Stability

[ghost]

Hi,

If I were to make a normal rocket like an Estes Alpha, for instance, but remove the fins, add 3/16" dowel rods (four of them) extended a foot down, and add some more body tube, and then add fins... would that work?

Another idea I had was to make a normal rocket (with a big diameter) and stick two engines in the nose but have them stick out at angles. I could then re-route the ejection gas and use rear ejection. Would this work?

Thanks!

[CPMcGraw]

Hi,

If I were to make a normal rocket like an Estes Alpha, for instance, but remove the fins, add 3/16" dowel rods (four of them) extended a foot down, and add some more body tube, and then add fins... would that work?

If you plan to put the parachute in the lower body tube, then yes. There's no problem with this idea. It makes a stable rocket because you're just lengthening the model. Watch your power, and run it through RockSim before taking it to the field...

Another idea I had was to make a normal rocket (with a big diameter) and stick two engines in the nose but have them stick out at angles. I could then re-route the ejection gas and use rear ejection. Would this work?

Thanks!

Yes, Professor Goddard, you can make a "tractor" work. But not very well. That's why everyone else in the business went to designing "pushers" with the motor at the rear. The problem is having all of the mass behind the CP. Rockets do not make good pendulums.

You've been looking at Goddard's first liquid design again, haven't you? He placed the tanks at the bottom and piped the fuel and LOX up to the top under pressure. He also never really had success with it, for many reasons. It didn't take him long to figure out why pushers were better than pullers...

[ghrocketman]

Both of your questions would work as long as you keep the proper CG/CP relationship for the rocket.
Perform a swing test to determine stability....if it is not stable, add nose weight until it is.

[ghost]

Actually, for my first design (an 2nd), I meant something like the attached image. Actually, I've never really looked at Professor Goddard's designs. What were they? Also, with the design w/ the balsa, would it be possible to remove fins and just add weight? This way, the rocket acts like a pendulum and should stay nose up and tail down. What do you think?
Thanks

[CPMcGraw]

Actually, for my first design (an 2nd), I meant something like the attached image. Actually, I've never really looked at Professor Goddard's designs. What were they? Also, with the design w/ the balsa, would it be possible to remove fins and just add weight? This way, the rocket acts like a pendulum and should stay nose up and tail down. What do you think?
Thanks

Diagram #1 shows the thrust of the model shooting down through the lower tube. Not a good idea. The tube is made of paper, and the thrust is made of fire. Fire loves paper, but paper hates fire. Take this from someone who has watched a motor eat through a piece of steel plate. The flame from those motors is not just a simple fire, but more like a cutting torch! Very hot, very aggressive, very destructive if disrespected.

Diagram #2 is do-able, because the thrust is vectored away from the tube. Using two should balance out if the motors are exactly identical. As for stability, pendulums are not flight-stable. The acceleration alters the way a pendulum behaves. It doesn't work at all the way it would in a zero-acceleration attitude on the ground. The forces of acceleration cancel out the "corrective" movement of the pendulum. It might fly, and it might look cool as a novelty; but as a practical means of powering a rocket it's not.

[ghost]

For the first diagram, it wouldn't work even if I were using A or B engines and the wooden rods were a couple feet long? What if I had two motors pointed outwards in the place of one?

Also, what motor cut through steel?!!? I would love to see a video of that!

Thanks.

[Royatl]

Diagram #1 shows the thrust of the model shooting down through the lower tube. Not a good idea. The tube is made of paper, and the thrust is made of fire. Fire loves paper, but paper hates fire. Take this from someone who has watched a motor eat through a piece of steel plate. The flame from those motors is not just a simple fire, but more like a cutting torch! Very hot, very aggressive, very destructive if disrespected.




If there is enough distance from the exhaust to the lower tube, he shouldn't have too much of a problem as most model rocket motor exhaust plumes aren't that long and aren't that wide. There have been designs like this from time to time. I don't think they're very efficient, but many new rocketeers come up with this at some point, thinking its entirely novel.

[ghost]

Do you (or does anyone else) know how long the flame is? I'm not looking for efficiency, just cool designs... ALTHOUGH, I could stick another engine in the lower section... :rolleyes: :eek: :D

[moonzero2]

Gentlemen,
Great discussion. I lurned something. Thank you

[jflis]

You've been looking at Goddard's first liquid design again, haven't you? He placed the tanks at the bottom and piped the fuel and LOX up to the top under pressure. He also never really had success with it, for many reasons. It didn't take him long to figure out why pushers were better than pullers...

