"LowBotomy" --- a MPR Parkflyer

[ckreef]

For this project I had a few goals in mind:

1) Low altitude MPR capable of launching in local ball parks (altitude range of 250' - 750')
2) Based on the low cost AT 24/40 E & F reloads
3) Stable, sturdy and easy to use (multiple back to back launches)

Just 3 simple goals but you would be surprised how difficult those goals were to design. After many, many hours in OpenRocket, checking the web vendors for parts and with a few tricks up my sleeve I think I have it figured out ........ so ........ let the build begin

Pictured below is the basic OpenRocket screen shot and the basic parts. There are a few tweaks I still need to make to the design and a few parts I still need to buy but I have enough to start the build for now.

...................... more to come

 

[Chuck]

Looks cool! I like your fin shape.

Good Luck!

~Chuck

 

[GrossApproximator]

Lovin' it. I'm going to be watching this one.

 

[ckreef]

I work 11 hours a day so I find the only way to get a project done is if I commit to doing a little something every single night and before I know it the project is complete.

This is my little something for tonight.

MMT sanded and ready to accept epoxy.
Forward centering ring epoxied in place and drying.
Aft end sanded and ready to receive the AeroPack 24mm retainer.

 

[Machet_Man]

Looks like a great solid design to me! Lord I love OpenRocket! I love the shorter design of this particular rocket and like Chuck said the shape of the fins are really cool. Can't wait to see the completed bird!

 

[ckreef]

I appreciate your comments on the fin shape. In regards to the fin shape:

I like the looks of longer sleeker fins.
The back scallop design I copied off some rocket I saw while cruzin the web because I thought it looked good.
The back angle I tried to put forward enough so even at a moderate angle of impact the bottom of the BT would hit first.
I squared off the top tip because pointy tips have less area and break easier compared to being squared off.

This is all trying to keep with the goal of multiple back to back launches.

The actual fin dimensions had a lot to do with stability compared to altitude on various motor simulations. Will post the simulation data and actual simm file as I come closer to completion.

 

[ckreef]

Got off work early today so ...................

I double checked my measurements especially the motor mount area then cut the coupler into 3 usable sections.

7/8" for BT bottom stiffener.
2 3/16" for nose cone shoulder.
2 15/16" for baffle assembly.



The NC came with these funky ridges that I really didn't like so I cut the bottom ridge off completely. I then did some sanding until I could get the couple piece to slide on it. I would have liked it to mate all the way to the front but because of the way they did the NC mold you couldn't sand down the forward ridge. It should work the way it is. The nose cone assembly isn't finished but wanted to get some work done on it and give it a test fit.

 

[Spooks]

Looks like a cool project!!

Watchin this one!!

 

[ckreef]

Note: Some of these build techniques I would NOT use if I was building a High Altitude, High Performance rocket, but ...... since I am building a Low Altitude, Low Performance rocket ...... weight is my friend

Pictured below is the ejection baffle assembly. I used 3/4" holes so I can easily shake out the ejection cap after recovery. The holes will be mounted offset/opposing so the ejection gasses/particles have to "S"nake their way through the assembly. I epoxied the nut onto the eye bolt. We don't need that coming loose over time.

Please Comment: If 3/4" holes is not big enough to properly pass the ejection pressure up to the nose cone w/ a 3" BT, let me know now. It will be much easier to make those holes bigger before I epoxy the assembly into the BT.

 

[MG13]

Since I do most of my launches in the park in front of my house, I'm most interested to see how this pans out.

 

[Handeman]

Please Comment: If 3/4" holes is not big enough to properly pass the ejection pressure up to the nose cone w/ a 3" BT, let me know now. It will be much easier to make those holes bigger before I epoxy the assembly into the BT.

3/4 should be fine, but since this isn't a structural component, there isn't anything lost by making them bigger and it won't hurt anything. I wouldn't move them any closer to the center of the CRs, but widening toward the outer wall would work.

 

[ckreef]

Good deal on the 3/4" should be fine as I have now installed the assembly.

I used a Albright knot to join the kevlar to the nylon w/ some heat shrink to finish it off. I attached the kevlar to the Baffle eye bolt w/ a slightly modified Butline Hitch and heat shrink.

I measured for the baffle location at 12". I have an extra 1 - 2 inches up front as I might go for an Altimeter 2 in the BT up by the nose cone ......
I then cut a scrap piece of 2" PVC x 12" L. and put this on the floor, Ejection Assembly on top w/ the Eye Bolt/shock cord facing down ......
I spread some epoxy in the appropriate location inside the BT using a small disposable paint brush w/ a metal rod taped to it to extend it's length ......
I then pushed the BT over this arraignment. Once the BT went down to the floor the Baffle assembly was exactly 12" inside the BT. Quick, easy and accurate ......
I then used the extended brush from both the forward and aft ends to add some more epoxy.

 

[ckreef]

The heart of my LowBotomy project will be the ability to configure the rocket at launch prep for a desired altitude. Since altitude is mainly a thrust to weight issue I knew I needed a system for easily changing out the nose cone weight. I have a plan forming in my head on how to accomplish this so latter today I'm off to the store to see what parts I can round up.


