Need help designing a model rocket

[BABAR]

The first post says the payload has to be deployed at a specific altitude. No mention of whether it has to be deployed at apogee, or if it can be deployed during descent.

Good point.

@dee , can you post the entire rule set?

 

[NTP2]

If they are not giving the thrust curve then they are really making your life hard, be careful when simulating to use a lot of motors in your impulse range so you can be comfortable that it will work with whatever they give you.

 

[lr64]

It's not much more than gambling if they don't properly characterize the motor. Do they reveal the weight?
I could see making a series of drag cones in different sizes, each weighted so as to maintain a constant stability margin when bolted on the back. One could watch how high other rockets launch with the mystery motor before choosing which cone. Then figuring out how to get a place in the back of the queue. ;-)

 

[cls]

Some more considerations. I'm just noodling here...

Going up is one thing. Coming down is another. The game in model rockets is coming down safely, and reuseably.

Convention is final recovery velocity is around 22 feet per second, 7 meters per second

Carrying 1 kg of payload / ballast in an otherwise lightweight and narrow airframe means high ballistic coefficient. Lots of mass per area, not much atmospheric drag, so high terminal velocity. That's considered dangerous, or rude.

Without direct experience (failures), building a recovery system that can handle shock loads and land gently could be troublesome.

1kg payload is 1 liter of water, 1000cc. So 10cm long, 10cm area, or 3.6cm diameter. That's pretty narrow, I suggest something shorter and wider, 54mm. That will fit a good variety of commercial J and K motors, so many different thrust curves to choose from to match the mission flight profile.

Easier to pack the recovery system, too.

Seems the allowed 180cm max length is very generous. An experienced rocketeer could probably fit it in half or so. But, no reason to be stingy about the length, it makes packing etc easier.

 

[Ronz Rocketz]

Or a quart of water in freedom units...

I'm imagining the competition tables with numerous J and K motors lined up. The rockets are weighed and measured, then the official selects the motor. There's also a big box of wading for them to use. Attach the chute to the water bottle, insert into their rocket, and then off to the pad. I wonder how they figure out if they've gone over the 3200ft limit?

@dee thank you for posting. We Americans have our own rules and it's fascinating to find out how others do what they do.

 

[user 48716]

Good point.

@dee , can you post the entire rule set?

I've listed the main rules here:

• Rocket Dimensions & Mass:
  • Max rocket length: 180 cm.
  • Rocket body mass depends on the material:
    • Aluminium: ≤15 kg (including motor).
    • Cardboard: ≤10 kg.
    • PVC: ≤11.5 kg.
  • Lift-off mass must be within ±5% of the design value.
• Motor:
  • Teams must use one of three motors provided by the organizers.
  • Motor mass is approximately 3 kg, with a max impulse of 2800 Ns.
• Payload:
  • Must carry and deploy a 1 kg payload at 1000 m altitude ± 100 m, the payload has to be about 40 cm long and 15 cm in diameter and will probably be a CanSat.
  • The payload must return safely using parachutes.
• Flight & Descent:
  • Use a parachute or alternative descent mechanism (e.g., streamer or glider).
  • Descent rate should be 2-5 m/s.
  • All rocket parts must descend tethered together.
• Structure:
  • Rocket structure must withstand 15g acceleration and 30g shock.
  • The design must be verified through Finite Element Analysis (FEA) and other tests.
• Avionics & Telemetry:
  • Mandatory sensors: position, altitude, pressure, temperature, orientation, power.
  • Use XBEE/Zigbee radios for communication.
  • Real-time data must be transmitted and displayed in a ground station.
• Software:
  • Telemetry and flight software must maintain state throughout the mission (e.g., ascent, payload deployment).
  • Onboard systems must store data in case of telemetry loss.
• Launch Pad:
  • Teams must build their own launch pad with a launch angle between 80° and 85°.

 

[user 48716]

Or a quart of water in freedom units...

I'm imagining the competition tables with numerous J and K motors lined up. The rockets are weighed and measured, then the official selects the motor. There's also a big box of wading for them to use. Attach the chute to the water bottle, insert into their rocket, and then off to the pad. I wonder how they figure out if they've gone over the 3200ft limit?

@dee thank you for posting. We Americans have our own rules and it's fascinating to find out how others do what they do.

