5:1 thrust ratio question

Hey folks. When you guys talk about 5 to 1 ratio, is that max thrust or average thrust.. or something else entirely?
One reason I ask: Wildmans site says the Wildman Sport can fly on an E motor. Mine weighs in at 25 oz. An Aerotech E30 meets the 5:1 for max thrust but not for average thrust.
Will that cause the rocket to tumble or with it reaching max thrust quickly will it sustain enough velocity to carry on?

Mike

Tumble is lack of stability. That is independent of thrust to weight ratio.

Insufficient speed can cause low stabilization. The rule of thumb 5:1 is just a rule of thumb to get what in most cases is enough speed at the end of the rail to achieve enough stabilization that the rocket goes roughly where it is pointed. Or has a chance to, if it has suitable stability margin.

The longer the rail, the lower T:W ratio one could get away with. 3:1 is the lowest we are allowed to use and it had better be a LONG rail on a dead calm day with a very stable rocket!

The T:W ratio matters really mostly for just getting enough velocity off the rail. So, the initial thrust, during that first half second to a second, is what matters most. That's what gets it off the rail.

Wind has a strong effect on the rocket when it leaves the rail. The more wind, the faster the rocket needs to leave the rail to be stable. The greater the side wind, the more the center of pressure on the rocket moves forwards. It doesn't take a whole lot to get the CP moved too close to the CG for comfort, for a rocket that is otherwise stable enough. So the more wind, the higher T:W required for launch.

Gerald

You'll be ok, but my guess is that you will need the 4 second delay.

I'd run a few sims first. Have you run a simulation?

Greg

Also, I wouldn't launch it in winds beyond 10mph. It could weathercock. A lot.

Greg

I think you want 5 to 1 off the pad. Once it is at a stable speed, any thrust helps but is not required. Think vmax and W9 reloads, they are gone in a fraction of a second but the flight continues vertical and stable, possibly for quite some time.

Thanks. That's what I wanted to know.

I haven't run any sims using that motor. Didn't really think about putting her up on an E til recently.

I've been considering a longer rail for more motor options but it's on the back burner for now.

Thanks again.

if you're gonna do an E it should be a vmax/warp 9 or maybe white thunder. cut the delay a lot because 400 feet ain't much.

I tend to fly sport sized rockets on G motors....much better flight. H's are fun with a tracker
There's also 1 6 grain and 2 6g xl I motors......the I 204 would be the wildman way to do it. (hey, it was fun with my drago, never saw the deploy but because it had my tracker it fell out of the sky less then 200 feet from me...might have something to do with the chute I use, too.....)

Use the motor guide at thrustcurve.org to do a quick sim. The rocket has a 29 mm mount but you could use a 24/29 adapter which will increase the weight by around 1 ounce. You could use an E30-4 but only in winds up to ~5 mph with a long rod or preferably a rail.

I personally consider F motors the minimum for this rocket.

Bob

It'll be flying from a rail for sure. Pretty much just curious after I saw it said an E on the site. Was just using my Wildman as an example. I was wanting to know what th 5:1 thrust referred to. My Wildman will be going up on an F42 for its maiden flight, hope that will keep it in sight. All sims check out as does swing test.

Thanks for the info and input

Mike

Quick and easy shortcut:
Liftoff wt (with motor) in lbs. X 22.5 < avg impulse.
Ex.: 4.0 X 22.5= 90
Therefore G100 should lift it fine. H110 will too, just will go higher. Poorer aerodynamic designs, short and stubby rockets should use a little higher in avg impulse for a more stable liftoff.

A couple things here though...

Average impulse doesn't matter. What matters is the impulse (thrust) to get the rocket up to speed by the time it runs out of rail. An example of why average shouldn't be used is if one has a progressive motor (thrust increases over time) one might think it is ok based on average impulse but instead it may not be. Another example might be most any hybrid. These usually have regressive curves. One might not think it would be ok but a hybrid burn profile is usually fairly regressive. It has its strongest kick at the very beginning of the burn. So a hybrid may be fine when by average thrust one might mistakenly think it would not be. Moonburner, C-slot, D-slot; these are also regressive. By average thrust one might think they are insufficient, but by looking at the thrust in the first half second one may find otherwise. Ditto dual thrust motors.

