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The ISS space station orbits at an inclination of about 50 degrees. That's in between polar orbit, which would be useful for Earth observations, and the ecliptic which would be useful for synchronous or interplanetary (Lunar) missions. Paying fuel for 40 or so degrees inclination change from the LEO orbit of the ISS sounds fuel expensive .

But minisats are anyway shipped to the ISS and launched from there, with the help of several systems now installed there. How helpful is that for the satellites compared to being launched directly as secondary payloads?

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    $\begingroup$ Are you sure you meant 'inclination change from LEO' and not 'inclination change from ISS' orbit'? $\endgroup$ – Hobbes Sep 8 '15 at 18:55
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The reason that Cubesats are launched from the ISS has more to do with launch availability. It's rather difficult to launch as such a small secondary payload. Furthermore, for small satellites, one has to be close to Earth (To reenter in a reasonable amount of time, and it assists with the communication budget), which is even more difficult to get. Virtually all of the launches that are low enough to have the FCC required 25 year reentry period of time are either to the ISS, or spy satellites. Spy satellites don't like having secondary payloads (Although there have recently been a few, see this link), thus the ISS is one of the best, if not the best, options for a satellite orbit. Note that if one has the required fuel, one could launch as a secondary launch to most Earth observing satellites, which there are a large number.

Okay, so if you are going to be a secondary payload heading to the ISS, why actually launch from the ISS? The reason is that the space around the ISS is carefully controlled. By launching from the ISS, they can do so at a time and in a manner that will ensure that they don't re-collide with such objects. They might choose to do so right before a thruster firing, for instance. It also makes it simpler to launch, as no rocket control is required to separate the payload, and the satellites can remain as any other cargo. Launch systems are complex devices, launching directly to the ISS avoids having one of these systems.

Cubesats are missions of opportunity, and can't afford to choose their launch trajectories very carefully. Bottom line is, they do what they can, and virtually all of them I'm familiar with don't care that much about what their orbit is (Just that they have a high enough inclination for them to control from their ground station). All cubesat missions I'm familiar with have waited years on the ground to launch. Mostly they just want to make it to orbit, they don't care about the inclination of the orbit very much.

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    $\begingroup$ +1 for "Spy satellites don't like having secondary payloads" $\endgroup$ – called2voyage Sep 8 '15 at 19:05
  • $\begingroup$ Ironically, NRO has launched several cubesats, see spacenews.com/… . What the rocket does is performs a second burn, bringing it away from the classified payload's orbit, but still... $\endgroup$ – PearsonArtPhoto Sep 8 '15 at 19:33
  • $\begingroup$ Atlas V seems to be a leading chritsmas tree of secondary payload service provider. Boeing/Lockmart doesn't fly their Atlas V to the ISS. They either go to GEO or to polar orbit as far as I know. Why would they launch to 50 degrees inclination? And Atlas V does have secondary payload on "spy sat" missions! @called2voyage $\endgroup$ – LocalFluff Sep 8 '15 at 19:47
  • $\begingroup$ Items launched on the ISS were included as payload to the ISS on something like a Dragon, and then launched from the ISS. Unless a customer specifically requested 50 degrees inclination (Which can happen), Atlas wouldn't launch there. $\endgroup$ – PearsonArtPhoto Sep 8 '15 at 19:51
  • $\begingroup$ ... and if the rocket malfunctions and can't do the 2ndary burn I'm sure the NRO's contract says in large bold print "sucks to be you". Although in this case it probably helped that the NRO owned 7 of them; and presumably vetted the 7 NASA owned to make sure they couldn't learn anything the NRO didn't want them to. $\endgroup$ – Dan Neely Sep 8 '15 at 19:51
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CubeSats are always secondary payloads and very rarely have a propulsion system. This means that you have to take the orbit you can get. It is not uncommon that the launch opportunity, which was planned years ahead during the design phase, changes shortly before launch. Therefore CubeSats are not designed for missions which require a specific orbit.

When you have the opportunity to launch from the ISS, there are at least two big advantage to every other launcher: Firstly, you know the inclination and the rough altitude (350 to 450 km) of the orbit. Secondly, you are guaranteed to reenter the atmosphere when launched from the ISS, which can not be guaranteed by launching with a standard earth observation satellite. These satellites orbit at above 600 km, while 600 km is approximately the maximum altitude if you need to reenter during 25 years after end of life without carrying a propulsion or drag system to speeden up your orbital decay.

For example Germany's space agency (DLR), is not funding CubeSats that are not guaranteed to reenter the atmosphere within the 25 years, because they are strongly following the European Code of Conduct for Space Debris Mitigation (which is good!). This leaves the ISS as one of the best launch possibilities.

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You are correct that a plane change from the ISS inclination of 51.6° to a polar or equatorial orbit is ludicrously infeasible. For example, assuming circular orbits the required ΔV is given by:

enter image description here

So a 50° plane change costs ~6 km/sec — that's almost as much as the original launch vehicle! (This assumes 7 km/sec orbital velocity, close enough to make the point.)

Cubesats deployed from the ISS because it’s available, and many cubesats are used to demonstrate new technologies, or as training for universities or start-ups. They don't care about the orbit; they care about orbit.

That said, useful Earth observation can be done from an ISS inclination, as most of the Earth's population lives within ±51° latitude.

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  • $\begingroup$ Is the delta-v required for inclination change still linearly proportional to the orbital velocity if the orbit is highly eccentric, and the change done at apoapsis? $\endgroup$ – LocalFluff Sep 11 '15 at 5:33
  • $\begingroup$ In fact the inclination of the ISS chosen, because it covers as many populated area as possible. $\endgroup$ – wheeler Sep 11 '15 at 10:58
  • $\begingroup$ @wheeler That and because 51.6° is the latitude of the primary Russian launch site (fun fact: not actually in Russia). $\endgroup$ – Adam Wuerl Sep 12 '15 at 23:17
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    $\begingroup$ @LocalFluff Definitely not, the equation for non-circular orbits is on just a little higher on the wikipedia page in my answer above. But fundamentally you're right that the inclination change is much cheaper at high altitudes when the velocity is lower. This is why for satellites going to geostationary orbits (GEO) that don't launch from low latitudes, the plane change (e.g. from 28.5°) to 0° inclination typically happens concurrently with the circularization burns at apogee. These burns transfer the vehicle from a geosynchronous transfer orbit (GTO) to GEO while changing inclination. $\endgroup$ – Adam Wuerl Sep 12 '15 at 23:20
  • $\begingroup$ @AdamWuerl Do you have a source on Baikonur's latitude influencing the ISS' inclination? Baikonur is at 46°. $\endgroup$ – wheeler Sep 14 '15 at 6:32

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