Long burn / end burn low thrust motors in sustainers?

I’ve been simming my next build, a 3” paper Ute Tomahawk, with the new Aerotech G11 and G12s and a question occurred to me.

Is it a good idea without active stabilization, even as an upper stage?

My sims show that the sustainer comes to Qmax and then just sits there for about 10 seconds. Thrust balancing drag - no net acceleration.

Now, not withstanding what that would do to an accelerometer based altimeter, is camping out at 200fps for such a long time a good idea? Is that just time waiting for the gravity turn to create a cruise missile?

I think both your concerns in the last sentence are valid.

Charles_McG said:

I’ve been simming my next build, a 3” paper Ute Tomahawk, with the new Aerotech G11 and G12s and a question occurred to me.

Is it a good idea without active stabilization, even as an upper stage?

My sims show that the sustainer comes to Qmax and then just sits there for about 10 seconds. Thrust balancing drag - no net acceleration.

Now, not withstanding what that would do to an accelerometer based altimeter, is camping out at 200fps for such a long time a good idea? Is that just time waiting for the gravity turn to create a cruise missile?

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It is a very good idea, if you want altitude. You just need to make sure that your rocket weighs no more than half the sustainer thrust, and that you get you rocket up to or above cruising speed quickly, headed in the right direction (vertical). Your 3" airframe provides ample room for as many D21 motors as needed for your initial acceleration. If your design, construction, and flying skills are not adequate, you could resort to active guidance, but if you lack the skill set required, you should avoid the long burn motors.

I’m planning my Ute Tomahawk as 38mm 1 grain (maybe 2) to 29mm 2 to 6 grain. So the booster will have plenty of thrust. The sustainer sims show that it would actually slow down under G11 power - it starts with more drag than thrust.

I have watched my upscale Vigilante arch over under Estes E16-E16 power. Started out straight and gently turned under thrust. Horizontal at deployment - stripped the chute right off.

I also tried a saucer (upscale Blender) on an F10 recently. Great boost, but as it built up spin, it slowed, turned, and took off cross range.

I love the idea of a long burning upper, but if she don’t stay vertical, that apogee deploy is going to be harsh.

I'm gonna say I'd love to see it. But don't do it; Cannot go well unless conditions are absolutely perfect.

Maybe the Bps system as a sustainer?

Wallace said:

Maybe the Bps system as a sustainer?

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I’ve thought of that too, though not for the build I have in mind. BPS Signal is built for 3”, and is limited to 40N. According to its website. I’d guess that out of the box, it might only work in the booster. My Nike is 3”. A Nike-Nike or Nike-Malamute would be cool.

I’m guessing that it would take some custom coding to be able to set it to air start mode, and detect first ignition, boost and burnout.

Wouldn't be easy, but it would certainly be a very cool challenge.

If possible, let me know next time you plan to attempt a staged launch @ Bong. I'd love to see and possibly help. I have a few low and mid power staged vehicles that have yet to fly that I would be happy to bring along for entertainment. Also have an staged HP kit that probably won't be built in the near future.

Wallace said:

If possible, let me know next time you plan to attempt a staged launch @ Bong. I'd love to see and possibly help. I have a few low and mid power staged vehicles that have yet to fly that I would be happy to bring along for entertainment. Also have an staged HP kit that probably won't be built in the near future.

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I'll try to remember. It won't be this fall - I pulled the Quantum from my Nike Apache for use in my Space Shuttle stack (or airstarting the pod motors in my NASA Pegasus - haven't decided which to do first.)

A few disjointed remarks:

Most of your gravity turning is going to occur at the beginning of flight, before the maximum velocity is reached. That’s where a first stage would comes in handy. If the first stage takes the second stage above its terminal velocity, the rocket will slow down during second stage burn, reaching terminal velocity asymptotically from above.

In a vertical flight, your terminal velocity is going to be proportional to the inverse of the square root of the drag coefficient. If most of the flight is spent at this speed, and if the coast distance is short, your altitude is going to be unusually sensitive to drag coefficient. The little wings on the Tomahawk are going to work sorely against you there. Consider a more efficient configuration for long burn, if you want altitude. (Of course there are perfectly valid aesthetic considerations when choosing long-burn motors.)

Altitudes of slow burning configurations that reach terminal velocity are also unusually sensitive to motor burning times. Thus, ambient temperature may affect performance noticeably. Conversely if the burning time were only .01 second (with a very high thrust-to-weight ratio), altitude wouldn’t be much affected if, on a particular flight, it turned out to be .02 seconds.

I agree with your latter remarks, but my experience is that the booster flight straight and the sustainer gradually turns.

Charles_McG said:

I agree with your latter remarks, but my experience is that the booster flight straight and the sustainer gradually turns.

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I realized the ambiguity of the statement some time later. Yes. Gravity turning happens later in the flight until, in the worst case, the rocket could be flying straight down.

What I meant to say is that the straightness of the flight overall can be augmented most efficiently by the addition of early speed or by an extremely long launch rod to keep the rocket from turning in early flight.

Larry Curcio said:

I realized the ambiguity of the statement some time later. Yes. Gravity turning happens later in the flight until, in the worst case, the rocket could be flying straight down.

What I meant to say is that the straightness of the flight overall can be augmented most efficiently by the addition of early speed or by an extremely long launch rod to keep the rocket from turning in early flight.

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Which is the source of my concern. I -love- to listen to those long burns. But 10-12 seconds? That’s a -lot- of time for a turn to develop, no matter how straight the boost.

I think the issue with the long burn can only really be mitigated by having a really stable rocket, and also making sure the booster puts it up a really high starting speed. Truthfully though, I think you'll have to do something else than the G11

@Wallace - I'll be flying my HPR 2 stager again at the next woosh launch, if all goes well.

Gotta agree with loving the long burners. The BPS videos on the G8 are almost spooky.

Charles,
Yeah. It hadn't sunk in that the burn time was 16+ seconds...
If I may ask, what's your launch weight (Specify with or without motor) and diameter? I'd like to run some numbers.
Thanks and Regards

It’s not built, but a 1/5.5 Ute Tomahawk is essentially a Ventris sitting on top of a Big Daddy.