AlignPlaneMFD How to use AlignPlaneMFD to achieve GTO ? - Orbiter-Forum

N_Molson · 06-14-2012, 02:30 AM

Hello,

The title sums it up. Let's assume I launch a rocket from Cape Canaveral (Equ. Inc. roughly 28°) and want to send a satellite in GTO (the satellite will perform the circularization burns).

The idea is to launch East (90°) and progressively shift from a heading guidance to an inertial guidance as the rocket acquires horizontal velocity.

The result of this is a sinuosoid on the MapMFD that crosses the equator over the Atlantic Ocean.

Now I want to reduce that 28° Equ. Inc. to 0° Equ. Inc. , because a true GSO stays right over a given spot on the ground and then right over the Equator (if you have an inclination, the satellite will draw an "8" above its station coordinates). For that, I use a restartable upper stage.

So this is where I'd like to use AlignPlaneMFD to plan the alignement burn. The ELS (Elements) button seems the right way to do this, because I don't want to align with a vessel but with an orbit. Of course I could put a DG in GSO and align with it, but this is no fun.

At this point I have two numbers to enter : the desired Inclination and the LAN. The LAN is given by OrbitMFD. The problem is then the other number. AlignPlaneMFD uses Ecliptic plane as a reference, while I use Equatorial plane as a reference. So entering "0" obviously doesn't work.

So the final question is : what I should enter there, and how to calculate it ? I tried Earth axial tilt (23.44°) but that doesn't work. I'm definitively missing something about orbital mechanics.

Subsidiary question : is it more efficient to perform a part of the alignement during the launch, by "bending" the trajectory off the velocity vector, or to launch straight ahead and make the inclination once in transfer orbit ?

sorindafabico · 06-14-2012, 03:05 AM

I know it's not the answer you want, but it's easier to use IMFD, I guess.

**Cras** · 06-14-2012, 03:13 AM

do you mind elaborating. I am interested in seeing how others get to GEO with 0 equatorial inclination. I have never tried it myself, I always just settle for the "figure 8" thing myself.

**Zachstar** · 06-14-2012, 05:44 AM

Just go to the moon first

In b4 Flytandem!

Tommy · 06-14-2012, 07:34 AM

Quote:

Originally Posted by sorindafabico

I know it's not the answer you want, but it's easier to use IMFD, I guess.

Quote:

Originally Posted by Cras

do you mind elaborating. I am interested in seeing how others get to GEO with 0 equatorial inclination. I have never tried it myself, I always just settle for the "figure 8" thing myself.

Well, I haven't tried it myself, but I seem to recall that you can target GeoSync with IMFD. If I have time (strongly recommend you don't hold your breath waiting) I will try to look into this further.

In the meantime, I wonder if the following might work:

Orbiter's stock MFD's assume a point gravity source, so if you use OrbitMFD in Ecl mode it should show you your inclination in the ecliptic frame - even while landed. It would show a LAN and Inc, which could be adapted to be used in AlignPlanesMFD if you sort of "reverse" things a bit.

As to whether it would be more efficient to "dogleg" during the ascent, probably not for most vessels. The problem is that you want to launch at a 90 degree heading, and by the time you cross the equator you are past your ApA (at least in most vessels - but the DGEx has such a slow ascent profile it may be the exception). I suspect that the most efficient method would be to have a low ascent profile, with the goal of having your initial ApA just after you cross the equator - that way you can make the plane alignment before the circularization burn (and your velocity is still a bit below orbital velocity).

IIRC, the one time I tried to reach GEO from KSC I basically set up a suborbital "hop" with a low ApA (about 90km), and then used aerodynamic lift to make the plane change, then another burn to establish my desired "parking" ApA (about 150k, since it doesn't need to be stable), then when I reached that ApA I burned again to establish a new ApA at GEO altitude.

That worked, but I imagine there is a better way to do it - that was a few years ago and my skills were pretty crude then.

Not that they are exactly "art" now ...

dgatsoulis · 06-14-2012, 07:41 AM

Question1: By placing a bunch of ships on different places at GSO and comparing the Eq frame and Ecl frame orbital parameters, I noticed that they always had an Inc of 23.44 degrees and LAN 179.98 degrees wrt the ecliptic. Those are the parameters you need to use in AlignPlanesMFD.

