Beginner looking for design tips

[Trekky0623]

Hello. I am designing a rocket for use at school, in which a rocket must be launched an then safely return an egg to the ground, with longer times getting more points. I think I have found ways to cushion the egg, but I need help with my rocket design. My basic design is attached as a picture.

Here are the guidelines, in case anyone asks.

- The pressure chamber must be 2 L or smaller.
- There can only be one pressure chamber.
- No piece of the rocket can fall unaided.
- No altering the structural integrity of the pressure chamber (e.g. no hot glue, etc.).
- No pieces within 2 cm of the neck of the bottle.

Any ideas for 1) a high flying rocket and 2) getting both the lander and the rocket to be retrieved, with the lander coming down as slowly as possible?

[Billkuhl53]

Nice diagram. From my experience, which is only slightly longer than one year, a system with a tomy timer releasing a plunger to push the nose is the most consistent method to get the parachute to deploy.

[rockets-in-brighton]

Hello Trekky0623, and welcome.

You say you have 3 objectives: (1) height, (2) safe landing, (3) longest possible descent time for the "lander" portion. I don't believe these are mutually exclusive so that's good - although complicated methods of parachute deployment tend to be heavier.

For (1) height: make the top of the lander, which acts as the overall rocket nosecone, more aerodynamic (less blunt). You can try strapping the pressure vessel with glass tape to increase how much pressure it can hold (although I suspect you have omitted a rule about maximum pressure allowed.) You should definitely add 3 or 4 fins, as close to the base of the rocket as you are allowed, to make it stable in flight.

For (2) safe landing: you have a parachute but you'll have to work out a means of getting it out at the right time. It's the #1 water rocket problem, and you should search this forum for ideas to suit your time and budget. There is also an excellent comprehensive recovery guide that includes non-parachute recovery techniques on the main Air Command site, you'll find it linked from this post http://www.wra2.org/forum/viewtopic.php?f=12&t=508. You might have to give your "booster" part a token parachute to fulfil the rule about unaided descent. This could just be attached to the top of the pressure vessel, underneath the lander, so that when the two separate the booster parachute deploys.

For (3) longest possible descent time of "lander": minimise the weight of the lander, do the opposite of getting good height (try to make it as unaerodynamic as you can), and make the parachute as large as you can reliably deploy.

Good luck!

[Tim Chen]

Does the booster and the lander have to land at the same time or together? If you only time the flight of the payload, then you can eliminate all the padding on the egg and use a huge parachute strapped to the egg by itself and a smaller chute tied to the booster. The egg will come down a lot slower if it isn't connected to the heavy rocket.

[Spaceman Spiff]

If you had a reliable way to wrap the parachute that would repeatably deploy some fixed time after being ejected from the rocket, you should be able to just skip a deploy system and set the chute on the ground beside the rocket. The launch would cause the parachute to begin to unwind and you calculate the time it takes to unroll and deploy to match the apogee time of the flight.

I don't have any suggestions for how you would go about wrapping the parachute, but I was watching George's new video and it really stood out that the parachute took a couple seconds to open up and stop the falling rocket. I'm sure there is a clever way to wrap it so that the unwrapping acts like a time delay opening for the parachute.

I'm not sure this idea has ever been tried before.

[The Mooseheads]

Hello. I am designing a rocket for use at school, in which a rocket must be launched an then safely return an egg to the ground, with longer times getting more points. I think I have found ways to cushion the egg, but I need help with my rocket design. My basic design is attached as a picture.

Here are the guidelines, in case anyone asks.

- The pressure chamber must be 2 L or smaller.
- There can only be one pressure chamber.
- No piece of the rocket can fall unaided.
- No altering the structural integrity of the pressure chamber (e.g. no hot glue, etc.).
- No pieces within 2 cm of the neck of the bottle.

Any ideas for 1) a high flying rocket and 2) getting both the lander and the rocket to be retrieved, with the lander coming down as slowly as possible?

Where do you live that you are participating in a Water Rocket competition? Are you from a warm climate this time of the year?

