Flight Question Calculating or determining a good ReEntry Anticipation Angle

[unknown_orbiter]

Hello all! I've been obsessed recently about doing a lot of math on paper to determine the optimum variables for orbital maneuvers. A couple things have been bugging me though, and no amount of research has given me a concrete answer.

In the DeOrbit program within the BaseSync MFD (v2.3), you have to enter values for "Reference Altitude", "ReEntry Angle", and "ReEntry Anticipation Angle" for the program to predict accurate burn times and locations. I understand that the Reference Altitude represents the altitude above the celestial body's surface at which point significant atmospheric effects are felt by the de-orbiting craft (80 km for Earth). I also understand that the ReEntry Angle is the angle at which you descend from your initial orbit towards your target landing area. And, I understand that the Anticipation Angle is the angle between your radial position at the point you intercept the Reference Altitude, and the radial position of the target landing area.

What I do not understand however is how to calculate appropriate values for those last two. Yes, I know that ~.7 degrees is a "good" reentry angle for the DGIV, but why and how can such a value be determined for any craft (I'm assuming based upon information such as lift coefficients and vessel mass). As for Anticipation Angle, I am fully lost after seeing so many people use so many different values. I've mostly seen 45 and 16.5 degrees. 45 seems to me like a very long distance from your target base to be ripping through atmosphere. Can someone give me some info on how to either calculate these two variables or at least how to choose them wisely?

Thank you in advance!

 

[TerraMimic]

I seem to be doing pretty well with these values in IMFD for XR-2, XR-5 and DG-IV. I even brought the XR-5 in heavy without any issues.

Alt - 80km
ReA - 0.2 degrees
Ant-70 degrees

And then I use Aerobrake MFD after the burn to bring me down near the base.

Got the info from this thread: http://www.orbiter-forum.com/showthread.php?t=24050

Edit: Accidently put the wrong link in there.

 

[boogabooga]

Why did you recursivly link back to this thread?

 

[Tommy]

Just to clarify, ReA is the angle at which you cross the Atmospheric Interface altitude (aka reference Altitude).

Because of this, ReA is partially dependent of where you have the AI. Having the AI set to 120k ("standard" for Earth - but 80k works better in Orbiter) will yield a higher ReA than if the AI is set to 80k (assuming that the ApA and PeA are the same in both cases).

Also, the higher the orbit you are in before de-orbiting, the higher the ReA will be - even if the PeA (after de-orbiting) is the same.

So, it doesn't matter much what you use for the AI altitude - within limits. Anywhere from 80k to 120k will work fine for Earth, just be consistent and use the same value every time.

ReA will vary depending on your starting altitude (and AI altitude). From a 200k parking orbit and a AI of 120k, an ReA of about 1.2 is fine - but for a direct lunar return you'll want a ReA of 6 or higher.

Anticipation will change depending on the vessel (different mass and drag) and on the velocity (which will be higher if you are coming from a higher orbit, of course). It will be shorter if you have a lower AI. It will also change depending on how you fly your re-entry, so there's an "individual adjustment" based on your re-entry technique.

Getting the numbers "just right" for a de-orbit can be important if you are flying a capsule - they have little lift so you can't adjust during the re-entry.

For a glider type vessel (includes the XR-series) then the de-orbit isn't crucial. You can use the same settings to de-orbit from 200k as you would de-orbiting from the ISS, and adjust for the differences during the re-entry.

It's better to develop your re-entry skills than it is to focus on de-orbiting "perfectly".

So, in order to get some numbers that work for you, you'll have to experiment. The DGIV is a good "test bed". Start from the ISS. Pick an AI, 120k is the default for the DGIV's computer and IMFD's Map. Try a d-orbit burn and lower your PeA to 50k, and see what the DGIV (or IMFD Map) tells you for ReA - and use that in the future. Then experiment with Anticipation until you get a value that allows you a nice "nominal" re-entry (not too aggressive, not to lame). Once you have those numbers, they should work for any starting altitude from 200k to 500k, If you are starting from a higher altitude, just increase the ReA (up to a max of 6.5 for a direct re-entry) and Ant (you'll need to experiment a bit to learn how much to increase Ant - but it shouldn't be more than 30 degrees extra if you are on a direct re-entry)

 

[TMac3000]

But the XR Series craft don't tell you what your current entry angle is. I always start from a 350-370 km orbit.

