Purdue USLI Seeking Design Opinions (and criticisms)

First, I am new to this forum. If I have placed this post in the wrong thread, feel free to move it.

Second, I am an AAE student at Purdue University. A team of students and I am building a high powered rocket (L-motor ish) for NASA's USLI (university student launch initiative) competition. The goal is to get a rocket to exactly 1 mile in altitude --> you lose points for going over or under.

Our team is really good with electronics. However, we are new to high powered rocketry, and we are ignorant to what will work and what will not in terms of aerodynamics and stability.

Basically, our current plan is to deploy rearward drag streamers after motor burnout, but before apogee. (The exact time of deployment will be determined by an onboard computer taking fancy sensor data and doing fancy calculations. Deployment will occur with black powder and an e-match.)

The hard part for us is designing external pods to contain the packed streamers. We don't know how exactly to keep the pods from flying off, yet keeping the rocket stable without adding too much weight to the back of the rocket.

Anyway, here are 2 of our ideas:

idea 1:




As seen, our rocket is 4 inches in diameter. There will be 6 external pods. 3 of the pods will be hollow tube fins. The other 3 pods will each have packed stramers. These pods will be topped with a nose cone. For extra stability, fins will be added through the tube fins. (are these needed.) The problem with the design at the moment is that there is no room for the rail buttons.

Design 2:







As seen, this design has the same 6 external pods. However, we make our own cap out of fiberglass instead of using nose cones. We think this will save weight and add to stability since the surfaces will act as stabilizers.

Comments? concerns? suggestions? criticisms? questions? We need all the help we can get.

That will be draggy beyond all heck, and will be fairly stable because of that.

How are you gonna mount it on a launch rail, though?

You might be better served by three much smaller pods just plain glued on the side of the main tube (perhaps at the fin roots). You would be able to use a smaller motor that way.

That said, streamers are going to have unpredictable drag; they give little drag when moving really slowly, but also when too fast. I doubt you'll be able to get enough data in the final flight configuration (trying to scale the streamers is gonna be next to impossible) to effectively model the rocket for a control system design.

You might be better served by butterfly valves in the tubes, but that again leads to the problem with launch guidance.

One way to keep the pods on without them flying off is to literally screw them into the airframe in multiple places. This will only go so far, though, as tube fins are very draggy and will rip no matter how you attach them at a certain speed. The extra fins are not needed if the tube fins provide enough margin of stability on their own. Otherwise, the fins would just add weight, drag, expense, and more potential failure points. As for the streamers after motor burnout...why? What purpose do they serve?

Yes, but we will only have 3 tube fins. The other 3 pods will have caps (either a nosecone, or a cap we make ourselves.) Will 3 tube fins provide sufficient stability?

Thanks for the screw advice. We plan to do that along with epoxying all adjacent tubes to one another.

The point of the streamers is to slow the rocket to 1 mile in altitude. We hope to load the rocket with a motor that will consistently take the rocket a little past 1 mile. The streamers will be deployed before 1 mile to slow the rocket down. We hope to determine if this will make a rocket more accurately hit 1 mile than if a motor were selected for 1 mile with a normal rocket. [It's part of our project, and we just want to see if we can do it. we have the electronic capability.]

The capped tubes will themselves provide stability, by simply moving the center of pressure waaaay back.

If you use the appropriate adhesive, you don't need to screw anything together (which would create stress concentrations).

in response to CarVac,

we have to launch from a standard launch rail as per NASA rules

Are you suggesting we only use 3 capped pods? No hollow tube fins? [Would this provide enough stability? Would this not make the pods vulnerable to flutter?]

We have gravitated towards streamers because we are not allowed to use parachutes. Streamers can also be deployed quickly and reliably with gun powder and redundant charges. It will certainly be a challenge [and perhaps one we can't overcome] to determine the average drag of a streamer as a function of airstream velocity. This is something we - unfortunately - have to accept about this current plan

What is a butterfly valve?

I know somebody going to Purdue that has his Level 2. I've e-mailed him to contact you, he would be a great choice to help you out. And yes, he's slightly nuts, hopefully I had something to do with that.

As far as the rail goes, there should be no problem running it through one of the open tube fins.

Adrian

What I1299 meant is that you don't need regular fins if you have the tube fins.

gotchya. i will drop the fins from the first design idea.

and yes, we will probably slide the rail through one of the open tube fins

I like the idea of tube fins. They fly straight in wind, they're different from anything else that the other teams are using, and they're simple to build. The only problem is the drag. The streamer deployment design seems like it would work better with a fast burning motor that would give the most coast time, but the rocket design might need a slow burning motor just to get it past a mile.

Tube fins add a lot of stability, and you don't need the "fin" inside the tube.

Your rail problem is easily solved -- with your six pods, three will be open. Mount the rail buttons inside one of the tube fins.

You have another potential problem you've not even mentioned. I'm guessing your reason for deploying the streamers is to create sufficient drag to try to hit one mile, exactly. What happens if your streamers don't deploy? How high is the rocket going to go? There are hard and fast altitude limits, and those running the event get very grumpy if you exceed it.

As one of the people who does the pre-flight reviews, I can tell you beyond a doubt that you're going to get asked hard questions about maximum altitude, should it not deploy, as well as the effects on stability if only some of them deploy, and not all of them.

Also, how are you securing the streamers so that they don't deploy under thrust, but are still able to deploy when you need them to?

Pod/tube fin attachment is easy -- epoxy the pods to the airframe, and to each other. There's an incredible amount of bonding surface there.

-Kevin

Tube fin rockets have a Cd =2.4 according to Larry Brand IIRC.

If so a 4" airframe tube fin rocket will apogee at 1 mile on a full L with a lift-off weight of 15 kg.

