Help a buddy out...(advice being solicited)

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Dad Man Walking

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Gentledudes and dudettes,

I'm interested in your thoughts/advice/warnings/random thoughts about a project I'm about to take on.
I'm helping a college student complete a rocketry/robotics challenge that he was engaged in during the school year. The challenge involved launching a rocket that would deploy a payload at >1000 feet. The payload is a robotic device--a rover--that has to detach itself from the recovery gear after landing, traverse potentially very rough terrain, stop to take a soil sample, and beam photos back to the rover operator. The rocket just has to get back down safely, separate from the rover.

The student team had a robot crew and a rocket crew. The robot got done but apparently the rocket crew didn't get too far. So the rover is home for the summer and the student wants to build a rocket and execute the mission before going back to school.

I've looked a couple of Arliss designs, but don't really want the complexity (altimeters) or need the re-usability of an Arliss rocket. My goals as the designer and flyer are to keep things simple and as cost effective as possible, while making sure that we get the payload out and get the rocket back down safely.

Some parameters and constraints:
  • Rover was designed to fit into a 6" or 7.5" tube, so we're using blue tube (7.5")
  • Rover weight is around 4 pounds
  • Maximum motor is a K. My sims of the basic airframe are showing a 2000-2500 foot flight on a mid-level K, which is perfect for our purposes.
  • I want to avoid using deployment electronics if possible. I'd like to use motor deploy to pop things open shortly after apogee, and get the rover clear of the rocket so that the recovery packages won't get fouled with each other.
I know how to build a big dumb rocket that will go up and come back down safely. My main area of concern is the payload deployment method.

My idea right now is to deploy the payload via a piston. My sketch of the basic layout is in the picture below.

The piston will be a 6-12" section of coupler tube with bulkheads on both ends. The piston will rest on a shelf or ledge made of a centering ring with most of the middle cut out.

The top centering ring will be about 3" beneath the piston shelf. I think this will provide enough room to hold the main and shock cord, and I want to minimize the volume that is pressurized with the deployment charge. My preliminary calculations indicate that a modest charge of just 1 to 1.5 grams will generate enough pressure to hit the piston with nearly 600 pounds of force due to the large surface area of the piston. But I have no idea what will happen as the piston moves through the body tube. The volume will expand significantly, the pressure will drop, and I don't know if that 600 pounds kick is actually enough to push the piston all the way out, or if it might just break things.

So ground testing will be done...

I am also thinking (hoping?) that the NC should be pushed out while the piston is pressurizing the upper chamber since there is no need for shear pins there. But if there's not enough energy to push the piston all the way out, maybe putting a drogue on the nose cone could create enough drag to pull the piston the rest of the way out?

I'm expecting the main parachute to be packed loosely in the lower compartment, so as long as I can get the piston all the way out I'm expecting that the shock cord and parachute will be extracted in short order. But should I be worried?

I want to make sure that the payload (the rover) can fall away from the rocket and deploy it's own parachute cleanly. I'm considering using a JL Chute Release and maybe a small drogue to slow the 4 pound payload until the main sets. Or maybe some sort of homemade deployment bag, where a drogue sets first and then pulls the main free. I've used the JLCR, but don't have experience with deployment bags.

One other thing I'm considering--not illustrated in the drawing--is to bring the NC and the booster down separately. If the deployment charge can push the NC free of the booster and the parachute is packed so that it will take a second or two to unfurl that might give time for the NC and the payload to fall away from each other before either sets a parachute. But is also means that the deployment charge has to do all the work in getting the piston completely out.

So that's what I'm thinking. Any feedback is greatly appreciated!

Rough drawing in the link

LeggyRockit.png
 
MHO, waay to complicated.
use motor eject
just above the motor is a chute for the rocket, then the rover package, and on top, I'd go with a streamer or small chute for the nose cone. Augment the ejection charge to really get things the ** out.
it really works.

