Recommended booster thrust to weight ratio?

[mikebpd221]

I've got my first two stage rocket nearly finished and I'm re-running a number of simulations to get a feel for what I can anticipate out of flight performance. My local field options are pretty tight so I am trying to keep altitudes under 3000 feet otherwise my recovery goes from occasional trees to full on forest. This presents some challenges to the event timing since there's very little coasting between boost and sustainer phases. Picking a relatively weak sustainer motor is a given unless I want to GPS tag a rocket eating tree. Where I'm more concerned is the choice of booster motor. Sustainer ignition is going to base off time delay with altitude and velocity gates (Raven 4) parameters so there will be a "no go" built in if things go sideways. My question is if there is a minimum recommended thrust to weight ratio. Obviously, higher is better for attaining greater acceleration and giving me more time in the programmed launch parameters. The 5:1 minimum T:W for single stage flight seems to be a bit weak in what motors I've simmed for my booster although they are great at keeping the altitude down. Is there a better safe minimum for two stagers? If it helps, basic specs on mine are 70" combined length, 1800 grams and stability margin of 3.5-4.75 depending on the motor choices. 38mm in the booster and 29mm in the sustainer. Thanks for the feedback.

 

[teepot]

On my two stagers I tell Thrustcurve that it only has 1 motor. Then it gives options for the total weight. Thrust to weight ratio and speed off the rail. Then I take the weight of the sustainer and put that in as a separate rocket. That at least puts me in the ballpark of total altitude. We fly off a dry lakebed so our recovery area is huge so I'm not to worried about altitude.

 

[Cameron Anderson]

Your need for a high thrust-to-weight conflicts pretty hard with 3000' total altitude.

Try to find a warp 9 motor for your booster that gets you off the rail at 70-80fps minimum and light your sustainer almost immediately after separation - no coast. Coast = altitude. And go as anemic on your sustainer motor as you can.

 

[Charles_McG]

I’ve been having success with about 12:1. Really cuts through the wind. CTI White Thunder motors with the right thrust for my PSII based sounding rockets puts staging at about 300’. Peak altitude of 2000-3000’ isn’t hard to arrange.

 

[Rocketclar]

Cris at Eggtimer recommends at least 10:1. I've built a couple of his Protons as my flight computers for my 2 stager. I, too, am constrained by waiver. I've flown to almost 3000' with an AT I-175 to a CTI G46 with a 0.2 sec delay (coast). It didn't drag separate, but I had stuffed wadding in the ISC so it wasn't damaged when the sustainer fired. For my sims, I like coming off the rail at >40mph since 2 stagers have a lot of fin area and I am trying to minimize weathercocking. Rocket is a Terrier/Sandhawk.

 

[Charles_McG]

Cris at Eggtimer recommends at least 10:1. I've built a couple of his Protons as my flight computers for my 2 stager. I, too, am constrained by waiver. I've flown to almost 3000' with an AT I-175 to a CTI G46 with a 0.2 sec delay (coast). It didn't drag separate, but I had stuffed wadding in the ISC so it wasn't damaged when the sustainer fired. For my sims, I like coming off the rail at >40mph since 2 stagers have a lot of fin area and I am trying to minimize weathercocking. Rocket is a Terrier/Sandhawk.

Sounds nifty. I've used the Eggtimer Quantum on a Nike Apache, and the Proton on Ute Tomahawk, Nike Tomahawk, Nike Nike Smoke, and Terrier Malemute. A Nike Recruit is up next. I've been using a really small separation charge - just packing some black powder into the protective cap that comes on the ematch and taping it over.

As I sim and fly these two stage sounding rockets, I'm getting the feel that for typical rail lengths, Gs should = windspeed.