What's the problem with ISS's orbit?

chris_in_space · Nov 8, 2005

I keep hearing on this board that the ISS is on a poor orbit, which isn't useful at all. So I want to ask the people who say this why they think so. What's the problem with the orbit? Also in this case what would be a better orbit for the ISS and why? If possible keep the better orbits in the range 0-5000km altitude cause a station at 40000km altitude could be nice but unfeasable or quite expensive with current or near future technology...

CalliArcale · Nov 8, 2005

<blockquote>In reply to:<hr />If possible keep the better orbits in the range 0-5000km altitude<hr /></blockquote> <smartaleck />Well, I forsee problems with 0 km . . . but imagine how easy resupply would be!</smartaleck> Seriously, though, most people who object are not objecting to the altitude. It's the inclination. The ISS orbits at 51 degrees. This is a significant inclination, but it is the optimal inclination for vehicles launching from Baikonur Cosmodrome. The optimal inclination from Kennedy Space Center is (IIRC) about 28 degrees. Any time you move away from the optimal inclination for your launch site, you are expending extra propellant to acheive what's called a "plane shift". Plane shifts are expensive. Shuttle can make a 51 degree orbit with a pretty big payload; for all its faults, the Shuttle really is a very capable vehicle. Soyuz, however, cannot make a 28 degree orbit with a Soyuz or Progress vehicle, and Proton cannot make that orbit with one of the really big payloads such as the Zarya or Zenit modules of the ISS. So Shuttle has to adapt. It basically just means Shuttle can't carry up as much payload as it could to a 28 degree orbit. It's also why the station has to be lower for Shuttle missions; at that inclination, it also can't make it as high a Soyuz at 51 degrees. ATV will have an even bigger plane shift problem; it will launch aboard an Ariane V from Kourou, French Guiana. This is an equatorial launch site, so zero degrees is the optimal orbit for there. This makes it an appealing launch site for commsats, especially geosynchronous ones, but not so much for flights to the ISS. However, higher inclination orbits do have considerable advantages for Earth observation, because the higher your inclination, the more of the Earth you will get to see. Because of this, spy satellites are almost invariably in polar orbits (orbits approaching a 90 degree <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

henryhallam · Nov 8, 2005

Station is an an orbit inclined at 51.6 degrees to the Earth's equator. Because the Earth is rotating, rockets can get a small boost if they launch in a direction that has some eastward component. This means that the amount of mass that a given launch can put into orbit is a function of the perigee, apogee, launch site latitude and orbital inclination. The lower the launch site latitude, the better. The lower the orbital inclination, the better. But there is one big caveat: The orbital inclination must not be LOWER than the launch site latitude, or you have to expend a lot MORE energy and get correspondingly less payload into orbit. So ideally you want an equatorial orbit (0 degrees inclination) with a launch site on the equator. This is difficult for logistical reasons, but NASA's launch site was chosen to be as far south as was practical in the US, hence Florida. Given that KSC's latitude is fixed at 28.5 degrees north, or thereabouts, the ideal ISS orbit for a payload point of view is one with a 28.5 degree inclination. This lets you get the best possible mass into orbit and gives long launch windows. Unfortunately, because the Russians have to launch from way up north, Station must be put in a higher inclination orbit so that they can reach it. This is one reason why rendezvous/assembly at ISS for a lunar or interplanetary mission is not necessarily a good idea: you lose a small, but significant fraction of your payload capacity by launching into such a high-inclination orbit. Highly inclined orbits do have some advantages though, such as being able to cover a greater area of the Earth's surface for observation, mapping and surface science.

