Weight and newtons and launch rods, oh my

[tdn]

The other night I was all ready to start work on my latest creation, a mid-powered (D/E) egg lofter. I had designed it in Rocksim, and everything looked hunky-dory. Before I got out the glue bottle, I decided to check the flight simulation and here's what I saw:

Launch guide data:
Launch guide length: 36.000 In.
Velocity at launch guide departure: 29.952 ft / s
The launch guide was cleared at : 0.347 Seconds
User specified minimum velocity for stable flight: 43.999 ft / s
Minimum velocity for stable flight reached at: 104.959 In.

(This was with a D12.)

105 inches? Could that be right? This is clearly a dangerous bird. The problem I'm encountering is that I'm not reaching the specified minimum required velocity for a 36 inch launch rod. I'm WAY under. Clearly I need more impulse, or less weight, or a longer rod (no jokes, please!). I could move up to a heavy duty 5' x 1/4" rod (which I need to get anyway), but that won't be enough.

So I'm asking for advice of 2 types: 1) Is there a good rule of thumb for weight vs impulse, and 2) any suggestions on a better design?

The rocket is basically this: Am 18 inch x 56mm airframe, topped by a 6 inch x 56mm payload tube, topped by a PNC-56A nose cone, and 3 small TTW balsa fins. There will be 2 interchangeable airframes, one for a single 24mm motor mount, and another for a cluster x 3 18mm mount. I'd like to use a 24 inch ripstop nylon chute.

The whole thing weighs in at about 11 ounces. Most of this weight comes from the nose cone (1.9 oz) and the egg (3 oz, includes padding). Even without the egg, it still comes out underspeed.

I can post the Rocksim files if that will help.

 

[Stymye]

I have launched heavier birds with a D-12, like the maxi honest John, fortunately the Honest John is a very stable rocket and can stay pointed up with minimal speed.
If it does fly, the bigger problem may be altitude, sounds like the recovery will be very close to the ground or served scrambled.

yes,post the rocksim file

 

[tdn]

The altitude with a D12-3 is around 250 feet, so I think that will be OK, as long as I have a 9' launch rod.

 

[tdn]

Dang, the file didn't attach. Let's try this again.

 

[powderburner]

other choices include:
---6 or 8 foot rail/buttons (seems like overkill)
---tower launcher built from 8 foot lengths of electrical conduit (kind of awkward to get to the contest?)
---bigger fins (pays off for low speed but costs you altitude)
---gamble (rhymes with "scramble")
Sounds to me like you already have a near-minimum amount of rocket. Low-power egg-loft is just always going to be one of those contests where you have a hard time getting a heavy rocket moving. Good luck!

 

[shreadvector]

What results do you get using 40 ft/sec instead of 44??

A 4 foot rod is too short for most heavy rockets unless they have high initial impulse motors. For tiny beginner models (Wizard sized) the standard Estes rod length (which is now around 30 inches) will work, but for the heavier plastic rockets I suggest a 4 foot steel rod.

Here is what our club uses:
1/8" dia x 4'
3/15" dia x 5'
1.4" dia x 6'

My MeggALOFTER kit flies fine (fine = bullet straight) on normal 3 foot rods (with egg), so perhaps you need a lighter model?

https://search.ebay.com/_W0QQgotopa...dsperpageZ25QQsosortorderZ1QQsosortpropertyZ1



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Originally posted by tdn
The other night I was all ready to start work on my latest creation, a mid-powered (D/E) egg lofter. I had designed it in Rocksim, and everything looked hunky-dory. Before I got out the glue bottle, I decided to check the flight simulation and here's what I saw:

Launch guide data:
Launch guide length: 36.000 In.
Velocity at launch guide departure: 29.952 ft / s
The launch guide was cleared at : 0.347 Seconds
User specified minimum velocity for stable flight: 43.999 ft / s
Minimum velocity for stable flight reached at: 104.959 In.

(This was with a D12.)