One of the problems (mistakes?) that Goddard had was his assumption that the "Center of Thrust" needed to be ahead of the "Center of Gravity"... Until he did a few test flights he really had little concept of what "Center of Pressure" was... But then, that's *why* we have test flights :)

jim

[JRThro]

For the first diagram, it wouldn't work even if I were using A or B engines and the wooden rods were a couple feet long? What if I had two motors pointed outwards in the place of one?

Also, what motor cut through steel?!!? I would love to see a video of that!

Thanks.
I've had a C6-5 burn a hole through an Estes blast deflector from about three inches away when the rocket got stuck on the launch rod.

[CPMcGraw]

I've had a C6-5 burn a hole through an Estes blast deflector from about three inches away when the rocket got stuck on the launch rod.

Bingo!

Cherokee-D, C6-5 in an adapter, and it was about 3" above the deflector. A lug slid down over a wrap of tape near the bottom, and it held the model through the thrust phase. Burned a good-sized hole in the plate and tried to take off one of the pad's legs, too...

[CPMcGraw]

If there is enough distance from the exhaust to the lower tube, he shouldn't have too much of a problem as most model rocket motor exhaust plumes aren't that long and aren't that wide. There have been designs like this from time to time. I don't think they're very efficient, but many new rocketeers come up with this at some point, thinking its entirely novel.

I keep a mental image of the "Lil' Augie" design in my mind, as that's what Ghost's design amounts to.

[Royatl]

I've had a C6-5 burn a hole through an Estes blast deflector from about three inches away when the rocket got stuck on the launch rod.

We use 1/4" thick cutting discs (the kind you can find in the power tools section at Home Depot or Lowes) for $5-10 each. At our last launch we had TWO rockets with E9 motors stick on the pad.

The first rocket didn't move much at all and the paper label on the bottom of the deflector was scorched, and there was a small depression on the top. The second rocket rocked back and forth during the burn and so didn't concentrate its burn as much on one spot, and the disc came through perfectly.

[ghost]

Thanks for all your help. If I were to use the 1st design and use 2 foot long metal rods, would this work. I imagine the flame isn't more than an inch wide and a foot long. Any suggestions?

[barone]

Thanks for all your help. If I were to use the 1st design and use 2 foot long metal rods, would this work. I imagine the flame isn't more than an inch wide and a foot long. Any suggestions?

Using the metal rods would be considered, at least IMHO, to be a major structural assembly. As such, they can't be metal (violates model rocket safety code).

[ghost]

Would plywood rods coated in epoxy work? Would this design even fly?

[maricopasem]

Would plywood rods coated in epoxy work? Would this design even fly?
I say try it. Stop asking questions and go for it. Find yourself a nice safe place to try it, of course, but build it right (read: without metal) and give it a go.

[Green Dragon]

ok, had to chime in here /

1) metal rods = NO NO , and would definately violate the safety code.

2) I have flown , and seen flown more than once - rockets exactly like you mention (# 1 case) , where the motor and recovery system is in atube at the front, then dowels ( we used standard 1/4" dowels ), leading back to a SHORT ( like 1" ) piece of tubing , at rear.
purpose of rear tube is to hold the dowels together, in our case, fins were on the dowels, but tube could be long enough for fins, I'd think.
the dowels will eventually get sharred somewhat, but should last 6-8 flights, maybe more.

as for the 'tractor' ( forward motors 'pulling ' the airframe ), I've seen that done as well, but have to think the non-parallel trust looses enough forward boost , that a standard rear mounted cluster of the same emgines would be higher peformance and simpler, too.
( plus, doubt two motors is good , due to possibly pitching side to side, ones I saw fly were three motros symetrically around the nose )


~AL

[PaulK]

Another idea I had was to make a normal rocket (with a big diameter) and stick two engines in the nose but have them stick out at angles. I could then re-route the ejection gas and use rear ejection. Would this work?One thing this requires is ignition of both (all) motors. If one motor doesn't light, it will do a very tight loop right into the ground. For a puller like this, I would make a custom harness by soldering wires to both igniters, and each other, to minimize the possibility of one engine not igniting.

[ghost]

Here are some photos of design #1 so far...
It still needs fins and high temp. foil tape...

[snaquin]

Here are some photos of design #1 so far...
It still needs fins and high temp. foil tape...

Ghost,

The first thing that came to mind when I viewed the images of your Design #1 is "Rack Rocket"

I realize your design wasn't intended for multi-staging but it certainly resembles the rack rocket concept. Ken Good the president of Tripoli wrote a fantastic article about rack rockets and you may find some of this article helpful because some of the designs mentioned also used metallic tape to overcome the obvious problems with a very similar type of design.

http://tinyurl.com/2yzo7s

They flew one rack rocket on two FSI F100's and an F7 as the third stage motor!

