Flight Question Entry angle

[henrik]

This is a pretty embarrassing n00b question for someone who's been flying Orbiter for years, but it's better to ask than to be baffled.

What baffles me most is re-entry. I know the basics and I can do a successful re-entry with DanSteph's DG-IV, but that's about it. My problem with any other craft is that I don't know how to calculate or see my entry angle anywhere. I know my entry angle should ideally be about .7 degrees, but without seeing it anywhere, the information is pretty useless.

All the tutorials that I've read use DG-IV as the spacecraft, which is not helpful because DG-IV has the entry angle prediction in one of its displays. What I'd need to know is how can I see it with other spacecraft as well.

 

[Urwumpe]

This is a pretty embarrassing n00b question for someone who's been flying Orbiter for years, but it's better to ask than to be baffled.

What baffles me most is re-entry. I know the basics and I can do a successful re-entry with DanSteph's DG-IV, but that's about it. My problem with any other craft is that I don't know how to calculate or see my entry angle anywhere. I know my entry angle should ideally be about .7 degrees, but without seeing it anywhere, the information is pretty useless.

All the tutorials that I've read use DG-IV as the spacecraft, which is not helpful because DG-IV has the entry angle prediction in one of its displays. What I'd need to know is how can I see it with other spacecraft as well.

Other vehicles need other reentry angles, also you often need to vary the angle if you come from extremely different orbits. The Space Shuttle for example 1.5° as normal reentry angle, but can be down to 1.0° for Hubble service missions, where the 1.5° would make it dive too deep in the atmosphere.

Apollo used 6° reentry angle, despite coming from the moon.

Calculating is pretty hard, but you can use IMFD for guiding you to a perfect reentry window. I could also write a MFD to display such data as the old Reentry MFD did (which doesn't work in 2010), but I don't have much time for such work.

I remember I once wanted to do a small plot, which shows you the right periapsis radius/altitude after deorbit, for an apoapsis, to get a desired reentry flight path angle in 120 km altitude.

The formula for Flight path angle is:

[math]\sin\phi=\frac{e\sin E}{\sqrt{1-e^2\cos^2E}}[/math]

or :

[math]\cos\phi=\sqrt{\frac{1-e^2}{1-e^2\cos^2E}}[/math]

e is eccentricity, E is eccentric anomaly, the position on the orbit ellipse using the center of the ellipse as reference.

 

[henrik]

Ah, so I didn't know even as much as I thought I did. Can't say I'm too surprised...

So how do I know the proper entry angle for the spacecraft I'm flying? I assume that that information is usually included in the documentation.

And thank you for the information about IMFD, I probably should finally learn how to use it.

 

[Urwumpe]

So how do I know the proper entry angle for the spacecraft I'm flying? I assume that that information is usually included in the documentation.

Yes it should. BUT, there is of course a formula around for approximating how deep you dive into the atmosphere, if you know your Lift-to-Drag ratio.

http://www-public.tu-bs.de:8080/~y0021684/pic/apollo11_reentry.png

Can be pretty helpful sometimes if you need to plan your reentry, if you already know from test flights, how much heat your spacecraft can take.

And thank you for the information about IMFD, I probably should finally learn how to use it.

I think I can try making a Reentry MFD, but you would have to be patient.

 

[Wishbone]

Why don't you take the boilerplate code from my Precession MFD? It is easy to clean old stuff and fit new formulas into it.

 

[Urwumpe]

Why don't you take the boilerplate code from my Precession MFD? It is easy to clean old stuff and fit new formulas into it.

well, I want to do more than just displaying data, I actually want a functional replacement, that does Deorbit and reentry guidance as well later...

It should then also have enough difference to Aerobrake MFD, which I personally find awkward to use.

 

[Wishbone]

Uh-huh. Am currently struggling with autopilot theory, Lyapunov, poles and stuff like that. Think that an autopilot public domain/open source library (orbital as well as atmospheric) would be a great thing for add-on developers.

 

[Urwumpe]

Uh-huh. Am currently struggling with autopilot theory, Lyapunov, poles and stuff like that. Think that an autopilot public domain/open source library (orbital as well as atmospheric) would be a great thing for add-on developers.

I don't think a full autopilot is the way to go (despite it being possible), I aim just at producing generic guidance screens so you know where to fly manually. :lol:

 

[Wishbone]

What is the difference between a generic guidance screen that accounts for man in the loop and an autopilot?

 

[Urwumpe]

What is the difference between a generic guidance screen that accounts for man in the loop and an autopilot?

The fact that I would need to implement that autopilot at the lowest level (thruster groups), can't use aerodynamic control surfaces easily and thus would require to implement special vessel code for every vehicle that I need to control. (IMFD does that by covering as many vehicles as possible and not being particularly effective, guidance for a DG during reentry would be impossible.)

A generic interface for turning guidance commands into vessel actuator actions would be a great leap for MFD programming (Like, the vessel DLL programmer defines how to get this rotation rate or how to rotate to this attitude, and the MFD just needs to define such commands)