Another option is deployable drag brakes. You can make an air brake from sections of airframe and locate them between the fins on a conventional rocket. If the forward ends are hinged, you can use a servo motor to push the aft ends out which increases the drag of the rocket.

The control is relatively simple. 16' below apogee the rocket velocity should be 32 fps, 48' below apogee the velocity should be 64 fps etc. (This asssumes minimal drag near apogee.) You results may vary, but the calculation is quite straight forward.

Bob

bobkrech said:

Another option is deployable drag brakes. You can make an air brake from sections of airframe and locate them between the fins on a conventional rocket. If the forward ends are hinged, you can use a servo motor to push the aft ends out which increases the drag of the rocket.

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I'm not certain the program allows fundamental design changes like that -- their project got approved based on what they submitted. I don't know that aspect of USLI well enough to know whether or not that's an admissible change.

The thing to remember is that this team must work within the confines of the program -- they submitted a proposal, and got that proposal approved. There are also factors that cause things to be more strictly controlled than a "typical" launch.

It's important to remember that this project isn't being built to be flown at a local club launch.

-Kevin

why bother with the streamers? just blow the main ejection charge when you get close to one mile. as long as the chosen motor apogees slightly above 1 mile, the stress on the airframe should be manageable.

Derek said:

why bother with the streamers? just blow the main ejection charge when you get close to one mile. as long as the chosen motor apogees slightly above 1 mile, the stress on the airframe should be manageable.

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Absolutely against the rules. Doing so dramatically increases the chance of a shred, and thus the chances of parts coming in ballistic.

-Kevin

Troj, thank you for your input. Very valuable.

1) it's good to know we won't need the fins. I'll excessively worrying about stability.

2) We have had this plan approved by the NASA RSO. You are correct about the max altitude though. We are under strict orders not not exceed 5600 feet. As such, we will put in a motor that will get us as far past 1 mile as possible, but never exceed 5600 feet --> even if the streamers don't deploy. This will most likely take a few test-launches where drag streamers are never deployed to close in on which motor we need to use for this purpose. (From what I understand, Rock-sim won't be the most helpful tool for sim-launching our design.) Basically, if we can get a motor that consistently and reliably places us between 1 mile and 5600 feet without drag streamer deployment, that will be a success.

3) The question of what happens when only 2 or 1 of the streamers deploys is difficult to answer. We don't have a good answer for this yet, but we will think about it. We will have redundant charges if that helps.

4) here are some sketches of the 2 proposed ideas. These explain how deployment occurs.




Ejection is fairly simple. A black powder charge ignites and kicks off the shear-pinned cap at the bottom of the tube. This cap has an eye bolt on it which will either be attached to the streamer shock cord, or be separately connected to the forward eyebolt.

5) epoxy is our friend. got it

Derek, thanks for the input. Kevin is, however, correct. Furthermore, the system that handles drag streamer deployment will be completely isolated from the recovery system.

We've looked at doing some sort of airbrake or servo control. We chose the drag streamer idea because deployment is quick and reliable. We were concerned with the reaction speed and reliability of a servo, or electronic-based control surface. It was also difficult for us to come up with ideas that would allow a servo to apply a considerable drag force to the rocket. (Servos cannot dish out the most torque.)

You don't need the servo to deploy the aerobraking bits, you use a spring or torsion rod for deployment, and the servo to restrain them during the boost phase.

G.D.

troj said:

Absolutely against the rules.-Kevin

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didn't realize that was against the rules. it seemed like too easy of a solution...

sorry!

Derek said:

didn't realize that was against the rules. it seemed like too easy of a solution...

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That's why I clarified in an earlier post that USLI operates on a more strict set of rules than a typical club launch.

Don't let that stop you from providing ideas to these guys -- the teams often don't have a high level of HPR experience, so outside suggestions are invaluable to them.

-Kevin

justice_purdue said:

1) it's good to know we won't need the fins. I'll excessively worrying about stability.

Click to expand...

RockSim can fairly accurately model tube fins, to help you with simulations.

4) here are some sketches of the 2 proposed ideas. These explain how deployment occurs.

View attachment 103594
View attachment 103595

Ejection is fairly simple. A black powder charge ignites and kicks off the shear-pinned cap at the bottom of the tube. This cap has an eye bolt on it which will either be attached to the streamer shock cord, or be separately connected to the forward eyebolt.

Click to expand...

You don't need pins on that top cap -- epoxy is sufficient, especially if you have a lip on it that recesses it into the tube like your drawing shows.

-Kevin

The reasoning for the rivets is that we would like to be able to pull that top cap out of the pod. This would make it easier to install the black powder charges and also to access the eye bolt. The rivets would then have 2 jobs:

1) prevent cap from spinning inside pod
2) hold the cap in place when the ejection charges blow. That way, the shear pins fail, not the rivets. We can use metal screws if plastic rivets aren't strong enough.

and yes, we don't have a ton of HPR experience - so any and all suggestions are helpful

Our team is still stuck on these 2 ideas: nose cone caps, or custom-made angled caps for our streamer tubes. Are there any opinions on which design seems to have more merit?

justice_purdue said:

Our team is still stuck on these 2 ideas: nose cone caps, or custom-made angled caps for our streamer tubes. Are there any opinions on which design seems to have more merit?

Click to expand...

Why not take the easy way out, and just nosecones of the appropriate size?

-Kevin

Instead of even nosecones or fancy angled caps, just use flat tube fins. Then you have to just cut a bulkhead and recess it. I've flown a tube fin that was flat - no angles. I would suspect covering three would create more drag, which would be good. Make sure alignment is good on the tube fins - they can cone and wobble if they are not symmetric.

Edward