General Question Reentry parameters

[Ilmars]

Hi:

I'm somewhat puzzled by the following 3 reentry parameters:

ReA : reentry angle
Ant : anticipation angle
RAl : atmospheric altitude interface

I notice that plugins like BasesyncMFD allow the entry of values for these parameters in an independent and somewhat unrelated manner.

I would have expected that one of the parameters would always be dependent on the other two.

For example:

If RAl and ReA are set, then the Ant value should result and be dependent
on the other two parameters.

And, if RAl and Ant are set, then the ReA should result and
likewise be dependent.

What am I missing, why is it possible to enter values for each of the
three parameters independently?

Any insight you may provide will be appreciated.

Thanks,
Ilmars

 

[sorindafabico]

If your deorbit burn was well done, the chosen ReA is your glideslope (distance in degrees of the velocity vector from the horizon line) at chosen RAl. Ant is the angular distance between your RAl point and your landing site in a (I suppose) ballistic trajectory. Ant is important to define your deorbit burn point. If smaller, you'll land before the target. If bigger, you'll land after the target.

The numbers change for each spacecraft and according to your orbit. Example: for a XR5 on a 300km circular orbit and an AoA of 40 degrees on reentry, I use ReA = 1.5, RAl 120km and Ant = 40. The numbers are not the same for a XR2. They have a similar looking (compared to a capsule), but are different enough to change the numbers.

If you use another RAl, you'll have to use another ReA, because the glideslope changes at each altitude, but the effect is the same if you put the equivalent numbers. On Earth, you can choose RAl between 80 and 120 as you prefer.

If you're in a capsule coming from The Moon, the ReA will be something like 6 and the Ant won't be so large. Bigger the reentry velocity, bigger the reentry angle.

Capsules generate less lift, so their Ant is smaller because they cover a smaller distance than winged vessels after they enter atmosphere. It doesn't matter if they're coming from the Moon or from LEO. Less lift, smaller Ant.

The bad (or not, depends on your point of view) part of my answer is: you'll have to guess the values for each spacecraft.

 

[Ilmars]

If you use another RAl, you'll have to use another ReA, because the glideslope changes at each altitude, but the effect is the same if you put the equivalent numbers.

Hi Arthur:

Yes, I understand that a different RAl yields a different ReA, and that the all the parameters differ depending on the vehicle.

But isn't the reentry angle (ReA) a function of the altitude and the anticipation angle? So, if you specify an altitude interface value and a value for the anticipation angle, there can only be one ReA for those values for a given vehicle. Is that not the case?

And, if you specify desired reentry angle (ReA), and an altitude (RAl), there should only be one anticipation angle that results from those values?

Ilmars

 

[Cras]

Re-entry altitude, set it dependent on the planet. 80km seems to work well for Earth.

for the others, use a combination that makes sense for what you want. The shallower the re-entry, the longer it will take, the more cross-range you may end up having. If you are flying an XR class vessel, and have low APU fuel left, you can go for a steeper descent.

 

[sorindafabico]

But isn't the reentry angle (ReA) a function of the altitude and the anticipation angle?

Sorry, I don't know. =/

The Ant depends on the capacity of the vessel to generate lift, also. But, if you want a nominal descent without changes on AoA, maybe there's some kind of relationship between these values (i never found).

If you choose a "wrong" Ant, you can fix it by changing AoA during descent (works better when the vessel is more lift-capable).

 

[Ilmars]

Thanks, Arthur.

I guess I may be trying to oversimplify things. My thoughts were that it might be rather like solving for a missing side of a triangle. If you know the values for 2 of the sides, the other can be derived.

But I guess when one factors in all the other issues, such as lift/drag etc between the different vehicles, the relationship between the parameters is not that simplistic or easy to generalize.

I'm still puzzling over it, though. If you set a value for the altitude, and a value for the reentry angle, then there can only be one value for the anticipation angle that will coincide with the target base. Anything else will overshoot or undershoot.


llmars

 

[Ilmars]

After reading the IMFD manual, full version, I finally realized the flaw in my thinking.

For some reason I had assumed that the reentry angle continued to be measured from the anticipation angle radial, to the target base. But it is measured from the deorbit burn point to the intersection of the atmospheric interface with the anticipation angle radial. So, it makes sense now why all three parameters have to be set.

Ilmars

 

[Tommy]

Yes, Ant is independent of ReA and EI. It will vary with the lift and drag of the vessel, as well as it's heat tolerance and even the pilot's technique.

 

[Ilmars]

Yes, Ant is independent of ReA and EI. It will vary with the lift and drag of the vessel, as well as it's heat tolerance and even the pilot's technique.

Yes, thanks Tommy, I see that now. My confusion was that for some reason I assumed that the trajectory path defined by the ReA continued to be plotted within the scope of the Ant angle, right down to the target. Which, is not the case.

Ilmars

 

[Urwumpe]

Yes, thanks Tommy, I see that now. My confusion was that for some reason I assumed that the trajectory path defined by the ReA continued to be plotted within the scope of the Ant angle, right down to the target. Which, is not the case.

Ilmars

You also have to include that the reentry angle is a function of vehicle aerodynamics (including mass) and heat shield capability, and thus more or less constant for a vehicle class.

You generally aim for a fixed set of these three values and correct any inaccuracies with piloting. For example, your Ant depends on how fast you can slow down, the faster, the shorter is Ant.

And PS: The reentry angle can really happen to be plotted in a straight line down to the target. If you do a steep ballistic reentry (like a ICBM warhead), the aerodynamic forces will dominate over gravity and you will travel in almost a straight line until low supersonic.