A Question Involving Orbital Decay, for You Learned Science Champions

[Kowbell]

Hi all.

As I was watching the THAICOM 6 live webcast today (congrats to SpaceX), I pondered orbital decay for a second.

Let me preface this blabbery and discourse with the forewarning and acknowledgement that I do not know everything about physics; indeed, I know not too much more than the "average" Joe - or, Jeb, I guess - who plays this game. I have looked up some information outside of KSP about astrophysics, orbital mechanics, and the likes, but I don't know everything. Which is why I inquire.

(From what I understand,) Orbital decay is the acting of particles in the "vacuum" of space on an object in orbit - say, a satellite. Because the continued force applied, the velocity slows more and more until the orbit coincides with the atmosphere - that is, where the atmosphere is thick enough to cause enough friction to make the satellite get really, really hot, eventually to the point where it asplodes, certainly rendering the satellite useless.

My question is, instead of using the occasional rockety burniness to fix the orbit to keep the very expensive, very heavy satellite in the target orbit, as I understand is usually done to space stuff which they want to keep in space, why not use consistently an extremely-high efficiency, extremely-low thrust engine - say, something along the lines of an ion engine? Or is KSP's rendition of ion engines markedly different from current real technology, in the attempt to make the gameplay more fun and simple? If so, why not use some real world equivalent that applies such a minute, carefully calculated thrust as to provide a force that combats the force of the particle friction?

What say you to this?

[K^2]

There have actually been satellites that do just that. GOCE is an excellent example. It's just that modern ion propulsion systems tend to be either big and heavy, or have very little thrust, as well as being rather pricey. Thus, limiting selection of missions where they can be used. But we'll probably see more and more of this as technology improves.

[Kowbell]

There have actually been satellites that do just that. GOCE is an excellent example. It's just that modern ion propulsion systems tend to be either big and heavy, or have very little thrust, as well as being rather pricey. Thus, limiting selection of missions where they can be used. But we'll probably see more and more of this as technology improves.

In regards to the very little thrust, do you mean so little that, even at full blast, it is weaker than the force of friction caused by the particles in space?

Reading that Wiki article, I see that GOCE was really low in orbit, meaning resistance was higher, so it'd have to push harder. But in higher orbits - say, even, Geostationary/synchronous - would it not be possible to use an extremely weak ion engine which lasted for many years, perhaps decades? Or is orbital decay not nearly as much a concern that high?

[DaveofDefeat]

In regards to the very little thrust, do you mean so little that, even at full blast, it is weaker than the force of friction caused by the particles in space?

Reading that Wiki article, I see that GOCE was really low in orbit, meaning resistance was higher, so it'd have to push harder. But in higher orbits - say, even, Geostationary/synchronous - would it not be possible to use an extremely weak ion engine which lasted for many years, perhaps decades? Or is orbital decay not nearly as much a concern that high?

I think he means that the ion drives and such are so heavy and expensive that you could put a cheaper, lighter normal rcs like engine that could keep your satellite in orbit way beyond its expected operational time frame.

[Scotius]

There is not much need for long term station-keeping means on a GEO satellite. Orbital decay so far from Earth is negligible during "lifespan" of the satellite placed there. It means any piece of technology in geostationary orbit will stop working due to technical problems long before it orbit decays - and then wreckage will keep hanging there for ages anyway.

[Kryten]

While the decay at GEO altitude might not mean anything there will re-enter anytime soon, most of the sats actually placed there have to stay in very specific orbital positions-and they do face enough perturbation from various effects that they they need to perform stationkeeping to stay in those positions-in most modern sats, with ion engines.

[K^2]

Station keeping in GEO requires very little thrust or total impulse, though. Orbital decay, on the other hand, is only significant at much lower altitudes, in hundreds of km above surface. There, it makes more sense to use conventional engines for occasional burns. This is quickly changing though, and might already be out of date. I was trying to find some info on Iridium NEXT, but I haven't found info one way or another on which propulsion they are going to use to maintain orbit. Current Iridium constellation uses occasional burns which you can actually see in the sky if you know when and where to look.

[Raven.]