105 inches? Could that be right? This is clearly a dangerous bird. The problem I'm encountering is that I'm not reaching the specified minimum required velocity for a 36 inch launch rod. I'm WAY under. Clearly I need more impulse, or less weight, or a longer rod (no jokes, please!). I could move up to a heavy duty 5' x 1/4" rod (which I need to get anyway), but that won't be enough.

So I'm asking for advice of 2 types: 1) Is there a good rule of thumb for weight vs impulse, and 2) any suggestions on a better design?

The rocket is basically this: Am 18 inch x 56mm airframe, topped by a 6 inch x 56mm payload tube, topped by a PNC-56A nose cone, and 3 small TTW balsa fins. There will be 2 interchangeable airframes, one for a single 24mm motor mount, and another for a cluster x 3 18mm mount. I'd like to use a 24 inch ripstop nylon chute.

The whole thing weighs in at about 11 ounces. Most of this weight comes from the nose cone (1.9 oz) and the egg (3 oz, includes padding). Even without the egg, it still comes out underspeed.

I can post the Rocksim files if that will help.

 

[DynaSoar]

Originally posted by tdn
User specified minimum velocity for stable flight: 43.999 ft / s
Minimum velocity for stable flight reached at: 104.959 In.

(This was with a D12.)

105 inches? Could that be right? This is clearly a dangerous bird. The problem I'm encountering is that I'm not reaching the specified minimum required velocity for a 36 inch launch rod. I'm WAY under. Clearly I need more impulse, or less weight, or a longer rod (no jokes, please!). I could move up to a heavy duty 5' x 1/4" rod (which I need to get anyway), but that won't be enough.

I use MPH instead of fps, but that's just a linear transform.

You're trying for 30 MPH. I usually try for 25 MPH (= 36.67 fps). On a D12 you're not hitting 25 MPH until 0.65 seconds and 10 feet up. Now, I'm using a generic flight profile calculator, but the answer is close enough (120 in. vs. 105 in. are both way long rods). You'd best stick with an E9 for single motor use.

Three C6s should be OK on a 6 foot rod. It'd be a close thing on a 5 foot rod, and it might be OK that last foot through the air while coming up to speed, but in that 1/10 second lots of things can go hinky.

This bird (https://www.rocketreviews.com/reviews/descon14/thunderchild.html) is the same diameter but weighs 2 ounces more than yours. It gets going just barely and sometimes at an angle on three C5s which have a bit more early kick than C6s. It does not do well on C6s. This is on a 6 foot rod.

If you can find C5s, I recommend them. In fact, if you can find some, tell me. I only have 4 left.

 

[cjl]

Definitely DO NOT USE AN E9. It has LESS initial thrust than a D12 (25N vs 30N). If anything, use an AT E15 or E30. I bet it would be fine at 20mph, (30fps) but if you really don't want to test that, then stick with AT motors. If you want D power, then put in an 18mm adapter and use a D24.

 

[Mad Rocketeer]

Originally posted by tdn
The whole thing weighs in at about 11 ounces. Most of this weight comes from the nose cone (1.9 oz) and the egg (3 oz, includes padding). Even without the egg, it still comes out underspeed.

A couple of suggestions:

(1.) Lighten your load.
A lighter nose seems to be indicated. With an egg up front, stability will likely be fine either way. Without one, better check. [OK. Always check.] You could make your own nose out of card stock and toughen it with CA.
What are you using for the fins? Balsa is light. Fiberglass is light but probably overkill for a rocket of this size / power.
Use lighter glue. Wood glue is light and very strong when used correctly. Epoxy is heavier and not needed for this bird.

(2.) Streamline.
With a requirement to fit an egg, you can't exactly get svelte, but you can do stuff to cut drag here and there. Make it mirror smooth. Shape the fins (rounded on the leading edge, tapered on the trailing edge, flat on the root edge. Use a tower launcher to eliminate lugs and buttons. Alternatively, bevel the launch lug (if you use one) and place it in a fin-tube joint partly embedded in the fillet. Make the fins rectangles or parallelagrams, and concentrate on span rather than chord length in order to get the most good for the necessary drag. Use only three fins. That cuts weight and drag.

Good Luck!

