Flight Question Maximum Entry Angle with Altitude - DGIV

[PaulG]

So, the DGIV can withstand reentry angles up to 1.2° correct? So, that angle, or the maximum safe angle must dependent on the altitude at deorbit burn right?

For example, lets say that I don't really care where I'm going to land, to make this simple, that I just want to concentrate on deorbit burn altitude and maximum safe reentry angle. Assume that I'm in the DGIV using the (D2) display for entry angle.

If I do a deorbit burn at and altitude of 250km, 0.8 ° really seems to push the limits on the hull temperature. But, it I burn at 150km, 0.8 ° is really pretty safe.

So, if I perform the deobit burn at 300, 250, 200, or 150 km, what is the maximum safe reentry angle?

Thanks.

 

[Oceanic]

when i do my reentry with the DG-IV, i reduce my orbit to ~220km alt, my reentry angle is about 0.8° - 0.9° (AoA 39° - 40°)
my hull temperature does not exceed 1800°C. so i guess at 220km you could use an angle of max 1.2°, but i'm not sure if the warning message will appear...

 

[PaulG]

Thats about what I am doing too....

For a while, I was starting at 150km or lower and hardly heating up at all. This is why I posed the question....

 

[Urwumpe]

Well, on demand:

The critical limiting factor for the initial reentry is the heating rate. The faster you are at altitude, the more energy you get as aerodynamic heat flux per second. You want to keep the DGIV well below this limit, so you have some tolerance.

Now, lets assume you reentry at normal attitude and lift vector up. In this situation, you will drop into the atmosphere until your vertical speed is neutralized. The faster you are at Entry Interface and the steeper your angle, the deeper you will dive into the atmosphere, with practically no control by you. Past the deorbit burn, you are in Newtons hand until you reached the level equilibrium glide and can afford diverting lift to the side.

So, yes, there is a relation between flight path angle (FPA) and speed at Entry Interface. And your speed at entry interface is of course depending on the orbit after the deorbit burn.

With some basic assumptions and simplifications, it should be possible calculating the entry interface limits, and thus calculate the resulting maximum orbit altitude for deorbit (apogee altitude of the post deorbit orbit) with an Excel spreadsheet.

But it would take me some minutes finding the formulas for the important values, as well as finding the maximum aerodynamic heatflux tolerable by the DGIV.

 

[PaulG]

Thanks... "On Demand" but much appreciated.

I'm not as interested in plotting that out, just knowing that assumption is in line is enough. That way, I have a better appreciation for what happens and what to expect. Thanks!

 

[Tommy]

I've brought a DGIV in at about a two degree re-entry angle and survived. A DGIV can withstand full re-entry velocity down to just under 65k alt, so it's more important what your slope is when you cross under 65k.

If you have a steeper re-entry angle, you can start at a lower AoA for better lift, and can start reducing your descent rate at about 75k altitude, with a target VS of -40 m/s at 65k alt. Then increase your AoA as required for normal re-entry, and keep the VS above -80 m/s.

That being said, I usually re-enter at about one degree.

EDIT......

I've been doing some flight tests, and so far am up to a 5.75 degree re-entry angle. Starting from the ISS, a de-orbit burn 6.25M from canaveral to attain 5.75 degrees PeA (PeA of -1065k, V < 7km/s), deadstick landing.

Remember, in order to get a steep re-entry angle, you need to burn off a lot of velocity in your de-orbit burn, so you'll be traveling slower when you hit re-entry interface. A re-entry angle this steep wouldn't work for a direct return - only re-entry from LEO.