I have a RockSim file on one of my back ups for my rack rocket design and I'm trying to locate it for you. IIRC my design used a rack of 24mm Estes D12 booster motors with an E9 on top.

.

[ghost]

Thanks for that article! The idea is essentially the same, but the engines in the upper and lower section of my rocket ignite at the same time.
Thanks!

EDIT: rocksim file attached

The rocksim file seems to work fine with the barroman and rocksim stability techniques, but the paper cutout doesn't seem stable. Did I design this correctly? Thanks!

[Gus]

Ghost,

The basic design of canted tractor rockets up ahead of dowels works just fine, both in large and small scale. ;)

[ghost]

WOW! Do you have a video of the model rocket taking off?

[Gus]

WOW! Do you have a video of the model rocket taking off?
Videos of the Pad Abort Test boilerplates and finished vehicles are viewable at
http://home.earthlink.net/~skristal/

There are a few other videos you may find interesting as well.

Enjoy,

Gus

[Green Dragon]

Ghost,

The first thing that came to mind when I viewed the images of your Design #1 is "Rack Rocket"

I realize your design wasn't intended for multi-staging but it certainly resembles the rack rocket concept. Ken Good the president of Tripoli wrote a fantastic article about rack rockets and you may find some of this article helpful because some of the designs mentioned also used metallic tape to overcome the obvious problems with a very similar type of design.

http://tinyurl.com/2yzo7s

They flew one rack rocket on two FSI F100's and an F7 as the third stage motor!

I have a RockSim file on one of my back ups for my rack rocket design and I'm trying to locate it for you. IIRC my design used a rack of 24mm Estes D12 booster motors with an E9 on top.

.


Steve,

neat post, thanks for the link - I'd never seen that before , nice info .

I've personally built and flown the ' Achilles ' rack rocket design ( as published in the Dec 1985 Tripolitan - drop me a note off list if anyone wants the plans or info ).

Mine was lost at a Battle Park launch , FSI D20 and four D12's , very nice five stage boost, and a unique idea. Never did get around to building a larger version, say, for the then-popular Silver Streak motors :-)

with regulations tightening up n HPR< might be a concept worth revisiting, might have to build another myself, if I can get access to a mill for a half an hour to slot the tubes.

~ AL

[ghost]

Next project: rack rocket
:)

[snaquin]

Steve,

neat post, thanks for the link - I'd never seen that before , nice info .

I've personally built and flown the ' Achilles ' rack rocket design ( as published in the Dec 1985 Tripolitan - drop me a note off list if anyone wants the plans or info ).

Mine was lost at a Battle Park launch , FSI D20 and four D12's , very nice five stage boost, and a unique idea. Never did get around to building a larger version, say, for the then-popular Silver Streak motors :-)

with regulations tightening up n HPR< might be a concept worth revisiting, might have to build another myself, if I can get access to a mill for a half an hour to slot the tubes.

~ AL

AL,

I've never seen a rack rocket in flight but the references made in that article describe a rather unique flight profile. Thanks for the offer for the plans. Your Tripolitan collection predates mine. I think my first issue was Aug 1988 and most of 1988 Tripoli was having magazine publishing issues (again).

I may build one for a rack of D12's and an E9-8 in the top but it would have to be one of the more simple racks, not a phenolic tube with milled slots. I like the idea of aluminum tubing for the struts but it would have to be flown independently of TRA or NAR and without insurance. I thought of slotting the bottom of the aluminum tubes and inserting thin G10 fins directly into the slots. It was a pretty good design. I still can't find my RockSim design file for the one I designed. I may have to resort to trying to find my notes :mad:

.

[jadebox]

This way, the rocket acts like a pendulum and should stay nose up and tail down.

You're in good company. Goddard made the same (incorrect) assumption. But, once in flight, all parts of the rocket are equally affected by gravity (remember Galileo's discovery). So the pendulum idea won't work.


Thanks for that article! The idea is essentially the same, but the engines in the upper and lower section of my rocket ignite at the same time.
Thanks!


I've played around with some ideas like that but haven't gone past "back of the envelope" doodling.

The trick is igniting the upper motor. You can't just hook your launch controller's leads to an igniter in the motor, because the leads are very likely to catch on the bottom section of the rocket. You could possibly snake wires from the igniter in the upper motor down one of the spars and hook up the leads below the rocket. Another method would be to build a special launcher with a stand-off next to the rocket which holds the leads for the upper motor's igniter at about the same level as the upper motor's igniter. That way, after ignition, the leads should fall away from the rocket. It might still take a bit of luck for the timing to work out right.

-- Roger