We're allowed to use either aluminium, PVC or cardboard for our rockets. The motors will vary depending on which material we use. We've been given the tentative lengths and diameters of all three variants and that the motor's total impulse mustn't exceed 2800 Ns.

Likewise!

 

[user 48716]

Some more considerations. I'm just noodling here...

Going up is one thing. Coming down is another. The game in model rockets is coming down safely, and reuseably.

Convention is final recovery velocity is around 22 feet per second, 7 meters per second

Carrying 1 kg of payload / ballast in an otherwise lightweight and narrow airframe means high ballistic coefficient. Lots of mass per area, not much atmospheric drag, so high terminal velocity. That's considered dangerous, or rude.

Without direct experience (failures), building a recovery system that can handle shock loads and land gently could be troublesome.

1kg payload is 1 liter of water, 1000cc. So 10cm long, 10cm area, or 3.6cm diameter. That's pretty narrow, I suggest something shorter and wider, 54mm. That will fit a good variety of commercial J and K motors, so many different thrust curves to choose from to match the mission flight profile.

Easier to pack the recovery system, too.

Seems the allowed 180cm max length is very generous. An experienced rocketeer could probably fit it in half or so. But, no reason to be stingy about the length, it makes packing etc easier.

The rules state the descent velocity has to be between 2-5 m/s. We're currently considering a dual deployment system to ensure a safe landing. The rules also state the rocket has to be less than 180 cm in length and that the payload has to be about 40 cm in length, and 15 cm in diameter.

 

[user 48716]

It's not much more than gambling if they don't properly characterize the motor. Do they reveal the weight?
I could see making a series of drag cones in different sizes, each weighted so as to maintain a constant stability margin when bolted on the back. One could watch how high other rockets launch with the mystery motor before choosing which cone. Then figuring out how to get a place in the back of the queue. ;-)

Yes, the motor mass is around 3 kg, and each team gets to choose from three variants based on the material of our rocket body (aluminium, PVC, or cardboard). I agree, it would be interesting to have more detailed specs upfront, but with the fixed motor options, it's definitely a bit of a guessing game to optimize performance.

 

[user 48716]

Can you use electronics? I know there are altimeters that deploy on the way DOWN, is there a way to deploy a streamer or something to “promptly” slow the rocket on the way UP at say 900 meters? (Obviously you don’t want a sudden stop that will disassemble the rocket mid flight).

How many practice shots do you get?

Yes, we're allowed to use electronics. While we’re not currently considering a streamer, we’ll definitely look into that option. Our current recovery system is a dual deployment system. We assumed the rocket would slow down by apogee, which is why we didn’t consider a streamer for ascent, initially.

The organisers weren't very clear on that. Given the successful submission of our report detailing a viable design, I think we'd be allowed to test it a couple of times before the main event.

 

[user 48716]

Cardboard is probably a good choice. It's relatively easy to work with. PVC is quite heavy for its stiffness and strength.

Is the 10kg a maximum weight, or a minimum weight? I should think there'd be no need for a maximum weight as the rockets just wouldn't perform if overweight. As a minimum weight, 10kg seems absurdly high.

I had just assumed that the apogee was supposed to be 1km. Tim Smith has a valid point, at least if that's how the actual rules are worded.

Where I live, wood is more affordable than aluminum or PVC. It's also available in different densities, though balsa gets kind of expensive, at least per board foot. Paulownia is quite light, if you have it in your area. Spruce, if you have something like that around, is traditional for aircraft, and is strong for its weight. Birch and maple are heavy and strong. I imagine that the species of wood available over there may be different than those found here in the northeastern part of the USA.

A good design that leaves enough time for building and testing is better than a perfect design that doesn't. If testing is allowed, it will probably be helpful. An extra rocket, just in case the first has a problem, might be good, if there's time.

Thank you, we're likely to proceed with that.

10 kg is the maximum weight permissible, both the motor and the payload included, if we're using cardboard. The maximum weight allowed differs according to the material we choose (aluminium, PVC or cardboard).

Yes, the apogee is 1000 m ± 100 m, we have to deploy the payload in that range.

Those sound like good choices but I'm afraid we're not allowed to use wood. I'll keep that in mind though!

Yes, that is what we're focusing on right now. The first step is a report, so we think we'll get ample time to refine our design before building it, given our report is accepted.