vogon13 · Nov 8, 2005

In my view, the biggest problem with the ISS orbit is that it is not around Mars. (subtle dig at pace of space exploration) <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

henryhallam · Nov 8, 2005

Forgot to mention that it's not a good idea to put a manned station at 4000km since that's smack in the middle of the Van Allen radiation belts. You have to go under about 500km (little radiation) or over about 40000km (a bit more radiation, but nothing like what you get in the Van Allen belts)

chris_in_space · Nov 8, 2005

Ok so some people find the ISS orbit useless because it can't be used for assembly for Moon/Mars missions. That's what I thought but I wasn't sure of. Now my next question. Once the station will be "completed", once the ATV and HTV will go on service and Russia will have Soyouz at Kourou will be it be possible to change the inclination of the ISS orbit to something like 23°? I know that orbital inclination changes don't usually require much energy but I don't know exactely what the thruster's impulse on the service vehicules are? So does someone have an idea or could someone make a quick calculus to know if such an orbital inclination change could be feasable with a reasonable amount of service vehicule launches?

henryhallam · Nov 8, 2005

I know that orbital inclination changes don't usually require much energy Very much the opposite, I'm afraid. Orbital inclination changes require a huge amount of energy, especially for low orbits. If I did the maths right then it would require a delta-V of (2*sin(inclination/2)*orbital velocity). Sin(28) is close to 0.5 therefore the delta-V required is about 80% of what was needed to get all the mass up there in the first place! So it is not practical to change the inclination with a standard chemical rocket. If you use an ion engine then the mass of propellant alone would be close to half of the mass of the ISS, the engines and power supply (biiig solar arrays, nobody's going to want to attach a nuke to the station) would add a lot more, it would take an age and basically the advantages of having the station in the new orbit wouldn't be worth the effort. If a new station is built, sure, put it in a lower-inclination orbit, but it's not worth the hassle to move the existing one.

SpaceKiwi · Nov 8, 2005

Thanks guys, I've learnt a little here today too. Chris, just to talk around the edges of what henry has already expertly explained, changing the inclination has been a regular if misguided conversation topic around here during my time. Wanting to move Hubble alongside ISS so it can be serviced was the last time this came up I think. As henry said, it would be cool if we could do something like that but it is unfortunately not practical. <div class="Discussion_UserSignature"> Who is this superhero? Henry, the mild-mannered janitor ... could be!-------------------------------------------------------------------------------------------Bring Back The Black! </div>

chris_in_space · Nov 8, 2005

Strange, in Orbiter space simulation it allways seemed to me that inclination changes don't require much energy. But well maybe it was because of some special orbit configuration or because of the unrealistically high performances of the thrusters. Indeed I checked out a little bit and you're right with the formula you gave. In this case inclination changes for the ISS are not very likely...

krrr · Nov 8, 2005

Launching from a lower latitude site (KSC, Kourou) into a higher inclination orbit is not really a problem. You lose a few percent payload capability, because you can't fully use the kick (~460 m/s) you get from the Earth's rotation at the equator. There might also be range safety issues, of course. Launching from "higher up" (e.g. Baikonur) into a lower inclination low earth orbit (28.5 or 0 degrees) is indeed next to impossible. Once in low earth orbit, plane changes (both towards lower and higher inclinations) are very expensive. For the ATV, we have the first situation, meaning it's not a big deal. After all, the ATV is 20 tonnes, and the Ariane V's payload capacity for an equatorial orbit isn't significantly higher.

henryhallam · Nov 8, 2005

To clarify / further confuse: With the same launch vehicle, a low latitude launch site (KSC or Kourou) beats a high latitude launch site (Baikonur) into a high-inclination orbit (ISS).