 

[teflonrocketry1]

This is a heavy rocket as pointed out by the other experts above. To decrease the weight and improve the performance, shorten the main body tube (by about half), since the stability margin is now around three calibers (worst case).
I fixed some errors in the file (RockSim version 7); You had the nose cone shoulder specified as its base length and the nose cone weight was wrong. The parachute you specified was to heavy at 1.6 ounces. I also swapped out the brass launch lug for a paper one. I get a launch guide departure of just under 23 mph on a 36 inch launch rod and an altitude of 357 ft AGL on a D12-3.

Bruce S. Levison, NAR #69055

 

[r1dermon]

good call cjl. if i was in your situation, id invest in a 24/40 casing, and grab some E reloads. E18's put on quite a show, plus, if you compare the price of the reload vs. the price of an estes E9...its actually cheaper. then if you decide that you want that bird to really fly, pop in an F39 and let her rip...that's one of the better options. rather than tinker around with stability issues and other things.

 

[SwingWing]

Originally posted by tdn
here's what I saw:

Launch guide data:
Launch guide length: 36.000 In.
Velocity at launch guide departure: 29.952 ft / s
The launch guide was cleared at : 0.347 Seconds
User specified minimum velocity for stable flight: 43.999 ft / s
Minimum velocity for stable flight reached at: 104.959 In.

Most of the suggestions so far have focused on ways to increase the speed of the rod, (lighten, more impulse,streamline), what about finding ways to be stable at the lower speed? (swept fins, larger fins, higher aspect ratio)
Why was 44FPS chosen as the minimum safe speed for this rocket? Is that just the rocksim default? Is this number conservative or liberal (sorry, no politics)? Is there some reference or calculation on how to determine minimum safe speed off the rod/rail?
It seems an eggofter, with its payload weight far forward, would be a fairly stable bird to begin with, so maybe it doesn't need blazing speed to be stable?

 

[r1dermon]

well, if its underpowered it wont be going too straight. i think the best bet is to grab some AP, and ditch the BP.

 

[tdn]

Originally posted by SwingWing
Why was 44FPS chosen as the minimum safe speed for this rocket? Is that just the rocksim default? Is this number conservative or liberal (sorry, no politics)? Is there some reference or calculation on how to determine minimum safe speed off the rod/rail?

Good question, to which I don't know the answer. It's a Rocksim default, and is also the same speed recommended by Harry Stine.

 

[SwingWing]

Originally posted by r1dermon
well, if its underpowered it wont be going too straight. i think the best bet is to grab some AP, and ditch the BP.

Stuffing a bigger motor in it is certainly one way to increase the speed off the rod, my question is, how do we design it to be stable at the given off the rod speed. Or better yet, how do we determine IF it is stable at the given off the rod speed.

 

[r1dermon]

we....we....WE FLY IT!!!! lol, jk. why not do a swing test and clock it with a laser or radar?

 

[BobH48]

Originally posted by r1dermon
we....we....WE FLY IT!!!! lol, jk. why not do a swing test and clock it with a laser or radar?

If you do a swing test with 5' 10" string and make one revolution per second, the rocket will be traveling 36.652 fps (close enough to 25 mph)

circumference = 2 * pi * R (radius of circle)

Check your stability and if it's stable, use a shorter string (still at 1 rev per sec) until its no longer stable.

Then just use the formula to find out how fast it was moving for minimum safe stable speed.

 

[r1dermon]

yeah, but you'd need to clock yourself exactly to 1rev per second. margin of error would probably be a couple MPH due to time clock issues. i dont think it'd make a huge difference though. good idea.

 

[tdn]

Originally posted by teflonrocketry1
I fixed some errors in the file (RockSim version 7)

Sorry I didn't get back to you sooner with my thanks on this. Tinsel and shopping and rebuilding Christmas presents that flew into trees and all.

This looks like a winner. I'll look at it more closely and begin construction...

...oh, maybe tonight?

Thanks Bruce!

 

[polaris]

Here are some rules of thumb. Look under Motor Selection for thrust and GLOW guidelines.