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MaverickLV

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Ok, well, I have had this idea for awhile but never thought to ask anyone else about it. Knowing my luck with "new" ideas (if this even is new) there is probably some giant flaw somewhere but I'll give it a shot anyway.

Anyway, after inserting some nose weight in my dad's 1/4 patriot from PML and running the rocksim again for it, this time with a lower altitude because of the added weight, I had an idea:

What if a design could be made to turn a small rocket that was relatively inexpensive into a very versatile bird that could handle very large motors without being lost? In essence I wanted to be able to take a 4" kit, make some mods, and be able to fly it on an h242 to 1000' or, if dad was going for his level two cert., put a J350 in it and still have it go to only 1500'. Therefore eliminating the need for a large field, large rocket with dual deployment (expensive) or low winds/high enough cloud cover (cooperative weather).

So, here is the basic design for the bird. Start with a 4" diameter kit that is 5-6' long. For this example lets just say it is a 1/4 scale patriot from PML with a 38mm MMT. The kit is about 57" long and has a 12" payload section.

Then, you make the modifications: Pick four pieces of 1/2" or 3/4" PVC tubing or even copper plumbing tubing about 8-10" long from the hardware store. Drill holes in the CR's for two of these tubes to go through. So it would kinda look like a three-a-breast cluster. Cap the forward ends of the tubes and glue them, as well as the MMT, in place in the CR's. Continue the rest of the fin can assembly with the fins, etc. Make sure they are really on there and if necessary 'glass them. Now, with the CR's and tubes all glued in place, put either male or female threaded closure onto the end which sticks out the aft CR. This way, you should be able to drop something in the two pieces of PVC and then close them off so that they form two small cylindrical compartments that are tightly sealed.

Then you need to make modifications to the payload bay. First glue everything together like you would and then get two extra bulkheads and drill two holes in them just like in the fin can (except no MMT). Epoxy them in place but this time seal the end of the tube which will face aft and put the threaded closures on the forward facing ends. Attach the NC with plastic rivets so it can be removed.

Now, before your flight, run Rocksim with the motor you are going to use, lets say a J350 and keep adding weight, using a "MASS OBJECT", to the point you want to have the CG at until the altitude is where you want it. Lets say 2000'

Now figure out the weight you added to get it to that point and measure out that much weight in lead shot. Place the lead shot in the two sets of tubes alternating how much goes in the fin can set and how much goes in the payload set so that the CG is where you want it. Then insert some Styrofoam to keep the shot from moving around at all, it must be very tight. packing Styrofoam with one end sealed w/ epoxy to keep shot from creeping down through the foam, would be good. Then seal the tubes with the closures.
Voila!

A very heavy rocket, but small and inexpensive, capable of flights on anything from an H123 to a K700 (54mm MMT) all to the same altitude if wished. Just a little change of weight!

So, what do ya think, is this possible?

Wow, that was a mouthful, if anyone gets confused or needs better explanation please tell me and I'll try to explain it better.

Dave
 
I had thought of doing something like this, but on a smaller scale. For instance, the Estes Honest John goes nowhere on a D12 and is perfect for flying on a football field. I wanted to achieve this same effect with the use of an F or G powered bird. I am currently in the process of building my MAXI (2.6" dia) Gyroc and am hoping that it won't get lost on an F20. Of course, this is a different scenario and an inappropriate example since its weight will adversely affect the helicopter recovery. But I understand where you're going with it. I have been wanting to achieve low altitude with high thrust for the "low, slow, smoky show" effect. I'm sure its been done before.

We're all great theorists here. I think you have a GREAT idea. I definitely like the idea of controlled ballast bays or compartments. I figure if the numbers are all in check, go for it. Stability is stability. Thrust is thrust. If it all checks out, I don't see why not.
 
Hmmmm......

Maverick, from first glance, I sure don't see any flaws in your theory. It sounds like it would work to me.

I don't know if I'll do it for my L2 bird (it'd be tough to find room in this one at 3" ID tubes with a 38mm MMT), but that certainly sounds like it would be worth trying!

I would guess that as long as the CG/CP relationship worked out properly, it would be just fine.
 
Interesting concept....here are two things I see to look out for.

1. Better build it strong if its going to get real heavy.
2. Get it too heavy and you may have problems loading a big enough chute in the 4" tube. I've fallen victim to this phenomena! Some people can pack huge chutes in small spaces, but I have problems doing so.

I suggest you test your theory/design! :D
 
Maverick...

