Flight Question Venus Aerobrake

[Pioneer]

What would be a safe altitude for the XR2 to aerobrake in Venus's upper atmosphere? :feedback:

 

[dgatsoulis]

It depends on the velocity.

Velocity ___ Alt
-------------------
14 km/s → 96 km
13 km/s → 95 km
12 km/s → 94 km
11 km/s → 93 km

With those, you shouldn't get a temp more than 2000 C°.
Make a save before you try. It's been a while since my last Pericytherion.

 

[wingnut]

It depends on the velocity.

Velocity ___ Alt
[...]

Did you calculate this? If yes, could you post the background information about how you got the results?

 

[dgatsoulis]

Did you calculate this? If yes, could you post the background information about how you got the results?

That one was from a note I had made a great while back.
But if you want to make the calculation, there is a way.

Doug has released part of the XR2's hull temp code (thanks Doug!):

XR2 nose temp in Kelvin = effectiveOAT + ((1.4 * 3.1034e-10 * 0.642) * ((atmpressure / 2) * (airspeed^3)))

[Note: effectiveOAT, "Effective Outside Air Temperature", is computed from GetAtmTemperature() from the core, but of course the affect it has on hull temperature is only a factor at all where there is enough static pressure to make any difference. The effect it has varies by static pressure.]

(This is for the nose hull temp, but it will do fine, since it's the one with the highest temperatures)

The Periapsis Velocity for Venus is ~ the Airspeed because of Venus' slow rotation. So you only need a way to find the static pressure at the altitude you want, and you will know the hull temp.

For most of the planets/moons with an atmosphere, you can find the pressure at an altitude with this:

(from Orbitersdk\doc\API_guide.pdf)

Earth, Venus and Mars however, have custom atmospheric models, so you need a table with the static pressure at a range of altitudes that correspond to typical reentries.

It is pretty easy to make one for yourself.
-Run a scenario in Orbiter.
-Pause
-Scenario Editor
-Bring in a DG and place it at the planet/altitude you want.
-Unpause and pause again (really quick, it's better to be at x0.1 time accel)
-Make a note of the STP from the Surface MFD
-Repeat the last three steps for the range of altitudes you want.

Here is a table I have made for Venus:

ALT (km)|Pressure (Pa)

85​

|

86.781718​

|

86​

|

68.604082​

|

87​

|

54.076894​

|

88​

|

42.499266​

|

89​

|

33.298619​

|

90​

|

26.008301​

|

91​

|

20.248931​

|

92​

|

15.724136​

|

93​

|

12.208767​

|

94​

|

9.480090​

|

95​

|

7.361888​

|

96​

|

5.717444​

|

97​

|

4.440682​

|

98​

|

3.449321​

|

99​

|

2.679496​

|

100​

|

2.081652​

|

Let's test it for say... 12 km/s at 95km alt.

(1.4 * 3.1034e-10 * 0.642) * (atmpressure / 2) * (airspeed^3) =
(1.4 * 3.1034e-10 * 0.642) * (7.361888 / 2) * (12000^3) = 1774.2048742728 K = 1501.05 °C

You will notice that I am not taking the effectiveOAT into account. For that static pressure the difference is negligable.

Another thing to remember is that you don't magically "pop" into the atmosphere (unless you use the Scenario editor), but you gradually go through the upper layers, before reaching periapsis, making the peak hull temp even lower than the prediction.