because they are not needed for the primary principle to hold.
« Last Edit: 12/14/2014 06:45 am by Burninate »

Offline Burninate

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Re: Time to Raise the ISS Orbit?
« Reply #9 on: 12/14/2014 06:49 am »
I would venture to suggest instead that the ISS and LEO space stations in general, would do better off with a *lower* orbit at which debris is not a problem, but that they should be propelled with generous amounts of efficient ion thrust to maintain that orbit.

It's very unfortunate that this is the case, but that's the cost of not managing your planet's orbital debris situation better.  A space station that has to last decades has to think about longer-term risk, and I have observed no signs of progress on mandatory deorbit functionality.
« Last Edit: 12/14/2014 06:53 am by Burninate »

Offline Prober

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Re: Time to Raise the ISS Orbit?
« Reply #10 on: 12/14/2014 12:22 pm »
More radiation at higher altitude since you're dipping more into the Van Allen Belts (specifically the South Atlantic Anomaly), but yeah, there probably is some merit to raising the orbit. It also improves the business case for launching cubesats. It does, however, lower the resolution of optical instruments on Station (while increasing their field of view).

As I understand the ISS mission is to reduce Radiation risks for BEO missions.   This is their task.  The ISS has had 10 plus years of research at the lower levels and should have an extensive database to work off of.  It should be an easy progression of research to raise the orbit for further research.  Remember deep space missions will not have the earths protections to hide in.
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Offline Danderman

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Re: Time to Raise the ISS Orbit?
« Reply #11 on: 12/14/2014 03:06 pm »
What would be the down side of moving up to 425 kilometers immediately? In this case "immediately" means after a decision has been made by the board to do so.

Both Soyuz and Progress can operate up to 425 kilometers in accessing the station.

Offline Hog

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Re: Time to Raise the ISS Orbit?
« Reply #12 on: 12/14/2014 11:28 pm »
In the interim, she's down to almost 410km, losing 4km of altitude in the last 30 days. Last April she was at a high of 418km.
She was beauty as she flew over at 325 km back in the Shuttle days.
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Offline Lee Jay

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Re: Time to Raise the ISS Orbit?
« Reply #13 on: 12/14/2014 11:34 pm »
What would be the down side of moving up to 425 kilometers immediately?

Every km you raise the orbit reduces the payload capacity of the visiting vehicles, but you conserve fuel for overcoming drag.  There's on optimum somewhere.  Are you sure the optimum is higher than it is now?

Offline erioladastra

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Re: Time to Raise the ISS Orbit?
« Reply #14 on: 12/17/2014 01:10 am »
One of the great cost drivers of the ISS program is the provision of propulsion to maintain the nominal ISS orbit. Right now, that nominal orbit is about 400 kilometers, and the Russian Progress vehicle supplies tons of propellant every year that is used to maintain the orbit.

One of the constraints on the altitude is that Progress and Soyuz cannot easily operate above 400 kilometers altitude; however, the introduction of Soyuz-2 as a launcher for these vehicles would probably resolve this issue - unless there are avionics issues that preclude operation of the vehicle at higher altitudes.  There is also an issue that Soyuz would have a slightly steeper re-entry, from, say, 500 kilometers altitude.

The use of Soyuz-2 would allow Progress to carry as much cargo to 500 kilometers as it currently does to 400 kilometers on Soyuz-U; the cost in prop for Progress to raise its altitude by 100 km is about 100 - 200 kg; however, the launch vehicle could inject Progress to an initially higher altitude with no loss of cargo mass.


There is probably a formula that could tell us the cost of raising the ISS orbit from 400 kilometers to 500 kilometers vs the savings in propellant at the higher altitude.

There are also some additional benefits in operating at a higher altitude, including more time in sunlight, less orbital debris, less damage from debris due to slightly lower velocities at altitude, shorter rendezvous times, greater field of view for photography, less blockage by the Earth for science experiments, etc.  Ground stations might gain an extra minute of  visibility, which is important to the Russians.

Gaining altitude does not have to happen all at once - the station could be raised to 425 kilometers to facilitate the current Soyuz and Progress operating regimes, and then the apogee could be raised to 500 kilometers, followed by the perigee, after the transition to the "MS" variants of these vehicles and Soyuz-2.

Another issue is that the orbital inclination could be slightly reduced, since the station would be able to "see" farther north at a higher altitude.



Actually, they are likely going to lower the orbit due to debris concerns and other issues.  More sunlight is not necessarily a good thing as it leads to thermal issues. 

Offline erioladastra

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Re: Time to Raise the ISS Orbit?
« Reply #15 on: 12/17/2014 01:11 am »
More radiation at higher altitude since you're dipping more into the Van Allen Belts (specifically the South Atlantic Anomaly), but yeah, there probably is some merit to raising the orbit. It also improves the business case for launching cubesats. It does, however, lower the resolution of optical instruments on Station (while increasing their field of view).

As I understand the ISS mission is to reduce Radiation risks for BEO missions.   This is their task.  The ISS has had 10 plus years of research at the lower levels and should have an extensive database to work off of.  It should be an easy progression of research to raise the orbit for further research.  Remember deep space missions will not have the earths protections to hide in.

*A* research goal.  But unless you go WAAAAY Up, which ISS can't do, you really won't scratch the type of environment astronauts to mars will. 

Offline robertross

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Re: Time to Raise the ISS Orbit?
« Reply #16 on: 12/17/2014 01:52 am »

Actually, they are likely going to lower the orbit due to debris concerns and other issues.  More sunlight is not necessarily a good thing as it leads to thermal issues. 

interesting

So I imagine they still have enough margin for power generation from the degraded batteries & solar panels? Or would this be AFTER the first battery replacement?

Offline asmi

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Re: Time to Raise the ISS Orbit?
« Reply #17 on: 12/22/2014 04:44 pm »
I seem to remember someone from ISS management told that post-Shuttle raising orbit to where it is today doubled amount of DAMs they had to execute.

Offline Comga

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Re: Time to Raise the ISS Orbit?
« Reply #18 on: 12/22/2014 07:12 pm »
I seem to remember someone from ISS management told that post-Shuttle raising orbit to where it is today doubled amount of DAMs they had to execute.

Increasing debris density with altitutde is one of the points of my earlier post.

SpaceX cannot launch CRS-5 until January 6 because of the "beta angle cut-out" where the ISS is has thermal issues due to constant sunlight.  A higher altitude means a slower progression of the nodes, and longer "cut-outs".  Was this part of the calculation made by the original poster before calling for a higher altitude?  (PS I know the equations and the effect.  This is not a request for someone else to do the calcuation and debate that specific impact.)

As said previously, this is an extremely complex issue.  Armchair mission controllers haven't even thought of the questions, never mind optimizing across them.  In this case, rather than asking if this is time to change something, one could ask what went into this particular optimized value.
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Offline Hog

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Re: Time to Raise the ISS Orbit?
« Reply #19 on: 02/19/2015 07:43 pm »
Getting down to 402km.

ISS reboost by Progress M-26M engines in early March.

Graphic courtesy of Chris Peat at "Heavens Above".
Paul

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