Drag post supersonic question

[Pirate Roberts]

I'm trying to model atmospheric ballistic trajectories. I think I have a good approximation using excel (but not so much on the calculus). I modeled acceleration, thrust, weight, drag and have an approximation to drag through the sound barrier. I have assigned a drag coefficient that seems to work. My model works pretty good launch and through sound barrier. After SRB burnout I model a ballistic trajectory to descent back through the sound barrier as far as chutes out and my sim is over.

Problem : Ascent Velocity from immediately past sound barrier > 360m/s to engine burnout and Ap isnt matching my model and is out buy about 5%. My model is faster than flight test comming out of sound barrier

I've got TWR correct ie model matches KE readings. It took a bit but eventually I got a fudge for Thrust efficiency against atmospheric pressure with a decent stdev. I used a logistic curve to approximate the efficiency curve that KE reports in the VAB  (yeah whatever but it works)

I've modeled Kerbin Atmosphere and drag F_Drag=1/2.P.v^2.K where K is a drag coefficient I tweek till matches "reality" (well flight recording), I did flight recording by screenshotting every few seconds and entering the data back to excel. (maybe there is a quicker way but its ok).

I know its not that I got the sound barrier approximation wrong from 300-360m/s because my model matches recording all the way to 360m/s then starts diverging.

The brunt is : is there something happening with drag as you go supersonic (>360m/s) that I need to consider?

Thanks ahead to any help, I would like to show the graphs but I'm not sure how to post pics, Im hoping there is something silly I forgot.

 

PS I'm new to the game its my first and only career , never did sandbox have a ton of mods, killed three Kerballs and have started a science program that so I dont loose more greenies.  I play very slow but I'm really loving getting back into basic physics its really wonderful.

 

 

 

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Edited March 13, 2020 by Pirate Roberts
Typos minor spelling

[FullMetalMachinist]

20 hours ago, Pirate Roberts said:

The brunt is : is there something happening with drag as you go supersonic (>360m/s) that I need to consider?

What you're asking about is a little over my head, but I know there is a sharp increase in drag as you transition from about mach 0.8 to 1.2. You can try searching for "transonic drag" or something similar for more specific information. This is what I found after a couple minutes. 

 

[Pirate Roberts]

Thanks, I already have sound barrier in my model it seems to cover the sound barrier on ascent and descent quite accurately, it wasnt that hard to model I just made a simple drag multiplier between 300 and 360 m/s. I think it might be temperature. I read on the wiki that atmospheric temperature doesnt have much effect on the game but I think it does something. Its maybe nothing to do with comming out of the sound barrier its that the vessel is entering the stratospheric temperature gradient around 12500m, I'm going to add atmo temp profile see if that helps.

[Pirate Roberts]

Does the  RT-10 thrust change during the last 10 seconds or 10% of burn? Or is it consistent (or at least follow the same Atmospheric pressure to thrust curve)?

 

[Rocket Witch]

On 3/18/2020 at 3:26 AM, Pirate Roberts said:

Does the  RT-10 thrust change during the last 10 seconds or 10% of burn? Or is it consistent (or at least follow the same Atmospheric pressure to thrust curve)?

 

KSP's solid rockets don't have thrust curves — full power until empty, or whatever level you set the thrust limiter to on the part. Indeed the thrust will rise along with the Isp as atmospheric pressure drops, in exactly the same manner as the other rocket engines. The "flameout at 10%" thing in engine stats only applies to jet engine intake air. Once an air-breathing engine is only capable of 10% of its nominal max thrust (I think?) it stops working until it begins drawing enough air again.

Edited March 19, 2020 by Rocket Witch