The "ion" drive is an actual, real-life engine that's being developed. However, not only is it expensive, but it's a very young and unproven technology. Investing in a young technology is not a smart move for several reasons, at least not until its reliability has been proven more than what it has been thus far.

If you think about it, orbital decay is not a bad thing in some cases. If a satellite has stopped functioning, then the cheapest way to get rid of that piece of space junk is to let it fall back down to earth and burn up in the atmosphere. Of course, this may take a while, but it's like they say, "what goes up, must come down."

[Firov]

The "ion" drive is an actual, real-life engine that's being developed. However, not only is it expensive, but it's a very young and unproven technology. Investing in a young technology is not a smart move for several reasons, at least not until its reliability has been proven more than what it has been thus far.

If you think about it, orbital decay is not a bad thing in some cases. If a satellite has stopped functioning, then the cheapest way to get rid of that piece of space junk is to let it fall back down to earth and burn up in the atmosphere. Of course, this may take a while, but it's like they say, "what goes up, must come down."

You might be thinking of a different type of engine. VASIMIR plasma drives, perhaps. Ion engines and the very closely related hall effect thruster are actually extremely old and relatively simple. The idea was first developed in the mid 1910's, the first engine built and tested in the late 1950's, and the first suborbital test in the early 1960's. At this point they're pretty well understood, though they're still researching improvements.

The only major craft I can think of offhand that use ion propulsion would be the Deep Space One mission and the Dawn probe. Though I do know that there are a few geosynchronous satellites that use them for station keeping. Incidentally, I recall reading that annually, station keeping at geostationary altitude requires around 45 delta-V per year. I expect that for most satellites it's not worth it to have the extra power generation capabilities needed to run an ion engine over just having a conventionally fueled thruster for such a small amount of delta-v per year.

[AeroEngy]

You might be thinking of a different type of engine. VASIMIR plasma drives, perhaps. Ion engines and the very closely related hall effect thruster are actually extremely old and relatively simple. The idea was first developed in the mid 1910's, the first engine built and tested in the late 1950's, and the first suborbital test in the early 1960's. At this point they're pretty well understood, though they're still researching improvements.

The only major craft I can think of offhand that use ion propulsion would be the Deep Space One mission and the Dawn probe. Though I do know that there are a few geosynchronous satellites that use them for station keeping. Incidentally, I recall reading that annually, station keeping at geostationary altitude requires around 45 delta-V per year. I expect that for most satellites it's not worth it to have the extra power generation capabilities needed to run an ion engine over just having a conventionally fueled thruster for such a small amount of delta-v per year.

Boeing's new-ish line of satellites the 702 series uses a XIPS (Xenone Ion Propulsion System) ISP of 3400/3500s with thrust of 79/165 mN for low & high power modes respectively. This is basically the same propulsion system on Deep Space One. They actually use it for final orbital insertion and station keeping. This saves a good bit of weight over typical chemical systems (hyrdazine arcjet, etc.). Lighter is always better as the weight saving can either be used for cheaper launches or more payload space which is quite valuable. One of the drawbacks is due to the low thrust it can take a long time (1 month or longer) from rocket sep to final orbit insertion.

[PakledHostage]

A related question about satellites in GEO: Are there any requirements (i.e. international agreements or even just industry standard practices) that require satellites in GEO to carry enough fuel to allow them to be moved to a graveyard orbit when they've reached the end of their service lives? It would seem that real estate in GEO is valuable enough that this would be a good idea?

[Nibb31]

Yes. GEO slots and frequencies are regulated by the International Telecommunication Union. Before getting a slot, your satellite needs to have end-of-life provisions, including moving to a graveyard orbit.

[DerekL1963]

Yep. And end-of-life considerations are so important, they're often redundant. Not only is main propulsion system designed to function (and have enough fuel) through end-of-life and the move to the graveyard, frequently there's an entirely separate system specifically designed with enough delta-V to move it to the graveyard.

[Aghanim]

The "ion" drive is an actual, real-life engine that's being developed. However, not only is it expensive, but it's a very young and unproven technology. Investing in a young technology is not a smart move for several reasons, at least not until its reliability has been proven more than what it has been thus far.

