How long is your booster stage?

[Buckeye]

I know the answer to this question is simulate, simulate, simulate. However, I will ask anyway, since I am a total noob at electronic staging.

Is there a good rule of thumb for the length of the booster airframe or ratio of sustainer to booster? Just enough to fit your motor and recovery gear? Seems to me, shorter the better on the booster for the stability of the stack. I am referring to an electronic-staged rocket with same diameter and fin pattern for both booster and sustainer.

To my eye, 2:1 looks reasonable. This example from Wildman looks almost 1:1, which would make me nervous in non-ideal conditions.

 

[Rschub]

It probably depends on how much motor you want to stuff in there. Those 2 stage guys don't seem like they fool around.

I am with you, to my eye the first stage should be shorter, maybe it was all those direct staging Estes kits when I was a kid.

https://www.rocketryforum.com/threads/magg-nificent-two-stage-hpr-build-thread.179824/ This looks right to me.

I also am toying with a design of my own, that's a little more...sexy.

 

[robopup]

What are your goals for the project? It depends...

 

[boatgeek]

I’m not really going to help with the “it depends” answers, but just long enough to fit the biggest motor plus room for recovery is a really common answer. With both stages the same diameter, you’ll either need a separation charge or plan to separate them with motor ignition. If you’re using a separation charge, consider whether you want an altimeter and altimeter bay in the booster. That will add some length.

 

[Buckeye]

What are your goals for the project? It depends...

• Learn 2-stage with 29mm motors, maybe 24mm in the sustainer. 29/180 is probably the longest I would ever use.
• 2500 feet or so altitude to start so that I can see all the events
• Utilize 3" diameter lightweight cardboard airframe (BMS)
• No electronics in the booster. Motor eject
• Raven4 in the sustainer: Separation charge, sustainer ignition, sustainer apogee, sustainer main

Since my booster is not doing a lot, I figure that it doesn't need to be overly long.

 

[boatgeek]

• Learn 2-stage with 29mm motors, maybe 24mm in the sustainer. 29/180 is probably the longest I would ever use.
• 2500 feet or so altitude to start so that I can see all the events
• Utilize 3" diameter lightweight cardboard airframe (BMS)
• No electronics in the booster. Motor eject
• Raven4 in the sustainer: Separation charge, sustainer ignition, sustainer apogee, sustainer main

Since my booster is not doing a lot, I figure that it doesn't need to be overly long.

For those goals, I'd just go minimum length. Well, add a little margin because stuff always seems to take more room than you expect.

You might consider using heavier weight tubing--these rockets need a moderate amount of oomph to get off the pad nicely (Eggtimer recommends 10:1 TTW, and that's worked well for me), and lightweight tubing can crumple under those loads. It's not really a major problem, but might be cosmetic enough to annoy you.

 

[OC-Patrick]

"How long is your booster stage?"

...sounds like this belongs in the OFF-TOPIC forum.

 

[Buckeye]

"How long is your booster stage?"

...sounds like this belongs in the OFF-TOPIC forum.

...and it begins.

To be clear, I am referring to length as in distance, not length as in duration. Both are important, though.

 

[robopup]

@boatgeek already said just about everything I have. I will add that you're on the right track with lots of simulation though. One of the big things I've found with two stagers is that things seem much more interconnected - need more 2nd stage stability? Bigger fins on the second stage. But this probably is going to reduce the stability margin of the full stack etc... I think to do it properly, you really have to do your own math and do it on the actual as built rocket.

 

[SolarYellow]

@neil_w

Is the weirdness in OR where it seems to give strange results for longitudinally split fins worked out in the current beta, or is that still on the future work pile?

 

[neil_w]

If I'm thinking of the same thing you are, then everything is working correctly according to Barrowman and nothing is on the work pile. If you're thinking of something different that I am, then... well then I don't know.

 

[SolarYellow]

If I'm thinking of the same thing you are, then everything is working correctly according to Barrowman and nothing is on the work pile. If you're thinking of something different that I am, then... well then I don't know.

If you're thinking of this discussion,

https://www.rocketryforum.com/threads/gapped-fins.177028/page-3#post-2377282
then you're thinking of the same thing I am. No question that OR is running the Barrowman calcs correctly, but that linked post pretty clearly shows why the Barrowman method is likely not providing totally reliable results on pairs of fins that are very close together (small gap between them), as in close-coupled booster/sustainer fins that are relevant to this thread, and the advice to use sims to model stability of a multistage rocket.

It was good for me to refresh a little on it, as it's been most of a year. I guess I forgot the outcome, which is that there is a hole in the simple-model theories we rely on. There is as yet no alternative theory to build into OR or any other simplified rocket sim software available to us hobbyists, so obviously there would be nothing on the OR work pile. And it wasn't fair of me to sound like I was criticizing OR.

There are also papers out there explaining why Barrowman fails to capture wind-caused instability. Barrowman is a great tool, but it doesn't provide the full picture across all rocket configurations and all flight conditions.

Definitely, if someone (that especially means me!!!) is going to be playing with optimizing two-stage rockets, the thread linked above should be reviewed and work should proceed very carefully and thoroughly.

 

[Rocketclar]

• Learn 2-stage with 29mm motors, maybe 24mm in the sustainer. 29/180 is probably the longest I would ever use.
• 2500 feet or so altitude to start so that I can see all the events
• Utilize 3" diameter lightweight cardboard airframe (BMS)
• No electronics in the booster. Motor eject
• Raven4 in the sustainer: Separation charge, sustainer ignition, sustainer apogee, sustainer main

Since my booster is not doing a lot, I figure that it doesn't need to be overly long.

Good goals. I started with very similar. I've enjoyed working on the 2-stage project as I found it more technically challenging than my L3 project. I, too, like to watch my rockets plus our site's waiver is only to 6K anyway so my flights have been less than 3.5K. (Just because we can fly up to 6K doesn't mean it's really doable at our field with the surrounding constraints.) For my very first HP 2-stage flight, I tested everything except sustainer ignition. I did have a starter in the empty reload casing that fired. I tried relying on drag separation for a number of flights. Didn't get it. I had painted the inside of the ISC with high temp grill paint and packed the ISC with wadding to minimize burning it. That all helped but I found that a sep charge is the way to go. It doesn't take much BP. I used the Eggtimer Proton in sustainer for everything you plan with the Raven. I also had it connected to a telemetry module as I plug in the voice module from receiver into our club's PA so that the data are broadcast. Booster from start had electronic deployment. I had one failure with that so I added redundancy with another altimeter (Eggtimer Apogee). I alway put a camera facing down on the sustainer and watch the video in slo-mo. I did put another camera on the booster looking up but that didn't offer much as mostly saw sky when the sun didn't block out everything. I didn't answer you ? about length. My stuff is usually stretched from repairs or for handling my payloads or other constraints.
Anyway, good luck with your project!

 

[SolarYellow]

Dangit. This thread kicked off another rabbit hole of gap staging brainstorms.

 

[waltr]

My multi-stage booster are long enough to hold motor, recovery gear and any electronics needed and not longer.

 

[cls]

I guess you know this but here goes anyway. once you decide on booster length and get it working mechanically, one more consideration. delay between booster burn out and sustainer ignition. it's a trade off between reliability vs altitude. longer coasting means sustainer has lower velocity and therefore lower drag, so will make better use of its motor impulse. the risk is sustainer not staying vertical (shear winds, gravity turn) then flying way away instead of more up.

last weekend at BALLS a good fraction of HPR 2 stages didn't light the upper stage, the flight was not vertical. score one for tilt inhibit altimeters.