GG2014 Posted March 3, 2014 Posted March 3, 2014 I was wondering if there was anyway to not use a stick for somewhat large bottle rockets. I heard that having a seperate body tube as the rocket itself seperated from the rocket motor is the way to go so I may have a steady and vertical flight. I dont understand why making the motor itself as the rocket does not work. Can anyone confirm the method of making a rocket structure first then adding the motor under it as the way for a balanced flight? I guess it's almost exactly how model rockets work anyway.
leedrill Posted March 3, 2014 Posted March 3, 2014 im no expert by anymeans and i understand little about the flight of finned rocket motors all i know is it all relates to the balance point of the whole setup and the aerodynamics { im assuming that is the same as a sticked rocket it needs a low centre of gravity and the fins work the same way as the stick in that they have drag when the point of thrust try's to over take the centre of gravity dragging the motor straight again with air resistance } id love for some one to set my answer straight simply not by just trying to seem smarter than everyone else {is a complex subject but can be described a lot simpler than some people try to make it } also if anyone has one of the rocket simulator programs or a link to a free download could you point me in the right direction id love to have a play with one of them it might shine some light on the bits i know nothing about hope this helps GG and someone chimes in who can simplify it all for us both
GG2014 Posted March 3, 2014 Author Posted March 3, 2014 Thanks for the info! That definitely seems pretty accurate.
FireBirdHank Posted March 3, 2014 Posted March 3, 2014 You need to keep the center of pressure behind the center of gravity. The easiest way to calculate this is with the cardboard cutout method. Draw a silouette of your rocket on cardboard, then balance it (right angle to the length) on a thin rod. The location of where it balances on the rod is your center of pressure. Now do the same thing with your loaded rocket/motor/whatever you're trying to make fly. Where it balances on the rod is the center of gravity. The center of pressure should be a diameter of the tube or more behind the center of gravity. If it is, it will likely fly straight. It may be counter intuitive, but more weight on the nose (top) makes it more stable, of course more surface area on the bottom also increase stability (fins). There are many free flight stability programs out there, but the learning curve for most is fairly steep. The cutout method with trail and error with an inert payload may be easiest. 1
schroedinger Posted March 3, 2014 Posted March 3, 2014 FireaBirdHank explained pretty god how stick rockets work. The stabilisation of Finned rockets, is done like Leedrill already said done by having fins, which create a drag like at on arrow. These things need a high speed and a very fast acceleration to work proper. The Third method would be stabilisation by spinning. This is done at the so calld spinner rockets.
GG2014 Posted March 3, 2014 Author Posted March 3, 2014 By the way, just checking. When you say that the center of pressure should be behind the center of gravity, that means that the center of pressure should be more towards the bottom of the rocket than the center of gravity? What I initially thought was that the center of gravity should be exactly in the middle of the rocket. but then as bottle rockets go, it appears the center of gravity should be at the very bottom of the rocket where the nozzle is. I haven't heard much about center of pressure though.
schroedinger Posted March 3, 2014 Posted March 3, 2014 No, the other way around. The center of pressure (e.g. the Nozzle) needs to be more towards the top of ghe rocket, than the center of gravity
ddewees Posted March 3, 2014 Posted March 3, 2014 im no expert by anymeans and i understand little about the flight of finned rocket motors all i know is it all relates to the balance point of the whole setup and the aerodynamics { im assuming that is the same as a sticked rocket it needs a low centre of gravity and the fins work the same way as the stick in that they have drag when the point of thrust try's to over take the centre of gravity dragging the motor straight again with air resistance } id love for some one to set my answer straight simply not by just trying to seem smarter than everyone else {is a complex subject but can be described a lot simpler than some people try to make it } also if anyone has one of the rocket simulator programs or a link to a free download could you point me in the right direction id love to have a play with one of them it might shine some light on the bits i know nothing about hope this helps GG and someone chimes in who can simplify it all for us both Burnsim has a free "annoying" mode if you download it. http://burnsim.com
nater Posted March 3, 2014 Posted March 3, 2014 Google "openrocket". It is free and should do what you want.