The calculation for Rel.Inc is:

initial inclination

target inclination
$\Omega_i =$ initial LAN
$\Omega_t =$ target LAN

$a1=sin(I_i) \cdot cos(\Omega_i)$
$a2=sin(I_i) \cdot sin(\Omega_i)$

$b1=sin(I_t) \cdot cos(\Omega_t)$
$b2=sin(I_t) \cdot sin(\Omega_t)$

Rel Inc $\theta=arcos(a1 \cdot b1 + a2 \cdot b2 + a3 \cdot b3 )$

2: The dV required for a plane change is:
$\Delta_V$ = Required dV

= orbital velocity at time of burn
$\theta$ = Relative inclination

$\Delta_V=2\cdot\ V \cdot\sin \left(\frac{\theta}{2}\right)$

From

Geostationary transfer orbit - Wikipedia, the free encyclopedia

Quote:

For a typical GTO with a semimajor axis of 24,582 km, perigee velocity is 9.88 km/s and apogee velocity is 1.64 km/s, clearly making the inclination change far less costly at apogee. In practice, the inclination change is combined with the orbital circularization (or "apogee kick") burn, so additional V is required.[citation needed]
Even at apogee, the fuel needed to reduce inclination to zero can be significant, giving equatorial launch sites a substantial advantage over those at higher latitudes. Kennedy Space Center is at 28.5 degrees north, the Guiana Space Centre, the Ariane launch facility, is at 5 degrees north latitude and Sea Launch launches from a floating platform directly on the equator in the Pacific Ocean. All have a significant advantage over Russia's high latitude launch sites.

Hope this helps

**C3PO** · 06-14-2012, 11:57 AM

You can save a bit of fuel if you do a small plane change in the GTO burn. If you launch from KSC (28.5° Inc) the optimum plane change is 2.2° during GTO insertion.

http://design.ae.utexas.edu/mission_...n_Planning.pdf

The difference is only 27.5 m/s so it doesn't matter that much in a deltaglider.

It might be worth mentioning that the Earth's sideral rotation period is not 86400 seconds (24 hours) but 86164.1 seconds.

N_Molson · 06-14-2012, 04:00 PM

Quote:

The difference is only 27.5 m/s so it doesn't matter that much in a deltaglider.

But I'm precisely not using a DG

Quote:

For a typical GTO with a semimajor axis of 24,582 km, perigee velocity is 9.88 km/s and apogee velocity is 1.64 km/s, clearly making the inclination change far less costly at apogee. In practice, the inclination change is combined with the orbital circularization (or "apogee kick") burn, so additional V is required.[citation needed]

I'm pretty sure that Proton-M, for exemple, doesn't do it that way, but fire numerous burns split over several orbits. And I foresee a problem with cryogenic stage like the Centaur : it takes several hours to get at Apogee and the LOX/LH2 combination is very sensitive to boiloff.

---------- Post added at 04:00 PM ---------- Previous post was at 03:21 PM ----------

Update : I just used Dgatsoulis figures and theoretically, it works well. The only problem is that the required Dv for the plane adjustement seems really huge to me.

Also, the first node on my trajectory is the Ascending Node, which, unfortunately, requires twice more burn time than the Descending Node (according to AlignPlanesMFD). It happen I don't have that much fuel (my payload is probably a little to heavy, but I doubt its the only reason) So the optimal solution would be to climb to the Apoapsis, and then use the Descending Node, pass the Periapsis, then the satellite waits the Apoapsis to perform the circularization burn.

Does someone knows how NASA/ULA/Air Force proceeds when delivering a payload to GTO, using cryogenic upper stages like on the Atlas-V and Delta-4 ?

**C3PO** · 06-14-2012, 06:26 PM

Quote:

Originally Posted by N_Molson

Also, the first node on my trajectory is the Ascending Node...

Are you launching from a pad south of the equator?

The usual profile should be to get into roughly circular orbit, coast to the equator where you get into the GTO. That way you'll place your apogee on the equatorial plane on the the "other side", where the remaining plane change will be done.

boogabooga · 06-14-2012, 08:49 PM

Actually, I've had good results getting satellites to GEO with IMFD. I've been working on a tutorial, I'll try to step it up to get it finished. I use the IMFD Delta-V tool in the P30 LVLH frame. The one point of advice I have is to perform GEO cirularization and plane change together in one or more burns. That's even more efficient than doing plane change in one separate manuver, then circularization in another.

Quote:

Originally Posted by N_Molson

But I'm precisely not using a DG

Does someone knows how NASA/ULA/Air Force proceeds when delivering a payload to GTO, using cryogenic upper stages like on the Atlas-V and Delta-4 ?

