Single Deployment - 4in diameter Rocket

I'm building a HPR for my level 1 certificate. I wanted it to be quite large and somewhat heavy so it doesn't go so high (1100 feet estimated from RockSim). When finished it will be 60 to 70 oz. with motor. Its 84in tall(7ft) and 4in in diameter. It consists of 16.5 inch long nose cone and two 34in long tubes. My plan is to place a bulkhead at the middle (bottom end of the top tube) and a 36in parachute resting in the bottom tube(motor-tube) above a baffle. The motor mount tube is 17 inches long and 38mm in diameter, but I'm only putting a 3 grain P-38 cesaroni casing with a h100 (2 grain) motor.

Layout:

[---nose----][-----upper---][bulkhead][---lower-----------]
[-----|----------------|-----------|-------------|--------------]
[-----v----------------v----------v-------------v--------------]
<--16.5in--|--------34in--------||--[p]--[ b ]--[==mm==]|


[p] = Parachute
[ b ] = Baffle
[==mm==] = Motor Mount


I know that I under estimated the parachute size - probably should have gone with a 50in+. Will have a decent rate of about 20fps...

I'm concerned with the recovery. Is the ejection charge in a h100 strong enough to deploy with this much area in the lower tube? Should I reconsider a new chute? First time HPR - just wanted to get some advise.

Thanks!

RockSim Layout:

The H100 should have enough black powder to do the job. Probably.

The parachute however is much too small. I'd go 48".

On the other hand, I would be slightly concerned about the liftoff thrust of the H100; I would go with a higher-thrust motor.

I second Carlo's concern about thrust-to-weight. This is likely to come up with your LCO or whoever is signing your paperwork. You want to make sure you can answer their questions about it, certainly if it's the rocket's first flight.

As far as volume, you're fine. This is about the same dimensions as a Mega Der Red Max, people fly them all the time with standard ejection charges.

Ari.

What about a shorty-fatty design? Its now 52in tall - top tube removed as well as baffle (replaced with wadding or cloth) - it now weighs 60oz. Still too heavy? I'm kinda stuck with this motor and I really want to launch this weekend.

Actually - I noticed I accidently left 10oz mass object in the nose cone... oops. The original design weight was 61oz and the shorty-fatty design is 50oz.

steven.malone said:

Actually - I noticed I accidently left 10oz mass object in the nose cone... oops. The original design weight was 61oz and the shorty-fatty design is 50oz.

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You shouldn't have any problems with the H100 with that design. You really need to look at the thrust curve of the motor you are using. The H100 has about a 115 Ns of initial thrust and should be able to handle a 5 lb rocket with a 5x thrust to weight ratio.

Handeman said:

You shouldn't have any problems with the H100 with that design. You really need to look at the thrust curve of the motor you are using. The H100 has about a 115 Ns of initial thrust and should be able to handle a 5 lb rocket with a 5x thrust to weight ratio.

Click to expand...

I feel it's really important to understand units. It is probably a simple typo in your post Handeman, but may be confusing to someone just learning this stuff.

Newton (N) is a unit of force, in this case thrust. Average thrust (100 N in this case) and initial thrust (115 N) are forces. Weight is a force (in this case 5 pounds-force ~=22 N). You can compare forces and get their ratio, e.g. 115/22=5.23 peak thrust-to-weight, or 100/22=4.55 average thrust-to-weight.

Newton * Seconds is a unit of impulse. If you measure thrust every second and add how much the motor thrusts in each of these seconds, you get the total impulse (286 Ns in this case). If a motor had constant thrust, you'd get total impulse by multiplying this thrust by burn time. You cannot compare Newtons and Newton-Seconds any more than you can compare acres and acre-feet. If you divide N*s by N you get seconds, rather than a dimensionless ratio. Dividing impulse by force is a valid operation; it answers the question "how long will my motor burn if it produces this much thrust all through the burn."

This was very confusing to me when I was starting out. Maybe it's obvious to Steven, but I want to stress this in case he has the same confusion.

Ari.

steven.malone said:

... I'm kinda stuck with this motor and I really want to launch this weekend.

View attachment 168679

Click to expand...

So the design constraint is the H100 motor.

The ejection charge should be fine in a 4" tube.

As others have said - 48" chute would be better.

In a 4" rocket the H100 should be fine lifting 50-55 oz (that's fully loaded ready-to-fly with the motor installed *real, not simulated* weight)

Between 60-70 oz it might be considered marginal. Over 70-80 oz might be considered unsafe.

If you are going for your L1, you should show up prepared, and be able to demonstrate the CG at least 1 cal ahead of the CP, and know what the thrust to weight ratio of your rocket/motor combination is.

Good Luck!!

Maybe I am missing something here, apparently you have Rocksim. Update the profile with the real weights and run a simulation. What does Rocksim tell you about decent rate when you simulate the flight?
I fly a BSD Thor with similar dimensions, and have not had a problem with motor deployment.

Final Design (Accurate Weight and CGs calculated)


The trial version of RockSim doesn't show anything after apogee. However, the parachute calculates the expected rate of decent which came to 16.7fps with the 36in parachute.

The long design is a bit too heavy - putting the decent at ~20fps.


I'll go with the shorty-fatty design for the cert and buy a larger chute and work on the full sized rocket as a dual deployment with e-bay next time.

I just ran OpenRocket and its estimates of CG/CF varies quite a lot from RockSim... The shorty-fatty design is unstable... however the long design is stable... Also notices the total mass is different...

I'll have to go over the specs again in OpenRocket to make sure they match real-life specs. I'll post the final results of the design after I get off work... Rocket is ready to paint, all components are built and ready to assemble... just have to make a final decision on which design to build. I have several photos of the process and I'll post a final photo of the finished rocket. Launch is this weekend - hope to have some good pictures and video.

Thanks everyone. I think I got enough to go on now and I'm a bit more confident its going to work.

This is going to be the best I can do... Added weight to the nose cone to get better stability. Final weight: 68.4oz. Rate of decent: 17.5m/s. I tried to contact people around where I live for a larger chute - no luck. I read where people have a decent upwards to 20m/s... Hopefully it will make the fall without damage.

steven.malone said:

View attachment 168744

This is going to be the best I can do... Added weight to the nose cone to get better stability. Final weight: 68.4oz. Rate of decent: 17.5m/s. I tried to contact people around where I live for a larger chute - no luck. I read where people have a decent upwards to 20m/s... Hopefully it will make the fall without damage.

Click to expand...

My descent rates are routinely 17-18 fps. I rarely have any damage.

qquake2k said:

My descent rates are routinely 17-18 fps. I rarely have any damage.

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Depending on where you fly :=) This is fine on the grass at Snow Ranch, but I would recommend lower descent on concrete at Moffett or playa at Black Rock :=)

Ari.

iter said:

Depending on where you fly :=) This is fine on the grass at Snow Ranch, but I would recommend lower descent on concrete at Moffett or playa at Black Rock :=)

Ari.

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Good point, Ari. It depends on your field conditions. I've never been to Black Rock or Moffett, just Snow Ranch and SARG. Of course I don't know yet what the conditions are at the new SARG launchsite.

I am now NAR Level 1 High Power Certified





Landing went well - however, drifted about 1.5 miles from launch site. Found it in a cow patty after squeezing through a barbed wire fence!

Well done Steven!

I notice a JawStand pad in the foreground in one of your photos :=)

Ari.

iter said:

Well done Steven!

I notice a JawStand pad in the foreground in one of your photos :=)

Ari.

Click to expand...

So that's what that thing was - ha! Never would have guessed...