If you think about it, orbital decay is not a bad thing in some cases. If a satellite has stopped functioning, then the cheapest way to get rid of that piece of space junk is to let it fall back down to earth and burn up in the atmosphere. Of course, this may take a while, but it's like they say, "what goes up, must come down."

I don't know, but it certainly sounds like a chicken and egg problem: If someone didn't want to invest on ion engine because it isn't reliable how do they get money to make it reliable?

[Kryten]

The "ion" drive is an actual, real-life engine that's being developed. However, not only is it expensive, but it's a very young and unproven technology.

The first ion engine flew in 1970, and the soviets used them operationally for station-keeping since the mid 80's. Heck, even the first deep-space totally ion-propelled mission was over a decade ago.

[Nuke]

the problem never was engines. there is such a backlog of experimental ion and plasma engine technologies, almost all have been ground tested and will probibly work, and just need to be tested in space to increase their trl.

the real hold up is that a lot of those systems use more power than is currently available on a space craft. you dont launch a spacecraft just to test a system, usually you piggy back that system on another mission (especially if the rocket used to launch it has some extra dv for a secondary mission). if said mission lacks the power supply neccisary to power the experimental technology, then there is just no way to test it, and the extra space gets allocated for something else (like cubesats for example). for example, the iss doesnt have the power to run the vasimr engine. but with a battery bank its possible to fire the engine long enough to reboost the station. the only reason they plan to test it is fact it can piggyback on a routine resupply mission.

once vasimr is proven, then we have a reason to start building spacecraft with better power supplies, and the other engines can piggyback on those missions for testing.

Edited January 8, 2014 by Nuke

[AeroEngy]

the problem never was engines. there is such a backlog of experimental ion and plasma engine technologies, almost all have been ground tested and will probibly work, and just need to be tested in space to increase their trl.

the real hold up is that a lot of those systems use more power than is currently available on a space craft. you dont launch a spacecraft just to test a system, usually you piggy back that system on another mission (especially if the rocket used to launch it has some extra dv for a secondary mission). if said mission lacks the power supply neccisary to power the experimental technology, then there is just no way to test it, and the extra space gets allocated for something else (like cubesats for example). for example, the iss doesnt have the power to run the vasimr engine. but with a battery bank its possible to fire the engine long enough to reboost the station. the only reason they plan to test it is fact it can piggyback on a routine resupply mission.

once vasimr is proven, then we have a reason to start building spacecraft with better power supplies, and the other engines can piggyback on those missions for testing.

Like I said before xenon ion systems are already in use. Boeing's 702HP spacecraft can deliver up to 18kW. It has 4x 25cm XIPS engines for both initial orbital insertion and station keeping.

[Nuke]

i never said they weren't. every time you improve a design you pretty much have to go back to a lower trl until the improvement is proven. there are lots of ways to improve upon the ion engine technology we already use (more isp, more thrust, better efficiency, higher power levels, better life expectancy), and each time that happens, we got to re-prove it. just cause we have ion engines doesn't mean we aren't playing around with new ones. hell were still improving chemical engines, using the same process.

Edited January 8, 2014 by Nuke

[Kowbell]

Yes. GEO slots and frequencies are regulated by the International Telecommunication Union. Before getting a slot, your satellite needs to have end-of-life provisions, including moving to a graveyard orbit.

What exactly is graveyard orbit? Or, more specifically, WHERE is graveyard orbit?

[Kryten]

Pretty much anywhere outside of GEO would count as a 'graveyard orbit' for those purposes. I don't think there's any real definition or regulation apart from the minimum distance out of GEO.

[Kerbin Dallas Multipass]

What exactly is graveyard orbit? Or, more specifically, WHERE is graveyard orbit?

http://en.wikipedia.org/wiki/Graveyard_orbit

They put old satellites there so they dont interfere or crash into the active ones.

[KASASpace]

Umm, vernier thrusters?

But seriously, it's nor really particle friction, it's atmo friction. Because the atmosphere still reaches so high into space, it's actually kind of difficult to just get out of the atmosphere, where you'd start running into the Van Allen belts, if I remember correctly. And that is a serious threat to radiation sensitive cargo, IE, humans, or other animals.