spacester · Nov 8, 2005

. . . it would require a delta-V of (2*sin(inclination/2)*orbital velocity). My understanding is that equation is the one to use, but one needs to be careful. First off, 'inclination' is more precisely 'change in inclination'. A trivial and obvious correction. But the interesting thing is that if you reduce the orbital velocity you reduce the plane change deltaV. The orbital velocity goes down as the orbital radius gets bigger. So the trick referred to earler is to go to a higher orbit with high eccentricity, and do your plane change burn at apogee when you're going slower. I've done the math, and it is in fact cheaper in some cases to spend the dV to pump up the orbit and then reduce it after the plane change. Seems like cheating, doesn't it? <div class="Discussion_UserSignature"> </div>

drwayne · Nov 8, 2005

It is indeed a very neat trick. It still requires quite a bit of energy to do, but not nearly so much as fighting mother nature.... Wayne <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

henryhallam · Nov 8, 2005

Glad that equation was right, I derived it on the back of an envelope from the difference in the velocity vectors at point when the source and target orbits intersect. There are plenty of neat tricks like you mention, that's what keeps the FDO in a job I suppose <img src="/images/icons/smile.gif" /> Apparently if you want to do both a plane change and adjust perigee and apogee, you can do some clever things by combining both manoevers into one burn, IIRC this can save more than the intuitive amount of dV. And things can be done to use masscons and the equatorial bulge to your advantage. I'd like to learn more about these tricks of orbital mechanics. I can just about follow "Fundamentals of Astrodynamics" now (though it's tough going, not exactly bedtime reading) but it doesn't mention many of these special techniques.

drwayne · Nov 8, 2005

There are some utterly fascinating orbits one bumps into references to, one of which I recall involves going around the moon on the way to the final orbit. Wayne p.s. A classic book by the way, on my shelf at work <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

strandedonearth · Nov 8, 2005

*general response, not specifically to drwayne* I once asked what dV would be needed to move the ISS into an equatorial orbit. SG answered, and while I don't recall the exact figures (4800ft/s?), it was about half the dV needed to reach orbit in the first place!

CalliArcale · Nov 9, 2005

<blockquote>In reply to:<hr />There was a piece on here that might have caused some confusion about the ease of plane changes. (I think it was Calli that brought it up when we noticed a 'funny' profile to a comsat launch.) There is a strange set of energy saving maneuvers done on some of those launches, but not practical for what we were talking about.<hr /></blockquote> Perhaps the strangest I've seen (and maybe this is the one you're referring to) was a Russian comsat that was launched into geostationary orbit. This is very difficult (but not impossible) from Baikonur, because it's at 51.8 degrees and the geostationary orbit is equatorial (0 degrees). So you have to look at either a really honkin' huge rocket or a puny commsat. For this reason and because much of Russia is so far north it has poor line-of-sight to the geostationary orbit, most Russian commsats are in Molniya orbits (named for the Molniya class of commsats). Molniya orbits are high inclination and highly eccentric as well, which means that while the commsat is not fixed in the sky, if you line up the orbit correctly and time everything just right, it will spend most of its time in Russian skies. Anyway, not long ago if I'm remembering rightly, Russia launched a geostationary satellite by sending it to the *Moon* for a gravitational assist. The Moon's gravity was used to acheive the plane shift. Inserting the spacecraft into the geostationary orbit was acheived using a conventional chemical rocket. So from some launch sites, you can actually get more payload to the Moon than you can to geostationary orbit! <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

dobbins · Nov 9, 2005

Actually Baikonur's latitude is 46 degrees north. The reason the Russians prefer a higher inclination than 46 degrees is a rocket launched at a lower inclination would overfly Mongolia and China and they don't want spent boosters falling on another country if they can avoid it.

thinice · Nov 9, 2005

Perhaps the strangest I've seen (and maybe this is the one you're referring to) was a Russian comsat that was launched into geostationary orbit. What is so strange here? Russia (USSR) launches satellites into geostationary orbit from mid 70s. In fact, there was Molniya satellite in geostationary orbit (Molniya-1C), followed by Raduga (Rainbow), Gorizont (Horizon) and Ekran (Screen).