FireBirdHank Posted March 4, 2014 Posted March 4, 2014 (edited) By the way, just checking. When you say that the center of pressure should be behind the center of gravity, that means that the center of pressure should be more towards the bottom of the rocket than the center of gravity? What I initially thought was that the center of gravity should be exactly in the middle of the rocket. but then as bottle rockets go, it appears the center of gravity should be at the very bottom of the rocket where the nozzle is. I haven't heard much about center of pressure though.Yes, the aerodynamic center of pressure needs to be behind the center of gravity. That center of pressure would be the cardboard cutouts balance point, it needs to be behind the actual balance point (center of gravity) of the completed rocket. schrodinger: You've got the right idea but wrong terminology. Fins do have drag of course, but they work by creating lift. GG2014: What you want to do can be done, but it's going to be more or less an amateur rocket with a firework payload on it. I haven't run the numbers but I'm pretty sure you'll need a longer body than you'd want, then there's just more trash coming down after, plus I think a fair bit more work than just attaching a stick. Here's a link to one of projects from a few years ago I do have a bit of experience making stable rockets. Edited March 4, 2014 by FireBirdHank
kpknd Posted March 7, 2014 Posted March 7, 2014 The way I always check the stability is to tie a string around the rocket at the CG (where it balances horizontaly) and swing it around in a circle. If it flies nose first, it's stable. 1
jl88 Posted March 13, 2014 Posted March 13, 2014 The spin stablised rocket are know to me as "stinger missiles" They can be made in any size up to 6 pounds with commercially available tooling and could probably be made bigger if you so desired. I have made hundreds of these with rcandy, bp and less successfully, whistle mix (damn buggers like to bust the spiral wound tubes I can get for cheap in Australia) They require no sticks for stabilisation but do require a pin (read cut down metal kebab stick or big nail) to launch off. The payloads have to be fairly small to guarantee a straight trajectory but anything up to 4 or 5 3/8 stars for a 1lb rocket seems fine. Adding titanium to the fuel makes an amazing whirlwind style launch at night. Best bit is there is generally no large falling debris. Here is a video.
SekndAmendment Posted March 19, 2014 Posted March 19, 2014 It's already been pretty well stated here but the stability of the rocket is directly related to the distance between and locations of the center of gravity and center of pressure. It seems a little counter intuitive but you need the center of gravity to be forward of the center of pressure. The reason for this is quite simple. Your thrust vector can be resolved and modeled as a single force being applied upward at the center of gravity. The drag force can be resolved and modeled as a single downward force and is applied at an approximate geometric center based on the surface area of the outside of your rocket. Drag pulls down from the center of pressure. Thrust pushes up from the center of gravity. If your center of gravity is below the center of pressure this means that you're pushing upwards from a location below where the drag is pulling downwards. If there is any misalignment of these two forces, it will want to rotate. Any disturbance to the rocket that upsets the alignment of drag and thrust will be greatly exaggerated by the rocket because it will tend to rotate farther in that direction. Now if your center of gravity is forward of the center of pressure then a slight misalignment between the two vectors still leaves the rocket pointed upward, but with perhaps a slightly different angle of attack than you originally wanted. The couple moment of the first rocket tries to reorient the rocket nose down, while the couple moment created by the second rocket wants to hold the rocket nose up. A slight cross-breeze will tip the rocket and it will recover its original orientation. The first rocket, when exposed to a slight cross-breeze will begin to exacerbate the problem because it naturally wants to rotate away from vertical instead of maintain it.
kpknd Posted March 27, 2014 Posted March 27, 2014 I have used the 1/4" I.D. motors whith small fins glued to them and a length of tube glued to the top for a heading, and a launch lug made from a small diamater plastic swizzle stick taped to the side. works great. Larger motors should work also, just take an assembled rocket and give it the swing test to make sure.
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