 

[user 48716]

The first post says the payload has to be deployed at a specific altitude. No mention of whether it has to be deployed at apogee, or if it can be deployed during descent.

I apologise for the lack of clarity. It has to be deployed at the apogee - 1000 m ± 100 m.

 

[NTP2]

100 meters is a pretty generous margin, I’d recommend cardboard it’s plenty strong I’ve heard of it going up to 30/40 Gs so it’ll take 15 like a champ. If you’re using FEA then you can optimize mass better than normal for hobby rockets, I’d think a I or J would be pretty good for that. Maybe make a way to change mass (eg a mass that goes in the nose but can be exchanged) so you can get the altitude where you need it with the test flights.

 

[Ronz Rocketz]

We're allowed to use either aluminium, PVC or cardboard for our rockets. The motors will vary depending on which material we use. We've been given the tentative lengths and diameters of all three variants and that the motor's total impulse mustn't exceed 2800 Ns.

Likewise!

Interesting. That's on the lower end of a L motor.

 

[NTP2]

Interesting. That's on the lower end of a L motor.

Probably for the aluminum rockets.

 

[cls]

the payload has to be about 40 cm in length, and 15 cm in diameter.

700cc? So, not just water, something denser.

Oh, one more gotcha... Polar moment of inertia. Happens to long rockets with heavy payload in front and heavy motors in back. Think about twisting a barbell in your hand. Needs overlarge fins to prevent and correct. You'll see it in the simulations, with a crosswind, lots of undampened roll and pitch and yaw.

 

[user 48716]

100 meters is a pretty generous margin, I’d recommend cardboard it’s plenty strong I’ve heard of it going up to 30/40 Gs so it’ll take 15 like a champ. If you’re using FEA then you can optimize mass better than normal for hobby rockets, I’d think a I would be pretty good for that. Maybe make a way to change mass (eg a mass that goes in the nose but can be exchanged) so you can get the altitude where you need it with the test flights.

Thank you, we're leaning toward cardboard right now! Yes, we'll be doing FEA and CFD analyses once the design is finalised. That's an interesting idea, we'll be sure to explore that.

 

[user 48716]

700cc? So, not just water, something denser.

Oh, one more gotcha... Pojae moment of inertia. Happens to long rockets with heavy payload in front and heavy motors in back. Needs overlarge fins to prevent and correct. You'll see it in the simulations, with a crosswind, lots of undampened roll and pitch and yaw.

No, it won't be water, likely a CanSat.

Thanks for the pointer! We'll check that out.

 

[ghostfather]

I've used the PML Intruder kit as a CanSat launcher, largely because it uses a piston for ejection. It's a 75mm airframe with a 38mm motor mount, and we launch two 500g CanSats to 1000m in a 4-grain I motor. It was only about 150 cm long, with a plastic nose cone. Weighs about 1.5 Kg empty, as I remember.
Using that as a beginning, go with a 75mm nose cone of your own design, and a 75mm airframe long enough to make it 180cm long. Four fins, and a 38mm motor mount, and a piston to push out the CanSats and parachute at apogee. Choose your engine to achieve 1000m with 1000g payload.

Sorry, this doesn't meet your design rules using a baby L motor, but you can scale it up using the same process. I always start with the dimensions and Impulse of the motor, and design around that. The airframe and nose cone can be scaled up, for example a 98mm airframe and a 54mm or 75mm motor mount. The idea is to keep it simple, and build it strong. Cardboard is fine, even for an L rocket, but I would add a couple of layers of fiberglass to handle the thrust forces. Plywood fins are strong enough, and easy to make. Four fins using through the wall mounting for strength. Plywood centering rings between the motor mount and the airframe.

I personally use lots of fiberglass nose cones and airframes for this class of motor, but it's very doable with cardboard and some fiberglass.

