General Question Making plane changes via Friction

[EliNaut]

Happy Holidays everyone....

I've been really cracking down on my orbital mechanics skills, and am looking to try something a little different away from the basics.

As the title says, i'm looking to make an orbital plane change by brushing the upper atmosphere. I know the basic outline of it; Make a retro burn to lower your Pe, almost like your performing a reentry; Utilize banking and a pitch with a decent amount of lift so you won't dive too deep into the atmosphere; use your engines to reobtain orbit. Alot like suborbital flights.

Basically what i'm looking for is some tips, mainly concerning what attitude to hold and how low I should maintain my Pe before diving in. I know some of those details may be craft specific, but I plan on trying this with an XR2 first, to make holding the attitude a bit easier, before giving it ago with a larger craft, say, Resolve.

 

[flytandem]

I've played around a fair bit with that but only with stock Deltaglider. If the craft has an L/D ratio greater than 1 (the DG has about 3:1 i think) then you only have to apply thrust to offset the drag while you have the ability to generate plane change deltaV using the lift. Bank angle would average about 90 degrees if you have about orbital speed. I usually did it at about 65Km agl or maybe a bit higher but not above 70 usually.

 

[Tommy]

I've done this in the DGIV. It has an advantage over the XR series for this manuever since it can hold a 90 degree bank at 15 degrees AoA.

I'm not sure what the XR's heat resistance is for this, in a DGIV you need to have zeroed your VS by 65k alt, you'll burn up at orbital velocity any lower. Most of the flight is at 85 degrees bank or 90 degrees bank, occasionally as low as 80 degrees. Any less bank and you'll have to lower your AoA to stay in the atmosphere, reducing the amount of lift you can apply to the plane change.

For an XR series, I'd try a PeA of about 70k. I'd forget about the attitude hold AP, just apply full upward trim and use bank to control altitude. You may want to experiment with moving the cog very slightly aft, but since you'll be at very high bank angles I'm not sure how well that will work.

The Align Planes MFD can be usefull for this. Watch the TN and Tth lines. Try to get both down to zero simultaneously. If your Tth is decreasing, but your TN isn't it indicates that you are turning too sharply - you may even see the TN increasing.
In that case, lower your bank and AoA to "loosen" your turn.

It takes some practice, but for large plane changes it can save you alot of fuel. For a Mir to ISS transfer (about 78 degrees RInc) you can do it using half or less of the fuel a standard plane change requires.

 

[EliNaut]

I've done this in the DGIV. It has an advantage over the XR series for this manuever since it can hold a 90 degree bank at 15 degrees AoA.

Gee you sure about that? From what I know it can maintain any attitude - its just heading you have to set manually.

 

[Tommy]

The XR series restricts banks at higher AoA's. Not sure where the cutoff for "higher" is, but I don't rember it ever letting me go more than 60 degrees of bank at 15 degrees AoA. This can be altered in the config, but the XR's autopilot isn't stable on my PC at the higher bank angles (with a 15 degree AoA). The XR series AP uses elevator trim and COG shifting to maintain it's AoA, and this causes problems at the high bank angles. The DGIV doesn't shift it's COG, just uses RCS and perhaps trim so it is less affected by the bank angle.

 

[HiPotOk1978]

I have use the atmosphere for plane changes in DGIV and all of dougs XR crafts. A few things to remember when your doing this

Your going REALLY FAST:
It will take some time in the atmosphere to perform this maneuver , forcing the change by pulling more G's in the upper atmosphere is tricky and your gonna most likely doing this manually.

Time in atmosphere = drag + heat:
the more bite you can get in the atmosphere the easier it will be to control, and the greater degree of change you will be able to accomplish, this has the side effect of losing h-velocity at a greater rate as well

Velocity Loss vs degree change
The more time you spend trying to change your plane in the atmosphere using your wings the more energy your gonna loose. It is not a free ride so your gonna have to weigh the loss of velocity vs doing the change with engines outside the atmosphere

Reasonable limit on amount of change:
This kind of maneuver is great for minor changes of a few degrees and if your using this for fine tuning your reentry, some of the above problems I have mentioned may be useful as well such as loosing energy.

My thoughts on the value of this maneuver? It is great as one of the last things your gonna do in orbit before touchdown at your favorite spaceport. Also good for getting that last little nudge into the orbit your looking for if your launching a probe or satellite to another planet and the craft has some sort of aerodynamic properties when doing aerobraking that would allow you to take advantage of atmosphere drag and lift

 

[Tommy]

It's also good for very large plane changes. A Mir - ISS transfer will use about 7km/s of Delta-V, maybe more. It's 78 degrees of RInc. I'm sure someone here can provide a formula for how much Delta-V a given plane change requires.

Using an aerodynamic plane change in a DG type craft you can do this plane change and need less than 4km/s Delta-V from the engines. I typically use engines to maintain speed when I get down to about 5 km/s, which uses a little fuel, and of course rebuilding the orbit takes some more.

It won't always be more efficient - theres a range from low to mid RInc where it's not, but I don't know where those cut-off points are.

 

[Scrooge McDuck]

So, I'm a little confused by the replies here if using the atmosphere could actually be more efficient at small plane changes. And also why it would be more efficient at large plane changes?

During (or after?) the atmospheric 'dive' you'll need to compensate the drag factor using the engines, so if we have a spacecraft with a real good L/D or glideratio, then you could do small plane changes too?

BTW, the dV needed for normal plane changes is

(using the orbital velocity and change in angle)

If we could somehow calculate how many dV is needed to compensate any velocity loss by atmpospheric drag, then we could see if it could save fuel.. Would it be possible to calculate this with as only input factor the L/D and the required delta-planechange angle?

But enlarging the ApA and doing the the planechange there could also save fuel, maybe even more...

 

[Tommy]

It can be slightly more efficient for very small plane changes since you won't be in the atmosphere long enough to loose much V. If you're doing it for a re-entry prep, you won't need to re-circularise anyway. For large plane changes it can be very much more efficient. A 90 degree plane change requires well over 10 km/s delta V in LEO, while an aerodynamic plane change of that magnitude can be done with about 4.5 km/s Delta V from the engines (to maintain speed and rebuild the orbit).
At the altitude you'll be doing this at, drag doesn't take much fuel to overcome, and there's no problem with letting you V drop to 3km/s or lower - it simply makes the plane change easier and more efficient (less lift required for the same rate of change = less drag)

Raising the Ap at a node, and doing the plane change there, can save fuel if you have the time, but by the time you've raised your Ap enough to get a real significant fuel savings, your orbital period will be measured in days, not hours. Don't forget that you'll need at least 1km/s dV (more likely 1.5 km/s), to raise the Ap enough, and the same to recircularise (although with proper aerobraking, fuel use for that is negligable).

In short, for any large plane change, aerodynamic plane changes are more efficient than a standard one in LEO, and much faster than using a high eccentricity PlC.

For extremely large plane changes (ove 100 degrees), you're probably better of making a full re-entry, and then ascending to orbit almost from scratch.

 

[EliNaut]

I did some playing with the stock DG. I was pretty surprised - I started out in perfectly lined up with the ISS's orbit and performed a "deorbit" burn. While holding a 90 degree bank, and as much pitch as the atmosphere would allow me, I was able to maintain at least 7km/s with small bursts from my engine and used small bank changes to keep my v/s very small (only using about +/-3% of fuel in the end) and ~61km altitude. Maintaining this pattern, I was able to cruise and climb into a 200x200 equatorial orbit.
I know what you might say - that this might not work for other vessels, since the stock DG's physics arn't that acute as say an XR2, but these results are a surprise to me nontheless.