If you use Aerobrake MFD, you never have to worry about missing your target, because it tells you where you will come down. What you worry about instead is burning up or skipping. You'll know the deorbit burn is wrong if keeping the endpoint on target requires an angle that's way outside of parameters and so will cause a burn-up or a skip

 

[unknown_orbiter]

Just to clarify, ReA is the angle at which you cross the Atmospheric Interface altitude (aka reference Altitude).

Because of this, ReA is partially dependent of where you have the AI. Having the AI set to 120k ("standard" for Earth - but 80k works better in Orbiter) will yield a higher ReA than if the AI is set to 80k (assuming that the ApA and PeA are the same in both cases).

Also, the higher the orbit you are in before de-orbiting, the higher the ReA will be - even if the PeA (after de-orbiting) is the same.

So, it doesn't matter much what you use for the AI altitude - within limits. Anywhere from 80k to 120k will work fine for Earth, just be consistent and use the same value every time.

ReA will vary depending on your starting altitude (and AI altitude). From a 200k parking orbit and a AI of 120k, an ReA of about 1.2 is fine - but for a direct lunar return you'll want a ReA of 6 or higher.

Anticipation will change depending on the vessel (different mass and drag) and on the velocity (which will be higher if you are coming from a higher orbit, of course). It will be shorter if you have a lower AI. It will also change depending on how you fly your re-entry, so there's an "individual adjustment" based on your re-entry technique.

Getting the numbers "just right" for a de-orbit can be important if you are flying a capsule - they have little lift so you can't adjust during the re-entry.

For a glider type vessel (includes the XR-series) then the de-orbit isn't crucial. You can use the same settings to de-orbit from 200k as you would de-orbiting from the ISS, and adjust for the differences during the re-entry.

It's better to develop your re-entry skills than it is to focus on de-orbiting "perfectly".

So, in order to get some numbers that work for you, you'll have to experiment. The DGIV is a good "test bed". Start from the ISS. Pick an AI, 120k is the default for the DGIV's computer and IMFD's Map. Try a d-orbit burn and lower your PeA to 50k, and see what the DGIV (or IMFD Map) tells you for ReA - and use that in the future. Then experiment with Anticipation until you get a value that allows you a nice "nominal" re-entry (not too aggressive, not to lame). Once you have those numbers, they should work for any starting altitude from 200k to 500k, If you are starting from a higher altitude, just increase the ReA (up to a max of 6.5 for a direct re-entry) and Ant (you'll need to experiment a bit to learn how much to increase Ant - but it shouldn't be more than 30 degrees extra if you are on a direct re-entry)

That's a great amount of solid information, thanks!

I have another question. For the XR class vessels, which require APU to be running for dynamic COG adjustment and control surfaces, the vessel weight is constantly dropping. Does the Aerobrake MFD take into account the vessels current weight? Because if that's constantly changing, then wouldn't the Aerobrake MFD glide path be dynamic as well, and therefor nearly impossible to get right without a lot of guesswork?

 

[TMac3000]

It drops because of use of RCS propellant, APU fuel, and O2. These are used so slowly that the change in weight is negligable.

 

[unknown_orbiter]

It drops because of use of RCS propellant, APU fuel, and O2. These are used so slowly that the change in weight is negligable.

... so it drops because I'm using the fuel... but it shouldn't because such weight loss is negligible? 0.o

 

[TMac3000]

APU barely uses 2 kg/min. RCS will use a little now and then. Oxygen use is very slow (as long as you are calm) It takes about 20 mins from EI to touchdown. Your weight does change in that time, but not enough to have a significant effect on your reentry