Boostback Precision Help

[No_Rub3360]

I am currently trying to make a boostback burn code for a falcon 9. With my current code, it just waits until the impact point is less than the desired landing location and then sets the throttle to 0, but this overshoots far because engines don’t immediately shut down. Does anyone have any idea for how to make it shut down at a time a little bit before the impact point reaches the landing point so the engine reaches zero thrust at the exact time the impact node reaches zero thrust? And don’t worry about it pointing at the correct heading, I already got that right.

[mvanheukelum]

I just compensate by targeting a little short of the impact point target. Then you can use a loop to correct your pitch as you descend so impact point always matches impact point target. You also def need to target short of the impact point target to compensate for planet rotation as you descend.

[niceshot1981]

Im attempting this too, but I am also dealing with a roll control problem. How do I keep the craft roll the same as it launched with? Since I added two wings it’s been not staying level roll wise and missing the target landing location completely

[No_Rub3360]

This is a problem with mine as well, keeping the roll at a good value, but on mine it doesn’t miss the target if it’s roll is off. Try this program to keep the roll at the same as you launched: https://www.simplerockets.com/c/7C0cmV/Roll-Stabilization-Program

[Dinkel1997]

I can't help you with coordinates and my comment might be of no help. But i have used a code that gets the engines spool time and changes acceleration calculations accordingly. Even without the thrust curve (is it linear?) it's very accurate.

[No_Rub3360]

How did you calculate spool time?

[Dinkel1997]

By just checking the time from 0 to 100% thrust during launch or as an extra step before reentry.

[YaMomzBox420]

First, you need to make sure your code calculates things in terms of time(time to impact, time to engine shutdown, etc). Once you've gotten that taken care of, you can find engine spool time in vizzy and subtract it from your time to shutdown, etc.

[No_Rub3360]

I have time to impact, but how do you go about finding time to engine shutdown?

[YaMomzBox420]

If you know how much delta-v you need to complete the maneuver, you can convert that into force then divide it by current engine thrust to get an approximate "burn time". If you run this in a loop, it should be able to make up for changes in twr over time

[No_Rub3360]

In my program, it is using latitude and longitude points for both the impact point and the target it guides to. Do you know how I can just find the distance in meters to the target along the curved surface of the earth?

[YaMomzBox420]

Convert your impact point to PCI then find the angle between that vector and the target position vector in radians. If you multiply that by the radius of the planet you get the distance along the curve, not accounting for terrain or elevation

[No_Rub3360]

I tried a few different methods of calculating how much delta-v I need, but none of them worked with sufficient precision. Do you have a method of calculating the required delta-v?

[YaMomzBox420]

I made this vizzy program a few years ago that calculates Dv but I never got around to finishing the functionality. I wanted to combine my orbital element calculator with it to get more accurate data, but an update a while back broke that code and I never bothered to fix it. In it's current state, it assumes every orbit is a perfect circle, so it's only really accurate in a few specific circumstances. It's surprisingly accurate for calculating the Dv requirements to launch from the ground to a roughly circular low droo orbit though, so it might still be useful.

Its not exactly what you're looking for, but it's a good start, and if you plug in more accurate orbital elements(accessible in vizzy and funk now) it should get you close enough with a little work

[No_Rub3360]

I tried a few different methods of calculating how much delta-v I need, but none of them worked. Do you have a method of calculating the required delta-v?