Try this:
http://www.ulalaunch.com/site/docs/p...sGuide2010.pdf

(Edit) and this
http://www.ulalaunch.com/site/docs/p...sGuide2007.pdf

Quote:

Originally Posted by C3PO

You can save a bit of fuel if you do a small plane change in the GTO burn. If you launch from KSC (28.5° Inc) the optimum plane change is 2.2° during GTO insertion.

http://design.ae.utexas.edu/mission_...n_Planning.pdf

Cryogenic stages have a few hour lifetime. If your're going to drop the stage anyway at the end of the burn, it makes sense to do as much plane change as you have the extra delta-V for. In mission literature, you see a lot of transfer orbits at 20 degree ish inclinations. BTW, I know Delta IV has a kit to extend the life of its second stage to apogee, but it is an extra, and I don't think it's often used.

N_Molson · 06-14-2012, 09:36 PM

OK according to the (very complete) Atlas-V user manual, my main mistake was to use a direct ascent flight plan, which according to them is not well suited for GTO missions (from Cape Canaveral). Ariane 5 performs direct ascent GTO missions, but of course Kourou is near the equator, which allows that.

I'm going to test a 2 or 3 burns flight plan.

**C3PO** · 06-14-2012, 09:54 PM

Quote:

Originally Posted by boogabooga

Cryogenic stages have a few hour lifetime. If your're going to drop the stage anyway at the end of the burn, it makes sense to do as much plane change as you have the extra delta-V for. In mission literature, you see a lot of transfer orbits at 20 degree ish inclinations.

In real life you have to have to adapt to the actual launcher/payload combination. If the first stage takes you higher than circular orbit, you take the DV that it gives. The main objective is to put the perigee on the equator so the kicker engine can do the circulation and the plane change in one burn. If you have a cryogenic upper stage you use any extra DV to reduce inclination. A GTO of ~20° sounds very plausible because doing a 8°+ plane change in LEO takes a lot of fuel.

boogabooga · 06-14-2012, 10:43 PM

Quote:

Originally Posted by N_Molson

OK according to the (very complete) Atlas-V user manual, my main mistake was to use a direct ascent flight plan, which according to them is not well suited for GTO missions (from Cape Canaveral). Ariane 5 performs direct ascent GTO missions, but of course Kourou is near the equator, which allows that.

I'm going to test a 2 or 3 burns flight plan.

I think there is also a interesting hybrid direct ascent/parking orbit mission profile in there called extended coast transfer (or do I have my terms confused?). The "parking orbit" is highly elliptical such that the initial burn to GTO occurs at a very high altitude, thus the final GTO has much less eccentricity.

At C3PO:
I'm not quite sure what you mean. What I was saying is that you can do more plane change "low" than 2.2 degrees even if it adds overall extra delta-V to the mission, because you may have extra delta-V to use "low" that will get thrown away anyway and be useless at apogee. This is due to the time limit of the stage.

Here is an example of both points. I'm looking at the mission booklet for the MUOS mission earlier this year. Here is the orbit at spacecraft separation:

Orbit at SC Separation:
Apogee Altitude: 35,786.6 km (19,323.2 nmi)
Perigee Altitude: 3,462.9 km (1,869.8 nmi)
Inclination: 19.0°
Argument of Pergiee: 179.0°

This shows that they did 9.5 degrees of plane change "low" and used an extended coast transfer technique to get perigee up above the LEO 200-300km ish altitude.

**C3PO** · 06-14-2012, 11:12 PM

I was actually agreeing with you.

If your cryo stage has to be used low, you use it there.

There are many ways of getting there, and it depends on the hardware. In the Atlas V user guide there are at least 3 different profiles, even one that goes beyond the GSO and back in again.

If you perform the GTO insertion high, you can have a perigee that's high. This leaves less DV for the kicker engine.

N_Molson · 06-14-2012, 11:33 PM

After some tests, the key seems to perform the GTO burn so that the node stays "ahead". The position to the equator seems to determine this. But that's a very unprecise, "trial & error" approach.

I'd like to have a "by-the-numbers" procedure that guarantees me the best efficiency. Which was the idea behind using AlignPlaneMFD. From what we said, it seems that this MFD is not relevant to plan a precise GTO (remember the title of the thread) ?

Any ideas on procedures that can optimize the launch profile in a GTO type mission ?

