Optimal Mass

[DPatell]

I just came in for a quick break from mowing the lawn, and while mowing I was theorizing.

I am playing with the idea of breaking the H and I altitude records, and despite long burning motors being best for altitude shots, I was going to use the Ellis Mountain H275 SU and Aerotech I600R reload.

I began thinking about optimal mass. In my case, with the fast burning motors, I want my rocket to be at optimal mass during the coast period of flight, correct?

I figured that since it will be coasting for much longer than boosting, I would make the model be at optimal mass during coast, which means that it would be slightly over optimal on the pad.

Ideas? Suggestions?

 

[shockwaveriderz]

Not sure I follow.....I was always under the impression that there is 1 optimal liftoff mass for a particular motor/rocket combination..... I do know the ballistic coefficient is important during coast.....don't know if there is an optimal mass for boost and or coast though.........

 

[rstaff3]

I agree with shockie. Optimum mass as I remember it is a function of the rocket design (drag coeff), mass at BO, and velocity at BO. Which of course makes it dependent on the motor, but not the initial mass per-se. IIRC you can maximize either altitude or coast time in optimal mass calcs, but now it elludes me what the difference is.

 

[shockwaveriderz]

Dan I don't see how with a high average thrust engine will ever be able to beat a lower average thrust engine as far as overall altitude is concerned unless the winds are very high and you need that higher average thrust to offset the wind....

But assuming a no wind conditions day ,an H50 would be idea for an H altitude attempt as its a long burn low thrust engine . Why do low thrust engines make a model go higher? well it has to do with the drag coefficient. Most people mistakenly thing that the CD during the typical flight of a MR/LMR/HPR is static, ie its doesn't change, but in reality the CD varies with velocity..... now it is true that in general for models below mach 1 the CD will be more or less constant within say 5-10% for 90% of the total flight time at low velocities....


see Velocity Dependence of CD's here:
https://www.ninfinger.org/~sven/rockets/ModelRocketry/Model_Rocketry_v02n04_01-70.pdf

13mb PDF!!!!

By the way the Ballistic Coeffcient of a rocket is its burnout weight(coast weight)/ aerodynamic drag factor which I believe is the CD x the reference surface area(cross sectional area) of the models body tube ..

The higher the better....

If I was going to do an H/I altitude shot and someday I might, I would launch from a piston tower and try and have an optimized piston launcher..... or perhaps use a DART although I don't know if either approach would be legal for the altitiude record.

 

[DPatell]

my theory was get it past mach and into thinner air as soon and as cleanly as possible. Both motors should get the model past mach within 100' of the ground, and should stay above mach for a good 5-6 seconds.

The current H record is held by the H268R because it is a full H. The H275 is 87% H, but I feel that the model I have could out-do the current record holder, but I guess we'll find out

I could try the Ellis I130 when it is released. No hardware lost in that case!

 

[rocwizard]

Originally posted by DPatell
No hardware lost in that case!

I am guessing that the pun was unintended

Something to keep in mind for any alt. attempt, long burners are good, but typically regressive. The ideal thrust curve is progressive because you want to have more thrust once the rocket has gotten up into thinner air and to velocity so all the motors power isn't wasted trying to get it off the pad, but rather where it can be the most useful. Progressive burners are seldom seen in the commercial world, so the next best thing is a neutral burner.