Multi-stage motor selection

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emckee

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For those who have played in the world of multi-stage rockets, I have a question: When selecting motors for two stage rockets, what sort of thrust curves do you seek for each stage?

The booster stage needs to have enough thrust to get the stack moving off the pad quickly in order to ensure stability... and you can generally get more altitude with a long burn in the sustainer, but other than that...

What characteristics are meaningful for each stage and why? What's your preference?

I'm not looking for a discussion of 75mm vs 54 vs 38 motors, unless there is some performance characteristic that you can get from one size but not the other (e.g. larger diameter motors tend to have longer burn times). Instead, I'd prefer the information discussed here to be generally applicable to anyone looking into flying a multi-stage rocket, from 29mm up to 98mm+. (Yes, I'm thinking of composite propellants, primarily)

Thanks!
-e
 
For the booster, I usually try to get 10:1 thrust ratio for the initial launch. I select the sustainer motor based on how high I want to get. I will then plug the combination into RockSim or OpenRocket to ensure my velocity at stage is sufficient.
 
What characteristics are meaningful for each stage and why? What's your preference?

The booster stage needs to have enough thrust to get the stack moving off the pad quickly in order to ensure stability... and you can generally get more altitude with a long burn in the sustainer, but other than that...

Seems like you answered this yourself....
Sufficient stability is assumed.
Beyond the basics you mention, it becomes personal preference, waiver limits, or other details such as launch-site base altitude or recovery limitations.

Set your goals and spend a few hours in front of the simulator and see what it takes to achieve the goal.
 
The OP asks about thrust curve.
For the booster motor you want a "square" curve with no long tail off. No C slots or moon burners, can cause issue with burn out detection.

If you want to go really high what works is to have 3 times the impulse of the sustainer (impulse not thrust). Sustainer thrust should maintain speed or small acceleration. Booster gets it to speed, after staging it just cruises. Unless going really high where the air thins out velocity is not your friend.

If not going for extreme altitude a short burn boost is good because the staging can be seen.

Long staging delays work very good in simulation but reality hates it.
 
I am not an expert my any count, but I have been looking into this exact question for a while.

What I personally came up with was; Imax or equiv in booster - longburn in sustainer, which matches what others have said. Dual thrust in booster would be bad. High av thrust in sustainer whilst still in thick air, also bad (drag and airframe strain)

just my 2c
 
For the booster motor you want a "square" curve with no long tail off. No C slots or moon burners, can cause issue with burn out detection.

I find a Finocyl geometry gives the best "square" curve. Agree - hard off the pad with a crisp shutdown to help drag separation.
 
I am not an expert my any count, but I have been looking into this exact question for a while.

What I personally came up with was; Imax or equiv in booster - longburn in sustainer, which matches what others have said. Dual thrust in booster would be bad. High av thrust in sustainer whilst still in thick air, also bad (drag and airframe strain)

just my 2c

I'm less of an expert than you, but wonder if you might've meant v-max instead of i-max? I-max is regressive, at least in the only 54 CTI available, where as VMax is square:

Can't seem to post images tonight...what gives? Oh well:

CTI IMax 54: https://pro38.com/products/pro54/motor/MotorData.php?prodid=3147L935-P
vs
CTI VMax 54: https://pro38.com/products/pro54/motor/MotorData.php?prodid=1408K2045-17A
 
Good info, thanks all.

I figure I have a general idea about the basics, and I can simulate the hell out of anything, but I don't know what I don't know at this point. Hence I'm looking to fill in the gaps... Trying to identify the "unknown unknowns", and at least turn them into "known unknowns".

The OP asks about thrust curve.
For the booster motor you want a "square" curve with no long tail off. No C slots or moon burners, can cause issue with burn out detection.

If you want to go really high what works is to have 3 times the impulse of the sustainer (impulse not thrust). Sustainer thrust should maintain speed or small acceleration. Booster gets it to speed, after staging it just cruises. Unless going really high where the air thins out velocity is not your friend.