06-14-2012, 02:30 AM	#1
N_Molson Addon Developer	How to use AlignPlaneMFD to achieve GTO ? Hello, The title sums it up. Let's assume I launch a rocket from Cape Canaveral (Equ. Inc. roughly 28°) and want to send a satellite in GTO (the satellite will perform the circularization burns). The idea is to launch East (90°) and progressively shift from a heading guidance to an inertial guidance as the rocket acquires horizontal velocity. The result of this is a sinuosoid on the MapMFD that crosses the equator over the Atlantic Ocean. Now I want to reduce that 28° Equ. Inc. to 0° Equ. Inc. , because a true GSO stays right over a given spot on the ground and then right over the Equator (if you have an inclination, the satellite will draw an "8" above its station coordinates). For that, I use a restartable upper stage. So this is where I'd like to use AlignPlaneMFD to plan the alignement burn. The ELS (Elements) button seems the right way to do this, because I don't want to align with a vessel but with an orbit. Of course I could put a DG in GSO and align with it, but this is no fun. At this point I have two numbers to enter : the desired Inclination and the LAN. The LAN is given by OrbitMFD. The problem is then the other number. AlignPlaneMFD uses Ecliptic plane as a reference, while I use Equatorial plane as a reference. So entering "0" obviously doesn't work. So the final question is : what I should enter there, and how to calculate it ? I tried Earth axial tilt (23.44°) but that doesn't work. I'm definitively missing something about orbital mechanics. Subsidiary question : is it more efficient to perform a part of the alignement during the launch, by "bending" the trajectory off the velocity vector, or to launch straight ahead and make the inclination once in transfer orbit ? Last edited by N_Molson; 06-14-2012 at 02:34 AM.

06-14-2012, 07:41 AM	#6
dgatsoulis Orbinaut	Question1: By placing a bunch of ships on different places at GSO and comparing the Eq frame and Ecl frame orbital parameters, I noticed that they always had an Inc of 23.44 degrees and LAN 179.98 degrees wrt the ecliptic. Those are the parameters you need to use in AlignPlanesMFD. The calculation for Rel.Inc is: initial inclination target inclination $\Omega_i =$ initial LAN $\Omega_t =$ target LAN $a1=sin(I_i) \cdot cos(\Omega_i)$ $a2=sin(I_i) \cdot sin(\Omega_i)$ $b1=sin(I_t) \cdot cos(\Omega_t)$ $b2=sin(I_t) \cdot sin(\Omega_t)$ Rel Inc $\theta=arcos(a1 \cdot b1 + a2 \cdot b2 + a3 \cdot b3 )$ 2: The dV required for a plane change is: $\Delta_V$ = Required dV = orbital velocity at time of burn $\theta$ = Relative inclination $\Delta_V=2\cdot\ V \cdot\sin \left(\frac{\theta}{2}\right)$ From Geostationary transfer orbit - Wikipedia, the free encyclopedia Geostationary transfer orbit - Wikipedia, the free encyclopedia Quote: For a typical GTO with a semimajor axis of 24,582 km, perigee velocity is 9.88 km/s and apogee velocity is 1.64 km/s, clearly making the inclination change far less costly at apogee. In practice, the inclination change is combined with the orbital circularization (or "apogee kick") burn, so additional V is required.[citation needed] Even at apogee, the fuel needed to reduce inclination to zero can be significant, giving equatorial launch sites a substantial advantage over those at higher latitudes. Kennedy Space Center is at 28.5 degrees north, the Guiana Space Centre, the Ariane launch facility, is at 5 degrees north latitude and Sea Launch launches from a floating platform directly on the equator in the Pacific Ocean. All have a significant advantage over Russia's high latitude launch sites. Hope this helps

06-14-2012, 08:49 PM	#10
boogabooga Orbinaut	Actually, I've had good results getting satellites to GEO with IMFD. I've been working on a tutorial, I'll try to step it up to get it finished. I use the IMFD Delta-V tool in the P30 LVLH frame. The one point of advice I have is to perform GEO cirularization and plane change together in one or more burns. That's even more efficient than doing plane change in one separate manuver, then circularization in another. Quote: Originally Posted by N_Molson But I'm precisely not using a DG Does someone knows how NASA/ULA/Air Force proceeds when delivering a payload to GTO, using cryogenic upper stages like on the Atlas-V and Delta-4 ? Try this: http://www.ulalaunch.com/site/docs/p...sGuide2010.pdf (Edit) and this http://www.ulalaunch.com/site/docs/p...sGuide2007.pdf Quote: Originally Posted by C3PO You can save a bit of fuel if you do a small plane change in the GTO burn. If you launch from KSC (28.5° Inc) the optimum plane change is 2.2° during GTO insertion. http://design.ae.utexas.edu/mission_...n_Planning.pdf Cryogenic stages have a few hour lifetime. If your're going to drop the stage anyway at the end of the burn, it makes sense to do as much plane change as you have the extra delta-V for. In mission literature, you see a lot of transfer orbits at 20 degree ish inclinations. BTW, I know Delta IV has a kit to extend the life of its second stage to apogee, but it is an extra, and I don't think it's often used. Last edited by boogabooga; 06-14-2012 at 09:14 PM.