One club i'm in flys pumkins - the small ones, for a parachute duration. One year I did that with a 4 inch rocket and pushed out a pumkin on a 13 foot parachute. the augment is the key. Shoot stuff across the sky so that the nose cone is away from the rover. size the chutes so that the rocket stuff is on the ground long before the rover, so nothing gets fouled.

or you could do a piston, or something equally difficult..or maybe even more difficult..
 
what does your sim show for a delay time for k motor?

Tony
Around 10 seconds to apogee. I was thinking that the delay should be set a bit longer so that the rocket is pointing down when the charge goes off.
 
A little before while there's still some fin control seems better, unless you can guarantee a backslide.

Once you start to tumble it may be awhile before it regains predictability.
 
I would consider putting the bulkhead in the bottom of the piston and a padded cradle for the robot. That will ensure the robot exits the BT in a known state and orientation. It also protects the robot somewhat. Or you can do it like cbrarick said. You don't really need electronic deployment or JLCR. You're going to be just over 1000 ft on a field big enough to fly K motors, main at apogee should be fine.

This sounds like a Battle of the Rockets rover contest. Talking with the test director, over the years very few of the robots ever move once they are on the ground. The rockets usually do well and the deployments usually work well, but the robots seldom survive the landings.
 
I would consider putting the bulkhead in the bottom of the piston and a padded cradle for the robot. That will ensure the robot exits the BT in a known state and orientation. It also protects the robot somewhat. Or you can do it like cbrarick said. You don't really need electronic deployment or JLCR. You're going to be just over 1000 ft on a field big enough to fly K motors, main at apogee should be fine.

This sounds like a Battle of the Rockets rover contest. Talking with the test director, over the years very few of the robots ever move once they are on the ground. The rockets usually do well and the deployments usually work well, but the robots seldom survive the landings.

You are correct -- Battle of the Rockets/Mars Rover III

Second paragraph of the Mars Rover contest description says "in the past, all rovers had some failure. Most failures were due to impacting the ground." I have a parachute that should bring the payload down at 20fps, or less if the rover doesn't weigh in at the maximum 2kg. Sounds slow for a rocket, but it's still like dropping the rover from a 6 foot ladder. Something else we'll ground test.

I wasn't keen on the idea of just wrapping the rover in a big nomex blanket and blowing it out of the tube, punkin' chunkin' style. My idea was to put the rover on the top of the piston, sitting in a cradle as you suggest. In the sims I have the piston 12" long in order to move the CG forward. Trying to avoid nose weight if possible if I can get by with longer cardboard tubes.

The rocket sims to around 2000 feet with a K535. The only reason for considering a JLCR was if we want the rover to fall away from rocket before setting the main. My TARC team had all sorts of problems this year trying to get the two rocket sections to set separate parachutes without fouling each other. So if I'm overthinking things, that is why. We'll do a test flight with a dummy payload to see how the deployment works.
 
You are correct -- Battle of the Rockets/Mars Rover III

Second paragraph of the Mars Rover contest description says "in the past, all rovers had some failure. Most failures were due to impacting the ground." I have a parachute that should bring the payload down at 20fps, or less if the rover doesn't weigh in at the maximum 2kg. Sounds slow for a rocket, but it's still like dropping the rover from a 6 foot ladder. Something else we'll ground test.

I wasn't keen on the idea of just wrapping the rover in a big nomex blanket and blowing it out of the tube, punkin' chunkin' style. My idea was to put the rover on the top of the piston, sitting in a cradle as you suggest. In the sims I have the piston 12" long in order to move the CG forward. Trying to avoid nose weight if possible if I can get by with longer cardboard tubes.

The rocket sims to around 2000 feet with a K535. The only reason for considering a JLCR was if we want the rover to fall away from rocket before setting the main. My TARC team had all sorts of problems this year trying to get the two rocket sections to set separate parachutes without fouling each other. So if I'm overthinking things, that is why. We'll do a test flight with a dummy payload to see how the deployment works.