[Andromeda]

If you had a reliable way to wrap the parachute that would repeatably deploy some fixed time after being ejected from the rocket, you should be able to just skip a deploy system and set the chute on the ground beside the rocket. The launch would cause the parachute to begin to unwind and you calculate the time it takes to unroll and deploy to match the apogee time of the flight.

I don't have any suggestions for how you would go about wrapping the parachute, but I was watching George's new video and it really stood out that the parachute took a couple seconds to open up and stop the falling rocket. I'm sure there is a clever way to wrap it so that the unwrapping acts like a time delay opening for the parachute.

I'm not sure this idea has ever been tried before.

I don't think that the parachute was taking so long to open. I was under the impression that he was intentionally allowing the rocket to freefall for a few seconds before deploying the parachute by setting the timer for a few extra seconds past apogee.

[air.command]

I don't think that the parachute was taking so long to open. I was under the impression that he was intentionally allowing the rocket to freefall for a few seconds before deploying the parachute by setting the timer for a few extra seconds past apogee.

That's correct. We set the timer to start ejecting the parachute at near apogee with the knowledge that the way this large parachute is packed it takes about 2-3 seconds to fully open. We didn't want to set the timer to two seconds past apogee because then in the additional 2 seconds the rocket would have gained too much speed and could damage the rocket and payload as we don't use a shock cord.

When we want the parachute to open right at apogee we start deploying the parachute earlier.

[Spaceman Spiff]

I don't think that the parachute was taking so long to open. I was under the impression that he was intentionally allowing the rocket to freefall for a few seconds before deploying the parachute by setting the timer for a few extra seconds past apogee.

That's correct. We set the timer to start ejecting the parachute at near apogee with the knowledge that the way this large parachute is packed it takes about 2-3 seconds to fully open. We didn't want to set the timer to two seconds past apogee because then in the additional 2 seconds the rocket would have gained too much speed and could damage the rocket and payload as we don't use a shock cord.

When we want the parachute to open right at apogee we start deploying the parachute earlier.

George,

Does the parachute take that much time because you pack it or wind it a special way, or is that just the nature of the size of the parachute and the speed at which it needs to be falling before it opens up.

In some of the NASA videos I have watched, they do something with the parachute lines that makes them open open the chute a little bit at first and then over time as the speed comes down the parachute opens more fully.

I want to do that with wrockets so they fall faster but slow down before they hit the ground so they don't travel so far in the wind.

[air.command]

George,

Does the parachute take that much time because you pack it or wind it a special way, or is that just the nature of the size of the parachute and the speed at which it needs to be falling before it opens up.

The parachute is packed very tightly with lots of folds and has a long main line that is wrapped lots of times around the chute. Deploying at apogee means the rocket is traveling quite slowly, and since the ejected parachute is in free fall just like the rocket there really isn't enough air drag to start unwinding it until the rocket picks up speed again.

In some of the NASA videos I have watched, they do something with the parachute lines that makes them open open the chute a little bit at first and then over time as the speed comes down the parachute opens more fully.

I want to do that with wrockets so they fall faster but slow down before they hit the ground so they don't travel so far in the wind.

'Reefing rings' are a common application of this. They are a little hard to control but you can make one out of a plastic shower curtain ring, or a keyring. Thread the parachute shroud lines through the ring and push the ring all the way up to the canopy. Attach and pack your parachute as normal. As the parachute starts filling, the ring will prevent the parachute from fully opening, but because the parachute wants to open it will start forcing the shroud lines apart and that will start forcing the ring down towards the rocket. As the ring moves down the parachute opens more. There are lots of variables that affect the performance such as line friction, air speed, length of shroud lines, ring diameter etc. I have not flown one so I can't speak from experience how well they work with water rockets.

[Spaceman Spiff]

George,

Does the parachute take that much time because you pack it or wind it a special way, or is that just the nature of the size of the parachute and the speed at which it needs to be falling before it opens up.

The parachute is packed very tightly with lots of folds and has a long main line that is wrapped lots of times around the chute. Deploying at apogee means the rocket is traveling quite slowly, and since the ejected parachute is in free fall just like the rocket there really isn't enough air drag to start unwinding it until the rocket picks up speed again.