06-14-2012, 11:33 PM	#15
N_Molson Addon Developer	After some tests, the key seems to perform the GTO burn so that the node stays "ahead". The position to the equator seems to determine this. But that's a very unprecise, "trial & error" approach. I'd like to have a "by-the-numbers" procedure that guarantees me the best efficiency. Which was the idea behind using AlignPlaneMFD. From what we said, it seems that this MFD is not relevant to plan a precise GTO (remember the title of the thread) ? Any ideas on procedures that can optimize the launch profile in a GTO type mission ? Last edited by N_Molson; 06-14-2012 at 11:48 PM. Reason: typo

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06-14-2012, 03:05 AM	#2
sorindafabico Orbinaut	I know it's not the answer you want, but it's easier to use IMFD, I guess.

06-14-2012, 03:13 AM	#3
Cras Spring of Life!	do you mind elaborating. I am interested in seeing how others get to GEO with 0 equatorial inclination. I have never tried it myself, I always just settle for the "figure 8" thing myself.

06-14-2012, 05:44 AM	#4
Zachstar Donator	Just go to the moon first In b4 Flytandem!

06-14-2012, 07:34 AM	#5
Tommy Orbinaut	Quote: Originally Posted by sorindafabico I know it's not the answer you want, but it's easier to use IMFD, I guess. Quote: Originally Posted by Cras do you mind elaborating. I am interested in seeing how others get to GEO with 0 equatorial inclination. I have never tried it myself, I always just settle for the "figure 8" thing myself. Well, I haven't tried it myself, but I seem to recall that you can target GeoSync with IMFD. If I have time (strongly recommend you don't hold your breath waiting) I will try to look into this further. In the meantime, I wonder if the following might work: Orbiter's stock MFD's assume a point gravity source, so if you use OrbitMFD in Ecl mode it should show you your inclination in the ecliptic frame - even while landed. It would show a LAN and Inc, which could be adapted to be used in AlignPlanesMFD if you sort of "reverse" things a bit. As to whether it would be more efficient to "dogleg" during the ascent, probably not for most vessels. The problem is that you want to launch at a 90 degree heading, and by the time you cross the equator you are past your ApA (at least in most vessels - but the DGEx has such a slow ascent profile it may be the exception). I suspect that the most efficient method would be to have a low ascent profile, with the goal of having your initial ApA just after you cross the equator - that way you can make the plane alignment before the circularization burn (and your velocity is still a bit below orbital velocity). IIRC, the one time I tried to reach GEO from KSC I basically set up a suborbital "hop" with a low ApA (about 90km), and then used aerodynamic lift to make the plane change, then another burn to establish my desired "parking" ApA (about 150k, since it doesn't need to be stable), then when I reached that ApA I burned again to establish a new ApA at GEO altitude. That worked, but I imagine there is a better way to do it - that was a few years ago and my skills were pretty crude then. Not that they are exactly "art" now ...

06-14-2012, 11:57 AM	#7
C3PO Donator	You can save a bit of fuel if you do a small plane change in the GTO burn. If you launch from KSC (28.5° Inc) the optimum plane change is 2.2° during GTO insertion. http://design.ae.utexas.edu/mission_...n_Planning.pdf The difference is only 27.5 m/s so it doesn't matter that much in a deltaglider. It might be worth mentioning that the Earth's sideral rotation period is not 86400 seconds (24 hours) but 86164.1 seconds.