What about using a JLCR or similar on the rover with a small drogue chute? You could use that to get the rover down relatively quickly and then open up a large chute for a softer landing. I think I've seen people successfully using the JLCR on chutes up around 60", which would be a really soft touchdown.

Another thought for separation is to have the nose cone coupler touching or nearly touching 3-4 posts around the perimeter of the rover. The posts would be hard connected to the piston and may be part of your cradle for the rover itself. At deployment, the piston pushes the nose cone off rather than having the rover do that job. When the piston comes to the end of its own recovery harness, the rover and nose cone pop off if they haven't already come free in the slipstream. I'm assuming the nose cone is recovered separately here, but that may not be necessary.
 
You are correct -- Battle of the Rockets/Mars Rover III

Second paragraph of the Mars Rover contest description says "in the past, all rovers had some failure. Most failures were due to impacting the ground." I have a parachute that should bring the payload down at 20fps, or less if the rover doesn't weigh in at the maximum 2kg. Sounds slow for a rocket, but it's still like dropping the rover from a 6 foot ladder. Something else we'll ground test.

I wasn't keen on the idea of just wrapping the rover in a big nomex blanket and blowing it out of the tube, punkin' chunkin' style. My idea was to put the rover on the top of the piston, sitting in a cradle as you suggest. In the sims I have the piston 12" long in order to move the CG forward. Trying to avoid nose weight if possible if I can get by with longer cardboard tubes.

The rocket sims to around 2000 feet with a K535. The only reason for considering a JLCR was if we want the rover to fall away from rocket before setting the main. My TARC team had all sorts of problems this year trying to get the two rocket sections to set separate parachutes without fouling each other. So if I'm overthinking things, that is why. We'll do a test flight with a dummy payload to see how the deployment works.

I would seriously consider a motor that only put you at about 1100 - 1200 ft instead of 2000. It doesn't seem like that big of difference on a field as large as BattlePark, but the contest is on a set date. If the wind is 15 to 18 mph ground speed, the contest will go on and that extra 800 ft of altitude could be disastrous. All conditions include weather, not just rough ground or corn stalks.
 
I would seriously consider a motor that only put you at about 1100 - 1200 ft instead of 2000. It doesn't seem like that big of difference on a field as large as BattlePark, but the contest is on a set date. If the wind is 15 to 18 mph ground speed, the contest will go on and that extra 800 ft of altitude could be disastrous. All conditions include weather, not just rough ground or corn stalks.
We'll launch this summer at a field in California so I'm not that concerned about 1000 vs. 2000 feet. (And we won't be dealing with corn fields, but alfalfa fields will present their own challenges.) And I agree lower is better in general. Initial designs were based around the Aerotech DMS motors and I wanted to keep the speed off the rail at >50fps. The K535 looked good, and that resulted in the 2000+ foot estimate. But I now see there are some RMS motors that will deliver the same kick with less total energy...J540R looks good for a safe launch and 1400 feet with the current sims. Great suggestion, thanks!
 
What about using a JLCR or similar on the rover with a small drogue chute? You could use that to get the rover down relatively quickly and then open up a large chute for a softer landing. I think I've seen people successfully using the JLCR on chutes up around 60", which would be a really soft touchdown.

Another thought for separation is to have the nose cone coupler touching or nearly touching 3-4 posts around the perimeter of the rover. The posts would be hard connected to the piston and may be part of your cradle for the rover itself. At deployment, the piston pushes the nose cone off rather than having the rover do that job. When the piston comes to the end of its own recovery harness, the rover and nose cone pop off if they haven't already come free in the slipstream. I'm assuming the nose cone is recovered separately here, but that may not be necessary.
We're definitely considering your suggestions. My two concerns are both related to the deployment and setting the recovery packages--making sure the deployment reliably gets the payload out of the body tube, and making sure that the parachutes & shock cords don't foul with each other.
 
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