In some of the NASA videos I have watched, they do something with the parachute lines that makes them open open the chute a little bit at first and then over time as the speed comes down the parachute opens more fully.

I want to do that with wrockets so they fall faster but slow down before they hit the ground so they don't travel so far in the wind.

'Reefing rings' are a common application of this. They are a little hard to control but you can make one out of a plastic shower curtain ring, or a keyring. Thread the parachute shroud lines through the ring and push the ring all the way up to the canopy. Attach and pack your parachute as normal. As the parachute starts filling, the ring will prevent the parachute from fully opening, but because the parachute wants to open it will start forcing the shroud lines apart and that will start forcing the ring down towards the rocket. As the ring moves down the parachute opens more. There are lots of variables that affect the performance such as line friction, air speed, length of shroud lines, ring diameter etc. I have not flown one so I can't speak from experience how well they work with water rockets.

Thanks George!

Now that you described it I know exactly what you are talking about. I have seen it myself and didn't know what it was that I was looking at. I had assumed that they had wrapped the lines in some special knot or with a special wrap that caused the lines to stay bundled.

I've also seen some videos where the lines really do stay grouped and they open up with a sudden "pop". I think in those cases they must use an explosive bolt or actuator to break a tie in the lines to make them expand.

We could do this with a servo on a water rocket. It could control an extra line that unhooked a loop around the shrouds. Do you think that could be easily done? Another way would be to tie a slip-knot around a bundle of shrouds and have a servo or a tomy timer wind the string and unhook the knot.

If you grouped your shrouds into branches 8 lines into 4 groups of 2 into 1 group of 8 the rocket, the point where 4 turns to 8 could be tied with the loop that released and caused the chute to expand.

[U.S. Water Rockets1]

Thanks George!

Now that you described it I know exactly what you are talking about. I have seen it myself and didn't know what it was that I was looking at. I had assumed that they had wrapped the lines in some special knot or with a special wrap that caused the lines to stay bundled.

I've also seen some videos where the lines really do stay grouped and they open up with a sudden "pop". I think in those cases they must use an explosive bolt or actuator to break a tie in the lines to make them expand.

We could do this with a servo on a water rocket. It could control an extra line that unhooked a loop around the shrouds. Do you think that could be easily done? Another way would be to tie a slip-knot around a bundle of shrouds and have a servo or a tomy timer wind the string and unhook the knot.

If you grouped your shrouds into branches 8 lines into 4 groups of 2 into 1 group of 8 the rocket, the point where 4 turns to 8 could be tied with the loop that released and caused the chute to expand.

NASA uses special lines connected along the skirt of the parachute that hold the parachute in a partially opened configuration to prevent it from blowing out under the full load of deployment. They use a purotechnic cutter to sever the line and thereby open the parachute. They have several of these on the Ares booster recovery system parachute and it opens in several stages. You can watch the parachute in this video and see how it looks. At the end of the video is an Engineer discussing how the system works and why they designed it the way they did.

http://fliiby.com/file/343977/b7hk9nq5by.html

A system of deploying the parachute on a water rocket in a similar manner looks like it it well within the skills of several of the designers we've met on the forum. The same effect could also be achieved in a simpler manner by having a system to deploy a second (or more) parachute at various altitudes.

We had designed a system like this in 2004, but it was overly complex and on a flight, the first parachute failed to deploy at an altitude of over 1100 feet, and the second parachute didn't have time to deploy before the lake came up and smashed into it. it was a total loss.

The system was hampered by the fact we were making FTC rockets and there's little room for the hardware inside. A multiple chute recovery would be more suitable for a larger diameter rocket like the large ones that George builds. A controlled "reefing" system like NASA uses may be applicable to an FTC rocket because of the space constraints.

The other benefit of a controlled reefing system would be to cut down on weight and increase altitude, since a multiple parachute rocket will have the extra weight of the second chute and the bay required to house it.

A system to control the reefing of a parachute sounds like an interesting project.