The mass limit wasn't the point. "About the same impulse" means about the same altitude, while "shorter burn" means higher thrust, thus better speed off the rail.
The F's peak early in the burn, so that helps
Certainly true, but there's one aspect I've struggled with. The 5:1 thumb rule is for average thrust. Is there a handy rule of thumb for first-quarter-second thrust? It obviously needs to be more than 5:1, and an actual simulation is obviously better than any rule of thumb. Still, thumb rules are really convenient when they work, so it would be nice to have one.The thrust during that first quarter second of the burn is much more important than the average thrust.
The 5:1 thumb rule is for average thrust.
Yes, I'm aware.Because that is the number stamped on the package, and the number fliers write on their cards. Average Jim Rocketeer can use that number most of the time and be fine.
The issue is that the two statements above contradict each other. In fact, most BP motors have such an early spike. What I'm saying is that it would be handy to be able to say "It doesn't meet the 5:1 average (named) thrust. But look, it passes the X:1 early thrust" without having to resort to the otherwise superior simulation results. One approach would be to look at that spike on typical BP motors; is it 3 times the average? Then use 15:1. Is it double? Then use 10:1. Then mostly the same BP motors pass or don't pass, and such a rule could be applied, as a a quick check only, to motors with different profiles.For lots of motors, there isnt too much difference.
[T]here are motors out there that have serious thrust spikes far outpacing the "average" thrust in the motor name.
and that's not 5:1.
Figures there'd be 25-30 mph winds with even higher gusts forecasted for launch day tomorrow. Rats.
A surgeon giving a lecture saidGlad to have fired up some good discussions on this. I'm a big proponent of doing our homework (especially for the big ones). It just takes that one to go badly. As a friend of mine used to tell his kids: "If you're going to do something stupid, at least be smart about it."
Figures there'd be 25-30 mph winds with even higher gusts forecasted for launch day tomorrow. Rats.
Reminds me of what I told my nephews. "I've known your father for a long, long time, so I know the day will come when you do stupid things with flammable liquids. It's in your blood. For goodness sake, use kerosene. Don't use gasoline, use kerosene." I then proceeded, as safely as possible, to demonstrate why. (And no one was hurt.)As a friend of mine used to tell his kids: "If you're going to do something stupid, at least be smart about it."
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