If not going for extreme altitude a short burn boost is good because the staging can be seen.

Long staging delays work very good in simulation but reality hates it.

Couple of questions on this...

Why would c-slots or moon burners cause burn-out detection issues? I presume it has to do with the slot tapering-off of the thrust?

Can you tell me more about the issues with long staging delays? reality vs simulation...

Thanks!
-e
 
Why would c-slots or moon burners cause burn-out detection issues? I presume it has to do with the slot tapering-off of the thrust?
....hard off the pad with a crisp shutdown to help drag separation.

Can you tell me more about the issues with long staging delays? reality vs simulation...

Gravity - where will your sustainer be pointing when it does light?
 
Why would c-slots or moon burners cause burn-out detection issues? I presume it has to do with the slot tapering-off of the thrust?
....hard off the pad with a crisp shutdown to help drag separation.

Can you tell me more about the issues with long staging delays? reality vs simulation...

Gravity - where will your sustainer be pointing when it does light?

So I take it to sim like crazy and errr a bit on the early side of sustainer ignition? (Disclaimer, I've never flown two stage APCP before but hope to someday.......)

I look at it as a couple of things. Does one want to go as high as possible or do they like to watch the staging process down low. I saw a K to L job one time that
was particularly entertaining. Got off the pad nicely and staged perfectly fine down low. Would have been picture perfect but the flier forgot to turn on the booster
electronics and it was a shovel recovery while the sustainer went to the "nether world" up high and came back o.k.

I had it fixated in my head that two stage meant heck bent for leather altitude but that's not necessarily the case. Getting a safe loft to a lower staging altitude would likely
entail as much planning for a "scream for the heights" flight. Kurt
 
Good info, thanks all.

I figure I have a general idea about the basics, and I can simulate the hell out of anything, but I don't know what I don't know at this point. Hence I'm looking to fill in the gaps... Trying to identify the "unknown unknowns", and at least turn them into "known unknowns".



Couple of questions on this...

Why would c-slots or moon burners cause burn-out detection issues? I presume it has to do with the slot tapering-off of the thrust?

Can you tell me more about the issues with long staging delays? reality vs simulation...

Thanks!
-e

With a long thrust tail off at some point the rocket is decelerating under thrust.

Long staging delay many forces wind shear, off vertical launch, etc all have more time to act.
 
A motor like an H999 as a booster on a 20-40 pound K or L upper is quite spectacular. Staging is real low and visible.

M
 
Again, thanks to all for the inputs.

The responses beg other questions about staging, separation methods, electronics, etc that I'll move to another thread.

At the moment, I'm not looking for extreme altitude. I just want to get some experience with staging on smaller motors (e.g. 38mm) so that I can translate that into a more powerful launch vehicle at some future date.

-e
 
I also like hard hitting motors to get off the pad, they take longer to arc over.
You want to be straight up when the upper lights.

My experience is that faster burning motors on the upper stage light quicker and more consistently.
If the upper stage will fly on its own to 800' on a 2-grain J, then that is all you need for a successful 2-stage flight.
 
Also keep in mind the velocity of staging.
In general, you don't want be transonic during staging.
 
Also keep in mind the velocity of staging.
In general, you don't want be transonic during staging.

Can you elaborate on why you don't want to be transonic during staging? I have seen lots of advice about not spending a lot of time transonic, but are there other issues specific to staging?
 
I have seen lots of advice about not spending a lot of time transonic

Same logic applies:
Why do something complicated requiring smooth, perturbation-free release of a sustainer during the period of highest instability, vibration and aerodynamic stress?
 
I have seen lots of advice about not spending a lot of time transonic

Same logic applies:
Why do something complicated requiring smooth, perturbation-free release of a sustainer during the period of highest instability, vibration and aerodynamic stress?

That makes sense. I had the idea you were speaking of a launch profile where the booster separated supersonic, then the sustainer dropped to transonic before lighting.
 