 

[Alan15578]

I've listed the main rules here:

• Rocket Dimensions & Mass:
  • Max rocket length: 180 cm.
  • Rocket body mass depends on the material:
    • Aluminium: ≤15 kg (including motor).
    • Cardboard: ≤10 kg.
    • PVC: ≤11.5 kg.
  • Lift-off mass must be within ±5% of the design value.
• Motor:
  • Teams must use one of three motors provided by the organizers.
  • Motor mass is approximately 3 kg, with a max impulse of 2800 Ns.
• Payload:
  • Must carry and deploy a 1 kg payload at 1000 m altitude ± 100 m, the payload has to be about 40 cm long and 15 cm in diameter and will probably be a CanSat.
  • The payload must return safely using parachutes.
• Flight & Descent:
  • Use a parachute or alternative descent mechanism (e.g., streamer or glider).
  • Descent rate should be 2-5 m/s.
  • All rocket parts must descend tethered together.
• Structure:
  • Rocket structure must withstand 15g acceleration and 30g shock.
  • The design must be verified through Finite Element Analysis (FEA) and other tests.
• Avionics & Telemetry:
  • Mandatory sensors: position, altitude, pressure, temperature, orientation, power.
  • Use XBEE/Zigbee radios for communication.
  • Real-time data must be transmitted and displayed in a ground station.
• Software:
  • Telemetry and flight software must maintain state throughout the mission (e.g., ascent, payload deployment).
  • Onboard systems must store data in case of telemetry loss.
• Launch Pad:
  • Teams must build their own launch pad with a launch angle between 80° and 85°.

And the minor rules? The devil is in the details. There are enough challenges to make it worthy of college credit. However, it appears to be all pass/fail. I see no cost function or performance index to minimize or maximize. How is the contest scored? The CanSat appears to be denser than I imagined. It must have some heavy batteries.

 

[techrat]

This is either an Aerospace Engineering challenge or some kind of ARC team. TRF seems to get these requests on a regular basis. Maybe we should just write up a general FAQ. But yes, I'm always amazed that someone starts off their foray into rocketry building something that flies on a "G" motor, without ever understanding the fundamentals. Has never built so much as an Estes kit. Which is why our group in NJ sees ARC teams flying rockets that have the fins hot-glued on or worse, using packing tape. And then they look confused when the rocket shreds itself after leaving the pad.

 

[Alan15578]

Those sound like good choices but I'm afraid we're not allowed to use wood. I'll keep that in mind though!

So, your rocket has to be 100% cardboard? A little bit of wood, plastic, and fiberglass can greatly improve a cardboard rocket. I'm having difficulty imagining a cardboard shock cord and parachute, and a 3D nosecone printed from wood pulp sounds interesting.

 

[neil_w]

Which is why our group in NJ sees ARC teams flying rockets that have the fins hot-glued on or worse, using packing tape. And then they look confused when the rocket shreds itself after leaving the pad.

That is a fault of the advisor, is it not? Unfortunately there are definitely some advisors who themselves have no experience in rocketry.

 

[NTP2]

This is either an Aerospace Engineering challenge or some kind of ARC team. TRF seems to get these requests on a regular basis. Maybe we should just write up a general FAQ. But yes, I'm always amazed that someone starts off their foray into rocketry building something that flies on a "G" motor, without ever understanding the fundamentals. Has never built so much as an Estes kit. Which is why our group in NJ sees ARC teams flying rockets that have the fins hot-glued on or worse, using packing tape. And then they look confused when the rocket shreds itself after leaving the pad.

they don't seem that bad (we do get that and a FAQ is a good idea), @dee can you promise not to use packing tape?

 

[WizardOfBoz]

I just checked my Madcow Super DX3 openrocket simulation. I added a 1kg mass near the nose cone. About 10 different motors got me to 3300' and above. All J motors which would be lower than the 2400N-s upper limit mentioned. I guess you could use a smaller diameter, lighter rocket to get 1kg to 1km, but I suspect that the 4" diameter is pretty close to optimal.
If you did use the Super DX3 or a scratch-build similar to it*, you could get your L1 and L2 with it (or do your L2 with a larger, heavier rocket to keep it low and slow) and then launch the 1kg object to 1km.
One thing. You mention "deploying" the 1kg payload at 1km. If you do that you MUST have a deployment system (chute, wings, whatever) to keep that object from coming down at velocity. Deploying a lawn dart at that height, to see how deep it penetrates the earth, is not an option.

*"To not know what has gone before is to forever be a child". That is, there is no sin in borrowing from the learning of others.

 

[user 48716]

The

And the minor rules? The devil is in the details. There are enough challenges to make it worthy of college credit. However, it appears to be all pass/fail. I see no cost function or performance index to minimize or maximize. How is the contest scored? The CanSat appears to be denser than I imagined. It must have some heavy batteries.

The original instructions doc is 50 pages long, I've summarised all the relevant details here. I've only avoided formal details regarding budgeting, team changes, faculty advisor details etc.