06-14-2012, 04:00 PM	#8
N_Molson Addon Developer	Quote: The difference is only 27.5 m/s so it doesn't matter that much in a deltaglider. But I'm precisely not using a DG Quote: For a typical GTO with a semimajor axis of 24,582 km, perigee velocity is 9.88 km/s and apogee velocity is 1.64 km/s, clearly making the inclination change far less costly at apogee. In practice, the inclination change is combined with the orbital circularization (or "apogee kick") burn, so additional V is required.[citation needed] I'm pretty sure that Proton-M, for exemple, doesn't do it that way, but fire numerous burns split over several orbits. And I foresee a problem with cryogenic stage like the Centaur : it takes several hours to get at Apogee and the LOX/LH2 combination is very sensitive to boiloff. ---------- Post added at 04:00 PM ---------- Previous post was at 03:21 PM ---------- Update : I just used Dgatsoulis figures and theoretically, it works well. The only problem is that the required Dv for the plane adjustement seems really huge to me. Also, the first node on my trajectory is the Ascending Node, which, unfortunately, requires twice more burn time than the Descending Node (according to AlignPlanesMFD). It happen I don't have that much fuel (my payload is probably a little to heavy, but I doubt its the only reason) So the optimal solution would be to climb to the Apoapsis, and then use the Descending Node, pass the Periapsis, then the satellite waits the Apoapsis to perform the circularization burn. Does someone knows how NASA/ULA/Air Force proceeds when delivering a payload to GTO, using cryogenic upper stages like on the Atlas-V and Delta-4 ?

06-14-2012, 06:26 PM	#9
C3PO Donator	Quote: Originally Posted by N_Molson Also, the first node on my trajectory is the Ascending Node... Are you launching from a pad south of the equator? The usual profile should be to get into roughly circular orbit, coast to the equator where you get into the GTO. That way you'll place your apogee on the equatorial plane on the the "other side", where the remaining plane change will be done.

06-14-2012, 09:36 PM	#11
N_Molson Addon Developer	OK according to the (very complete) Atlas-V user manual, my main mistake was to use a direct ascent flight plan, which according to them is not well suited for GTO missions (from Cape Canaveral). Ariane 5 performs direct ascent GTO missions, but of course Kourou is near the equator, which allows that. I'm going to test a 2 or 3 burns flight plan.

06-14-2012, 09:54 PM	#12
C3PO Donator	Quote: Originally Posted by boogabooga Cryogenic stages have a few hour lifetime. If your're going to drop the stage anyway at the end of the burn, it makes sense to do as much plane change as you have the extra delta-V for. In mission literature, you see a lot of transfer orbits at 20 degree ish inclinations. In real life you have to have to adapt to the actual launcher/payload combination. If the first stage takes you higher than circular orbit, you take the DV that it gives. The main objective is to put the perigee on the equator so the kicker engine can do the circulation and the plane change in one burn. If you have a cryogenic upper stage you use any extra DV to reduce inclination. A GTO of ~20° sounds very plausible because doing a 8°+ plane change in LEO takes a lot of fuel.

06-14-2012, 10:43 PM	#13
boogabooga Orbinaut	Quote: Originally Posted by N_Molson OK according to the (very complete) Atlas-V user manual, my main mistake was to use a direct ascent flight plan, which according to them is not well suited for GTO missions (from Cape Canaveral). Ariane 5 performs direct ascent GTO missions, but of course Kourou is near the equator, which allows that. I'm going to test a 2 or 3 burns flight plan. I think there is also a interesting hybrid direct ascent/parking orbit mission profile in there called extended coast transfer (or do I have my terms confused?). The "parking orbit" is highly elliptical such that the initial burn to GTO occurs at a very high altitude, thus the final GTO has much less eccentricity. At C3PO: I'm not quite sure what you mean. What I was saying is that you can do more plane change "low" than 2.2 degrees even if it adds overall extra delta-V to the mission, because you may have extra delta-V to use "low" that will get thrown away anyway and be useless at apogee. This is due to the time limit of the stage. Here is an example of both points. I'm looking at the mission booklet for the MUOS mission earlier this year. Here is the orbit at spacecraft separation: Orbit at SC Separation: Apogee Altitude: 35,786.6 km (19,323.2 nmi) Perigee Altitude: 3,462.9 km (1,869.8 nmi) Inclination: 19.0° Argument of Pergiee: 179.0° This shows that they did 9.5 degrees of plane change "low" and used an extended coast transfer technique to get perigee up above the LEO 200-300km ish altitude.

06-14-2012, 11:12 PM	#14
C3PO Donator	I was actually agreeing with you. If your cryo stage has to be used low, you use it there. There are many ways of getting there, and it depends on the hardware. In the Atlas V user guide there are at least 3 different profiles, even one that goes beyond the GSO and back in again. If you perform the GTO insertion high, you can have a perigee that's high. This leaves less DV for the kicker engine.