Can you elaborate on why you don't want to be transonic during staging? I have seen lots of advice about not spending a lot of time transonic, but are there other issues specific to staging?
I keep everything well below 750mph, not a speed junkie. I like watching the rockets stage without snapping my neck, LOL
Also keep in mind the velocity of staging.
In general, you don't want be transonic during staging.


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I had the idea you were speaking of a launch profile where the booster separated supersonic, then the sustainer dropped to transonic before lighting...

You might think about that too.
If the sustainer is going back to supersonic, you may want to modify the flight profile so that you don't pay the penalty to go transonic multiple times.
 
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Can you elaborate on why you don't want to be transonic during staging? I have seen lots of advice about not spending a lot of time transonic, but are there other issues specific to staging?

Why do something complicated requiring smooth, perturbation-free release of a sustainer during the period of highest instability, vibration and aerodynamic stress?

In addition to this, your Raven 3 programming gets harder as the rapid pressure drops could fool an altitude versus time logic. It'll specify a velocity versus time with pressure trigger logic or something specific for transonic. In addition at Mach 1 there are oblique shockwaves (Conical AIR version of concrete speedbumps for all sharp objects) in CFD that are visible at Mach 1 with rapid pressure changes. In addition before/after supersonic you have transonic flow causing turbulent flow boundary layer problems. Shockwaves can reflect. If airfoiled you can expect airfoil performance to rapidly change.
I remember from advanced compressible gas dynamics that the shock goes from supersonic to subsonic for normal shocks. But oblique shocks were related to the angle of object which is theoretical as a half wedge. Longer cones were better for that theoretically with lower angles.

In addition, "If a reducing diameter stage rocket design doesn't drag separate by maximum booster velocity then it will not drag separate", Dr. Sreenivas UTC Sim Center, the guy that did our comp rocket CFD. Now you have a burned out booster stuck to a sustainer stage with an unexpected unwanted CG shift massive aft of initial design expectations because you tried separation at transonic and the drag force depending on velocity wasn't high enough for drag separate.

Only designed and flown two L-1 hpr multistages for USRC SEDS 2017 comps, got 3rd. Avoid transonic like the plague. Avoid real spikey thrust curved motors on lightweight rockets with thrust specs exceeding class of motors your limited too. i1299N-P destroys interstages/disconnects batteries, lol.

Keep the design if its MD screamer and supersonic is unavoidable, completely supersonic stage separation and ignition (this is risky). I've tried a M1.5 stage separate/ignite, we destroyed a interstage. Structural has got to be top notch or its not gonna survive. You also don't want the fins to flutter and your sustainer could easily wind up around M2.4 or greater. So thermally your epoxies get expensive. So then the upper limit will be the fin design and flutter predictions on fins if supersonic stage separation/ignition is even viable or not.

Or slow it down. Subsonic booster stage separate, subsonic sustainer ignition, and let sustainer stage itself no booster attached accelerate under it's own power through trans and supersonic regions. Transonic is not a region you want to play around with multistage rockets. There's many ways to screw up a multistage, more than you think. Keep in mind the drag force depends on velocity squared. This was the second route I've tried.

Don't maximize thrust for specific impulse of class is what I learned first try about multistage motor selection on lightweight rockets.
Back to op, triangle thrust curves with high peak thrust at very steep slopes are bad for light multistage rockets, they will sim high, with stupid amounts of acceleration in excess of trackers or neck turning ability. Any mild thrust curve booster with a long burn sustainer will beat that.

Jim Jarvis has a youtube vid up of a sustainer failed ignition with sustainer laterally ripping itself off of a "good" interstage then flying vertically past a booster still burning. You see it from the rocket's point of view. Transonic and supersonic multistages shouldn't be taken lightly. Some real world behaviors are beyond CFD capabilities to simulate.

When it's really unstable in transonic you could change the fin angle of attack severely at high Mach number beyond fin joint limits and destroy fin by bending stresses or alter rocket course off planned path if it poorly separates at a bad angle.


Sorry for the long rant...
 
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