The contest is comprised of several levels, of which the first level is a preliminary design report submission, followed by a workshop, and the critical design report culminating in the event. Only teams that pass each round can proceed to the next. The preliminary design report will be scored based on the quality of the design, the simulation results, the tests we've used, whether our design adheres to the mentioned constraints and whether it includes the required details (sensors used, dimensions, specifications of each component, electronics, recovery system specifications etc.) for each section.

 

[user 48716]

I've used the PML Intruder kit as a CanSat launcher, largely because it uses a piston for ejection. It's a 75mm airframe with a 38mm motor mount, and we launch two 500g CanSats to 1000m in a 4-grain I motor. It was only about 150 cm long, with a plastic nose cone. Weighs about 1.5 Kg empty, as I remember.
Using that as a beginning, go with a 75mm nose cone of your own design, and a 75mm airframe long enough to make it 180cm long. Four fins, and a 38mm motor mount, and a piston to push out the CanSats and parachute at apogee. Choose your engine to achieve 1000m with 1000g payload.

Sorry, this doesn't meet your design rules using a baby L motor, but you can scale it up using the same process. I always start with the dimensions and Impulse of the motor, and design around that. The airframe and nose cone can be scaled up, for example a 98mm airframe and a 54mm or 75mm motor mount. The idea is to keep it simple, and build it strong. Cardboard is fine, even for an L rocket, but I would add a couple of layers of fiberglass to handle the thrust forces. Plywood fins are strong enough, and easy to make. Four fins using through the wall mounting for strength. Plywood centering rings between the motor mount and the airframe.

I personally use lots of fiberglass nose cones and airframes for this class of motor, but it's very doable with cardboard and some fiberglass.

That sounds like a good approach! Our constraints include the payload being 15 cm in diameter and the motor being about 98mm in diameter. We'll look into what we can do to handle the thrust forces. Thank you for your response.

 

[user 48716]

So, your rocket has to be 100% cardboard? A little bit of wood, plastic, and fiberglass can greatly improve a cardboard rocket. I'm having difficulty imagining a cardboard shock cord and parachute, and a 3D nosecone printed from wood pulp sounds interesting.

Not really, just the main body, from what I understand. We're allowed to use whatever materials we'd like for the parachute and the shock cord. The guidelines don't mention any specific material for the payload and avionics bays either.

Yes, the nosecone is what we're currently thinking about as well, we'll explore alternate options and see what works for us.

 

[user 48716]

This is either an Aerospace Engineering challenge or some kind of ARC team. TRF seems to get these requests on a regular basis. Maybe we should just write up a general FAQ. But yes, I'm always amazed that someone starts off their foray into rocketry building something that flies on a "G" motor, without ever understanding the fundamentals. Has never built so much as an Estes kit. Which is why our group in NJ sees ARC teams flying rockets that have the fins hot-glued on or worse, using packing tape. And then they look confused when the rocket shreds itself after leaving the pad.

A general FAQ sounds like a good idea, I'd definitely appreciate that.

I suppose this is more of an engineering challenge. We've worked with model rockets before but they were often using kits and not of this scale. We signed up for the experience really, we're not expecting to win. We'd just like to familiarise ourselves with the process.

 

[user 48716]

I just checked my Madcow Super DX3 openrocket simulation. I added a 1kg mass near the nose cone. About 10 different motors got me to 3300' and above. All J motors which would be lower than the 2400N-s upper limit mentioned. I guess you could use a smaller diameter, lighter rocket to get 1kg to 1km, but I suspect that the 4" diameter is pretty close to optimal.
If you did use the Super DX3 or a scratch-build similar to it*, you could get your L1 and L2 with it (or do your L2 with a larger, heavier rocket to keep it low and slow) and then launch the 1kg object to 1km.
One thing. You mention "deploying" the 1kg payload at 1km. If you do that you MUST have a deployment system (chute, wings, whatever) to keep that object from coming down at velocity. Deploying a lawn dart at that height, to see how deep it penetrates the earth, is not an option.

*"To not know what has gone before is to forever be a child". That is, there is no sin in borrowing from the learning of others.

I'll check that out. Our payload's required diameter is 15 cm or about 6".
Yes, we're currently considering a dual deployment system but nothing is set in stone yet.

Thank you so much, I'll keep that in mind.