2nd thing... Aerodynamic drag effects can be completely ignored for liftoff thrust considerations. Drag, in the subsonic range, is roughly a function of the square of the velocity. The speed is too slow. Mass and gravity have far greater effects on speed at the end of the rail than does drag.

Gerald

G_T said:

A couple things here though...

Average impulse doesn't matter. What matters is the impulse (thrust) to get the rocket up to speed by the time it runs out of rail. An example of why average shouldn't be used is if one has a progressive motor (thrust increases over time) one might think it is ok based on average impulse but instead it may not be. Another example might be most any hybrid. These usually have regressive curves. One might not think it would be ok but a hybrid burn profile is usually fairly regressive. It has its strongest kick at the very beginning of the burn. So a hybrid may be fine when by average thrust one might mistakenly think it would not be. Moonburner, C-slot, D-slot; these are also regressive. By average thrust one might think they are insufficient, but by looking at the thrust in the first half second one may find otherwise. Ditto dual thrust motors.

2nd thing... Aerodynamic drag effects can be completely ignored for liftoff thrust considerations. Drag, in the subsonic range, is roughly a function of the square of the velocity. The speed is too slow. Mass and gravity have far greater effects on speed at the end of the rail than does drag.

Gerald

Click to expand...

+1

Spot on.

Greg

For the velocity of the rail and the 5:1 rule of thumb I agree completely that the thrust until rail departure it's all that matters, but I want to point out the NFPA required 3:1 is based on average thrust and needs to be considered as well.

Yeah I looked at thrust profiles (currect term?) of different motors. The E30 and F42 both have the punch at the beginning to get it up to speed from a 4' rail. I guess these are both what would be called regressive, thrust peaks nearly instantly then drops off. An F26 has sufficient thrust but it takes longer to reach its max, I would need an 8' rail to use one of those motors.

Thanks folks think I got a handle on it.

Mike

Did a quick 2-dimensional simulation at 90 degrees and 70 degrees launch angle. (Used 2.2 inch diameter, 26.7 oz launch mass, .6 Cd, 36 inch launch rod)
Straight up it gives 351 feet altitude, which is consistent with the manufacturer's estimate. (33.4 ft/sec launch rod speed, 139 ft/sec max)
At 70 degrees, it gives 293 feet altitude and 259 feet throw-distance at apogee. (33.8 ft/sec launch rod speed, 141 ft/sec max)

Tried this because much of what is perceived as lack of stability from fins not working at low speed is actually just gravity turning. 2D simulation captures that much. Naturally, there are stability issues that require a 6 DOF simulation to represent, but I trust Wildman to have done a reasonable job there.

This doesn't seem like an impossibly dangerous configuration to me, simply because it doesn't go very far.
That said, it's rod speed is very low, and I agree that it should be launched in calm conditions.

As a reference, a 120-gram Big Bertha on a C6 gets to 176 ft/sec and reaches 456 feet aimed straight up with a (measured) Cd of .55.
Rod speed is about 59 feet per second. Comparable altitude but almost twice the rod speed

At 70 degrees the BB gave 373 feet altitude and 318 feet out

The BB is commonly launched on a B6. You'd think this would be worse than the Sport, but the rod speed is about the same as for the C6 (since the thrust curve looks about the same as the C6 up to that point).

Did that help? Guess I wandered... Best of luck!

-LarryC

Thrust curve says the minimum for this rocket is an F42. I don't know what rod length they use, haven't played with it that much. But using the 5:1 rule and looking at profiles it should go good on an F23FJ, which I wanted to use to begin with, but seems like my sims required an 8' rail which I don't have. I'll have to run em again whenever I get a chance.

Lots of info here. My thanks to you all. I just hope to fly it soon. I don't think she'll be going up on a G any time soon though. My launch sites are too small and I gotta see how it does on the F42 first. Maybe Tuesday. Fingers crossed.

MikeyDSlagle said:

Thrust curve says the minimum for this rocket is an F42. I don't know what rod length they use, haven't played with it that much.

Click to expand...

The default guide length is 3ft, but you can change it. If you save the rocket (or use the phone app), the guide length is saved for with each rocket.

Okay. Good to know. Last time tried saving rockets and settings I kept getting errors and never went back to try again. I'll take another look