What you propose 'could' work, but there are some things to be concerned with. There are other forces at work in a rocket launch besides thrust and weight. Importantly, the fins must produce forces to counteract any rotation away from the 'apparent wind' (note that I didn't say 'vertical'). The righting forces generated by the fins is determined by the size and shape of the fins, and the windspeed (and the angle of attack and other factors, but let's not get into all that...).

If a rocket is weighted well beyond it's design weight you create a potentially serious situation. The fins are now undersized relative to the mass they are trying to control. The rocket's acceleration and top speed are also much reduced, making the situation worse. Even if the CP and Cg are in the correct relative positions, the fins could be just "overwhelmed' by the mass of that heavy nose. A simple gravity turn could get started and not be able to stop.

The acceleration off the pad is the other issue. Even properly sized fins cannot keep a rocket pointed if the airspeed is slow, so check the speed off the launch rod in a sim program.

Finally, I mentioned "apparent wind'. A rocket leaving the pad at 20 mph upward, in a horizontal wind of 20mph, is actually going to 'feel' a wind hitting it at a 45 degree angle! That's a big angle of attack (and it will probably move the CP forward!) and it will cause the rocket to pivot into the wind. The rocket will continue to pivot into the wind until the nose has turned throught the wind and the fins start to generate rotational force in the other direction. If the rocket is really heavy and the fins small in comparison (and the speed slow), the rocket will need to turn a long way past the apparent wind to get the rotation stopped. It's not hard to get a heavy rocket nearly horizontal in a breeze.

The March 2002 edition of the LUNAR club's newsletter decribes an unfortunate demonstration of this effect.(https://www.lunar.org/). A heavily weighted rocket doing a long-range lawn-dart can do damage.

The best way to fly larger moters in a restricted altitude field is to stick to 'draggy' airframes (as in 'fat' rockets). If you're scratch building just have some fun and stick on some external pods and other stuff that will add more drag than weight. You want to accelerate quickly (light weight) but keep top speed and the coast time down (high drag)
 
Yeah, this was one thing I was concerned about. I thought it might be able to get too much momentum for the fins to correct if I added too much weight.

If I did build it, which is unlikely with the current shortage of funds:( , I would not be building it to take anything from an H242 to a K700. That was just the EXTREME. In reality it would probably be more of a great level two bird. One that could fly to 1500' on and I161 and with a little more weight (1-2 lbs.) fly to 2000' on a J350 for a lvl. 2 cert. That would be its purpose.

Also, it would not be flown in high winds and the fins would be VERY large compared to the airframe diameter. The thrust to weight ratio would also be safe and Blue loads would probably be used on most heavy flights.

Also, it would need to be built STRONG to prevent zippering which would otherwise easily happen. This means either 'glass or carbon fiber reinforcement plus lots of epoxy, etc. That would add plenty of weight as is.

So, maybe not the most practical design but still possible on a smaller, less extreme scale, with only moderate changes in weight.

Dave
 
I hadn't really though of the issue Dean raised, but I see his point. "Thinking out loud'.....

It seems to me it would work as long as you used a motor with a high enough average impulse to ensured good velocity off the rod. Or preferably a rail! Then, as long as the find are only correcting for perterbations in a flight with close to 0 angle of attack, they should work fine for high weight as well as low. Also assuming a calm day.

I wonder if the weight you need to get the low flights you want and a safe launch velocity are mutually exclusive for some engine/weight combinations? To keep a 4" x 6' rocket low on a K you might as well make it out of lead!

The other thing that comes to mind is what will work well if everything is perfect, can still be dangerous (i.e. what if the motor chuffs and gets a slow start?).

Interesting point of discussion though.
 
The concerns I raised were really concerning the idea of taking an airframe designed for one sort of flight profile, and adding enough weight to keep under an altitude restriction with a large engine. If you're going to build a rocket from scratch, especially for this purpose, you can take the appropriate steps to keep things safe.

Making the fins large enough is one good idea. I also encourage you to think beyond the traditional TFNC design so common in HPR. Rather than make the rocket "out of lead", you could build a really large UFO with normal HPR materials and get a great, but low, flight.

Other folks have taken up this challenge. If you haven't seen the "Rocket Team Vatsaas" site you need to know about it. These guys are doing some really fun and interesting stuff. Here's a link to my favorite. It flies on K motors to around 1000 feet, and is just about the coolest thing out there.

https://www.vatsaas.org/rtv/arsenal/bradrocs/napkin/napkinrocket.aspx

Check it out.
 
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