Good day you all.
I'm having some troubles with the precision of the burn using the built-in burn calculator.
I'm performing a TLI with a 220 KN Hydrolox engine to send some 20 tonnes payload around the moon.
And now comes the troubles. planning my burn, as i increase over 3 km/s, the precision of the burn goes from High to low, and the extimated "duration of the burn" exceede the "burn time" of the stage.
Is it related to the relatively low power of the engine?
thanks
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1,176 YaMomzBox420Pinned 3.4 years ago
@Matteovir well, I think Smurf research might be right, the accuracy would definitely go down as your craft orbits the planet because your directional velocity changes a lot and a long duration burn gives more time for small inaccuracies to compound into much larger ones. Half a degree difference in heading is only a few feet at a range of less than a mile, but a 1000 miles, it becomes over a mile of difference, and when you're actively changing course during that path, the difference is potentially even larger.
Also, RSS might not cooperate very well with the burn node tool. I think it's optimized for the stock Droo system, so an RSS system might push it just a little farther than it was meant to
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137 Matteovir3.4 years ago
@YaMomzBox420 thanks. I solved the problem increasing the thrust of my upper stage from 210 kN to 900(adding 4 engines instead of 1), as you suggested me. I found also a problem in attitude control, solved increasing gimbal a bit.
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137 Matteovir3.4 years ago
@SmurfResearchX thanks for answer. Just to be clear, I'm using a real solar system, so the earth I "real dimension", not droo.
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7,823 SmurfResearchX3.4 years ago
@Matteovir - it might be just the low twr during low orbit - i think the auto burn determines accuracy from how much the craft will change position/direction during the burn, it would have a major direction change (a 10 min burn time would be close to 1/4 of an orbit) - if you try a long auto burn in a higher orbit, or outside of droo SOI, they should still be able to get high accuracy - because the timing and direction wouldnt matter as much
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137 Matteovir3.4 years ago
@YaMomzBox420 thanks for your answer. But the situation is a bit different.
In my situation i have an upepr stage with a DV of 4100 m/s in LEO and a burn time of (let's say) 10 minutes. In the MAP mode, when planning an auto-burn, i select my DV of 3.5 km/s to reach escape velocity; so the burn duration increase, but when planning it increase more then the real stage burn time. It is as the AI cannot calulate precisely how long will the burn last, and so the precision of burn result in a LOW precision, both in planning and practically when performing it. -
1,176 YaMomzBox4203.4 years ago
Probably. If you need 4000 m/s ∆v, but your craft only has 3500 m/s, the burn node will be very accurate up to the point that your fuel would run out, then it's accuracy goes down because you would be drifting around without any way to maneuver(unless you have rcs or something). The only real fix is to increase your available ∆v by optimizing your craft. I can't tell you what would work on your specific craft, but I would try removing any dead weight, increasing engine performance, and improving aerodynamics if the craft goes through the atmosphere
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@YaMomzBox420 just to clarify how i solved this old-but-not-too-much issue. I found that just increasing the thrust of my engine is not a good and realistic solution. I'm now using an Hydrolox engine similar to RL10c of Centaur upper stage, wich has just 110 Kn of thrust, but is very efficient and reliable. The solution is that you cannot rely at all on the auto-burn function, so i ended up with hand calculation. Divide the residual burn time of your stage/remaninig deltaV of your stage; then multiply the result * the DV required for the calculated burn. The result will be the total duration of your burn. Divide it /2 and the result is the distance in seconds from the node of the burn, and the moment to start your burn. it is very precise, even for long burns for Earth-Mars transer.