CalliArcale · Nov 9, 2005

<blockquote>In reply to:<hr />Actually Baikonur's latitude is 46 degrees north. The reason the Russians prefer a higher inclination than 46 degrees is a rocket launched at a lower inclination would overfly Mongolia and China and they don't want spent boosters falling on another country if they can avoid it. <hr /></blockquote> Y'know, someone explained that to me once before, and I completely forgot it. Thanks for the reminder. *sheepish grin* <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

CalliArcale · Nov 9, 2005

<blockquote>In reply to:<hr />What is so strange here? Russia (USSR) launches satellites into geostationary orbit from mid 70s. In fact, there was Molniya satellite in geostationary orbit (Molniya-1C), followed by Raduga (Rainbow), Gorizont (Horizon) and Ekran (Screen). <hr /></blockquote> Yes, I know they've launched many spacecraft into geostationary orbit. However, this particular one was novel becuase of the *way* they did it. Instead of the usual pick-a-whopping-huge-rocket, they did a lunar flyby to make the plane shift. It struck me as very clever, very cool, and also very Russian. Instead of the sledgehammer approach, they came up with a more complicated trajectory that acheived the same effect with a smaller rocket. I wish I could remember which satellite it was.... It might even have been for a foreign customer. Russia launches a LOT of foreign commsats. The mighty Proton does a lot of lifting to geostationary orbit. The main reason for the Molniya orbit is not a cheaper launch; it's the fact that geostationary commsats become progressively less useful the further north you go, because you have a lot more of the Earth's atmosphere to get the signal through. Molniya orbits are more useful for customers at higher latitudes. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

shoogerbrugge · Nov 9, 2005

Calli, are you sure the Russians actually launched a bird from Plestek to Geo-stationairy Orbit. Because I knew they had a plan and did the maths, I wasn't sure they actually did it. I thought it was more of a failsafe when Biakonur became unavaileble. I do remember that somebody used that trick on a comsat that was placed in a to low orbit by Sea Launch. It was procured for scrap by someone and used the birds engines to fly it around the moon into GEO orbit, and it had a couple of usefull years left overthere. About the Molniya orbit. I believe it was chosen because this particular orbit overflies Russia a lot more then GEO stationed birds. Which makes it more usefull for Russian comsats. Thats why Russia only developed GEO based commsats after 3 or 4 generations of Molniya based sats.

krrr · Nov 9, 2005

they did a lunar flyby to make the plane shift. It struck me as very clever, very cool, and also very Russian. Instead of the sledgehammer approach, they came up with a more complicated trajectory that acheived the same effect with a smaller rocket. I wish I could remember which satellite it was.... It might even have been for a foreign customer. It was the HGS-1 satellite. The two lunar flybys were not planned. The people at Hughes (Americans!) came up with the idea after the satellite was left stranded in a 350x36000 km orbit because the Proton's Block DM upper stage had failed. The maneuver was done using the satellite's own propulsion. That said, it is indeed the case that lunar flybys could be used to inject satellites into GSO using less delta-v than the normal mission profile. It just hasn't been done yet (on purpose). Something in-between is placing the spacecraft in a super-synchronous transfer orbit, meaning a highly elliptical orbit with apogee of 100000 km or more. At apogee, the spacecraft is so slow that a plane change maneuver, combined with perigee raising, can be done at very little cost. After that, the apogee is lowered to the 36000 km required for geosynchronous orbit.

CalliArcale · Nov 9, 2005

I remembered it launching from Baikonur, but didn't remember that it was a Hughes spacecraft, or that it was a contingency operation! Wow! Coolness! <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

thinice · Nov 9, 2005

<blockquote>In reply to:<hr />The main reason for the Molniya orbit is not a cheaper launch; it's the fact that geostationary commsats become progressively less useful the further north you go, because you have a lot more of the Earth's atmosphere to get the signal through. Molniya orbits are more useful for customers at higher latitudes. <hr /></blockquote> Most of Russian population is concentrated in rather small European part of the country, no further to the north than southern Scandinavia, which is covered by geostationary comsats pretty well. Molniya orbit was designed mainly to bring the Soviet state media to remote areas of Siberia and Far East.

What's the problem with ISS's orbit?

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