Timing of Max Velocity versus motor burn out for low power rockets

[BABAR]

Does the average sport low power rocket frequently hit maximum velocity BEFORE propellant burn out?

I was just thinking that most low power rockets are pretty short burn times, and even though the spike is in the first fraction of a second, the rest of the burn seems pretty short. So are most of these rockets accelerating right up until propellant burns out and the delay/smoke/coast phase starts?

 

[lakeroadster]

Does the average sport low power rocket frequently hit maximum velocity BEFORE propellant burn out?

I was just thinking that most low power rockets are pretty short burn times, and even though the spike is in the first fraction of a second, the rest of the burn seems pretty short. So are most of these rockets accelerating right up until propellant burns out and the delay/smoke/coast phase starts?

Here's a screenshot of the Open Rocket flight data download for my Ahpla rocket. The Maximum Vertical Velocity is recorded just as the D12 motor burn out.

 

[neil_w]

Based on some quick tests in OpenRocket with the Simple Model Rocket example, I would say max velocity is generally hit at burnout. There might be exceptions.

 

[lakeroadster]

FWIW.. On my 4 stage Saturn V... the rocket is slowing down from the time the 2nd stage drops off...

And yes, I realize it is not an average sport low power rocket.

Point being you could design a low power sport rocket that is 2 stage, with a small second stage motor that is merely there to gain additional altitude, not additional velocity.

 

[tsmith1315]

To decelerate under thrust, drag plus gravity would have be higher than thrust, right?

So, most likely candidates would be some combination of a large diameter (or other draggy constituents) and regressive burn motor.

 

[cerving]

That is often the case, because most motors' thrust tails off a lot before burnout. Some motors develop their maximum thrust very early then tail off almost linearly... those are called "regressive burn" motors. The remaining low-level thrust isn't enough to overcome the drag induced by the earlier part of the burn, so you'll actually see velocity dropping even though the motor is still technically firing.

 

[BEC]

Now you're going to make me go back through a bunch of FlightSketch and AltimeterThree data and look.

My first impulse is to say that max velocity generally is right at burnout. On models flying with motors that have a long low-thrust tail (particularly A10T, current C5) that might not be the case. I know that when I fly my Checkmate (two-stage 13mm motor rocket) two stages, with an A10-0Ts in the booster and I look at FlightSketch accelerometer data from it, it looks as if it's only maintaining speed, not accelerating, for most of the burn. This is what it looks like visually as well. But they're atypical cases.

 

[Charles_McG]

I'm pretty sure you're not going to hit max V -after- burnout.

Less snarky, I think Estes motors have a pretty flat burn profile, so if they count as 'average sport LPR', then max V should be the moment before burnout.

 

[SolarYellow]

A10-T thrust curve is atypical in the high initial spike vs long, low sustain and Checkmate is a lot of rocket for that motor.

 

[BEC]

I'm pretty sure you're not going to hit max V -after- burnout.

Less snarky, I think Estes motors have a pretty flat burn profile, so if they count as 'average sport LPR', then max V should be the moment before burnout.

See attached overlay of most all Estes time-thrust curves. This compilation predates the re-release of the C5-3. Its curve looks lots like the A10 — big spike then really long, low tail.

 

[neil_w]

See attached overlay of most all Estes time-thrust curves. This compilation predates the re-release of the C5-3. It's curve looks lots like the A10 — big spike then really long, low tail.

Yeah, I could imagine either the A10 or the C5 maxing out velocity after the initial thrust spike, when used on high-drag rockets.

 

[lakeroadster]

Yeah, I could imagine either the A10 or the C5 maxing out velocity after the initial thrust spike, when used on high-drag rockets.

An Open Rocket simulation for an Estes Stovepipe, powered by a C5-3 motor is a good example of this:

Maximum Vertical Velocity........... 319 mph @ 0.72 seconds​

C6-5 Burnout Vertical Velocity 12.11 mph @ 1.72 seconds​

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[waltr]

It does depend on the motor's Thrust Curve.

Also, why is everyone looking at the tables in the SIm??
Plot the flight in the Sim - Put Acceleration, Velocity and Altitude on the graph. Then zoom in om the motor burn and it is very easy to see Velocity verse motor burn (acceleration).
OR Plot of Estes Goblin on a C11-5. Note that Accel curve when greater than Zero is increasing the Velocity. There is a short time as the motor burns out when accel goes negative. Peak Velocity is when Accel reaches Zero.

 

[lakeroadster]

.... why is everyone looking at the tables in the SIm?? ....

Different strokes... for different folks. There are lots of ways to skin this cat.

File this under "It's All Good"