Next round of U.S. optical spy satellites to start launching in 2018

[RonM]

A monolithic mirror of 4m-class would fit inside the fairing. What makes you think NRO will settle for anything less than that?

History, The Undersectretary for defense saying the next gen. will be 2.4m, the donated mirrors from FIA-I being 2.4m, Hubble being 2.4m, the KH-11 being 2.4m, the 2.4m tooling exists. They really seem to like 2.4m. Based on what has been published they will be sticking with it for at least the next two satellites. Remember these satellites last about 15 years, they historically have kept 4 in orbit, two where built and launched after FIA-I was canceled, with the two new satellites on order this will be the system until the mid 2020's.

It would will be insteresting to see what they do after that.

2.4m might be the optimal size. Since optical spy satellites have to deal with looking through the atmosphere, larger mirrors may not be worth the effort.

[gosnold]

Larger mirrors at higher altitude gives you more frequent revisits. But to transition to a new constellation with different orbits, you need to launch most of the new constellation before the old one is retired, or else you can lose coverage. Keeping the same kind of sat on the same kind of orbit makes the transitions to the new generation much easier.

[Burninate]

Good points. Anyway, it does not make sense to launch a low-flying SEP sat with a delta IV Heavy. It would need to weigh twice as much as Hubble. That's why I think a move to higher orbits is more likely.
Another option is the NRO wants to be able to change plane from one orbit to the next to be able to overfly a target directly in a crisis situation. That means hydrazine, because SEP would be too low thrust.

Hydrazine can't do significant rapid plane changes in practice, because the amount of propellant required grows to many times the mass of the satellite very quickly.  They would prefer to fly a larger constellation, or have their satellite last ten times as long in orbit, to actually using rapid-response plane change & phase change capability a few times per satellite.

[gosnold]

Good points. Anyway, it does not make sense to launch a low-flying SEP sat with a delta IV Heavy. It would need to weigh twice as much as Hubble. That's why I think a move to higher orbits is more likely.
Another option is the NRO wants to be able to change plane from one orbit to the next to be able to overfly a target directly in a crisis situation. That means hydrazine, because SEP would be too low thrust.

Hydrazine can't do significant rapid plane changes in practice, because the amount of propellant required grows to many times the mass of the satellite very quickly.  They would prefer to fly a larger constellation, or have their satellite last ten times as long in orbit, to actually using rapid-response plane change & phase change capability a few times per satellite.

You are right the plane change could only be used a dozen times or less per satellite. So with a constellation of 4 you would get around 40 plane changes to overfly time-sensitive targets over 15 years. It seems cheaper to me to stock up on hydrazine than to launch more satellites, assuming you can use SEP to compensate drag.

[gosnold]

Speaking of constellation architectures, here is an interesting paper by Airbus on LEO/MEO/GEO architectures for earth Observation.

[Burninate]

Good points. Anyway, it does not make sense to launch a low-flying SEP sat with a delta IV Heavy. It would need to weigh twice as much as Hubble. That's why I think a move to higher orbits is more likely.
Another option is the NRO wants to be able to change plane from one orbit to the next to be able to overfly a target directly in a crisis situation. That means hydrazine, because SEP would be too low thrust.

Hydrazine can't do significant rapid plane changes in practice, because the amount of propellant required grows to many times the mass of the satellite very quickly.  They would prefer to fly a larger constellation, or have their satellite last ten times as long in orbit, to actually using rapid-response plane change & phase change capability a few times per satellite.

You are right the plane change could only be used a dozen times or less per satellite. So with a constellation of 4 you would get around 40 plane changes to overfly time-sensitive targets over 15 years. It seems cheaper to me to stock up on hydrazine than to launch more satellites, assuming you can use SEP to compensate drag.

What do you base those estimates on?

[gosnold]

Good points. Anyway, it does not make sense to launch a low-flying SEP sat with a delta IV Heavy. It would need to weigh twice as much as Hubble. That's why I think a move to higher orbits is more likely.
Another option is the NRO wants to be able to change plane from one orbit to the next to be able to overfly a target directly in a crisis situation. That means hydrazine, because SEP would be too low thrust.

Hydrazine can't do significant rapid plane changes in practice, because the amount of propellant required grows to many times the mass of the satellite very quickly.  They would prefer to fly a larger constellation, or have their satellite last ten times as long in orbit, to actually using rapid-response plane change & phase change capability a few times per satellite.

You are right the plane change could only be used a dozen times or less per satellite. So with a constellation of 4 you would get around 40 plane changes to overfly time-sensitive targets over 15 years. It seems cheaper to me to stock up on hydrazine than to launch more satellites, assuming you can use SEP to compensate drag.

What do you base those estimates on?

The earth rotates 22.5° between each pass of a LEO satellite. So to overfly a target by plane change, you need to rotate your plane by half that at most, so 11.3°. On average, you don't need to use the maximum plane change, so you need 5.6° change. That's for one satellite.
If you have a constellation of 4 sats, you can set up their ground tracks so they are spaced equally, so you need 1/4th of the plane change capability of one sat.
So one of the 4 sats of your constellation needs to change plane 1.4° (on average) to be able to directly overfly a target within 24 hours.
Thats a delta-v of sin(1.4°)x8 km/s=200 m/s
If your satellite is half hydrazine, your delta-v is ln(2)*230s / 9.8m/s^2 =1500 m/s
So each sat can do around 8 plane changes. If you have 4 of them, that's 32 plane changes over the lifetime of the constellation (15 years for a typical satellite).

[Targeteer]

Interesting analysis from Ted Molzcan on the subject that expands on his comments used in the story that spawned the thread.  I won't post the text from Seesat-L after being "corrected" for doing it recently

Past and future KH-11 orbits

http://www.satobs.org/seesat/May-2015/0096.html

[kevin-rf]

Very nice read, thanks for sharing.

[zubenelgenubi]

Interesting analysis from Ted Molzcan on the subject that expands on his comments used in the story that spawned the thread.  I won't post the text from Seesat-L after being "corrected" for doing it recently

Past and future KH-11 orbits

http://www.satobs.org/seesat/May-2015/0096.html

The information in the article noted in the OP (from a certain web site to be referenced in this forum as little as possible) is really augmented by Ted M's post.  I really wish the article had gone into this greater explanatory detail; I don't know why the author did not.

(One of the main reasons I "lurked" NSF for a few years before joining L2, is for the synthesis of sources provided in the various topic threads, with an enviable signal-to-noise ratio therein.

Also--the direct commentary by the many learned veteran spaceflight experts.

And the grade-A articles.

There are other spaceflight web sites that I rarely use anymore--they just don't measure up!)

Regards,
Zubenelgenubi

EDIT Dec 3, 2019: NROL-71/USA 290 launch thread

[Star One]

Interesting analysis from Ted Molzcan on the subject that expands on his comments used in the story that spawned the thread.  I won't post the text from Seesat-L after being "corrected" for doing it recently

Past and future KH-11 orbits

http://www.satobs.org/seesat/May-2015/0096.html

Thanks for the info. I posted that as well but it was removed & I couldn't figure out why.