My thought process behind this:

More weight = greater stability number = lower altitude for a given motor. I did need to make sure I maintained an acceptable velocity off the launch rod and at ground impact. I needed to maintain this throughout the weight range I might use in the nose cone. As I ran all the different simulations my lowest velocity off the launch rod comes out to 30.4 ft/s (fly on a calm day). The highest ground hit velocity 24 ft/s (a little high).

Pictured below is a chart I made from my simulation data to help illustrate this. I used the data that was closest to 50' increments from the highest simulated altitude down to the lowest.

 

[cwbullet]

Like the design. Nice baffle. You can put a steel pot scrubber in the baffle to help cool the gas more.

 

[ckreef]

My shopping excursion was successful

Pictured below is all the parts needed for the configurable weight system including 8 weight pellets at 3/4 oz each. Stay tuned. Chicken is on the grill but after dinner I hope to get this assembly completed.

 

[ckreef]

I got the configurable weight system built and installed.

The weight system guts are built out of 1/2" PVC-1120 (thin walled) and cut down connectors. The PVC connections were solvent welded together and also epoxied w/ JB Weld. The main weight tube is a single tube that goes up through the bulkhead and into the threaded connector w/ a "dummy" coupler to aide in securing the assembly to bulkhead.

It was installed into the nose cone with the modified shoulder.

You determine how many weight pellets you want to use (3/4 oz each). Unscrew the weight tube cap and load the pellets. Add the appropriate spacer and then tighten the cap back on. Pictured is the spacers and some pellets to illustrate how the system works.

I have some new simulation data using this weight system and will post asap.

 

[ckreef]

As expected when I added the configurable weight system the basic rocket weight went up and the altitude went down. I knew this was going to happen but that's OK as I am building a Low Altitude Parkflyer.

As I add parts or complete assemblies I weight them on a digital scale and then add their actual weight to the simm so I get the most accurate simulation possible. After installing the configurable weight system and updating the simulation I ran a few basic numbers and this is what I came up with. I also changed the parachute size from a 24" w/ 6 lines to a 30" with 8 lines (will have to order that before maiden).

 

[etoyoc]

I had assumed that you would be putting more nose weight with the F motors and not the other way around.

What is the total mass of the rocket? Are you pushing the 5:1 stability guidelines with the full set of weights in the E18?

Where did your 3/4 oz weights come from?

 

[ckreef]

With that set of numbers I was looking at worst and best case scenarios for those 3 motors. The weights are pellet sinkers purchased at Wal-Mart.

I was good w/ everything until the extra mass for the weight holding system came into play. They say hind sight is 20/20 but it can also be a witch w/ a capital B :eyepop: I could have done things a little different but at this point it's glued up so I'll just push forward and maybe rebuild the NC in the future.

Yes E18 is no longer a valid choice. This morning I put the entire rocket on the scale along w/ the fin wood and a little extra to account for paint etc... I wanted to get as accurate of a simulation as possible. This new set of numbers represent flights from 250' - 450' in 50' increments. These flights seem reasonable although some of the delays could be better.

 

[etoyoc]

I do like the idea of the adjustable weight system. I wonder how using something as simple as a flip top tube (like M&M minis) and dog barf would work with fishing weights. Open the tube, put the correct amount of weight in and stuff with dog barf then shut.
I suppose part of the issue would be the tube popping open and dropping weights from above, but a retainer could be added without much weight.

 

[ckreef]

I suppose part of the issue would be the tube popping open and dropping weights from above, but a retainer could be added without much weight.

That was definitely my concern as 3/4 oz balls falling from the sky would not be a good thing. :y:

I will try and come up with a different weight holding system. If I can come up with something I really like I'll rebuild the NC. Was thinking maybe a 29mm body tube with the Estes screw on retainer "capped off".

 

[etoyoc]

That was definitely my concern as 3/4 oz balls falling from the sky would not be a good thing. :y:

I will try and come up with a different weight holding system. If I can come up with something I really like I'll rebuild the NC. Was thinking maybe a 29mm body tube with the Estes screw on retainer "capped off".

If you cap it with something solid enough, you could still put an eye bolt for the recovery setup there.

 

[ckreef]

Not to let a crappy NC deter me , lets move on. (I'm forming an idea for a rebuild )

Taking Handeman's advice I drilled a few more "pressure relief" holes in both the forward and aft end of the baffle while I still could.

I epoxied the forward end of the MMT assembly in place using the aft centering ring to keep it alined properly. Pictured is the aft centering ring w/ some masking tape "pull tabs" so I can remove it to install some fillets on the fins/MMT connection.

I then cut and sanded my fins.

Using my Fin Jig I installed the 1st fin. Waiting for the epoxy to set up.

 

[ckreef]

The 4th fin in the jig waiting for the epoxy to setup. The fin jig worked really well and the fin alignment looks spot on.

 

[ckreef]

With the epoxy basically setup on the 4th fin I figured I would give everything a test fit to see what it looks like. My initial observation is that the fins look a little tall to me. I am planning on cutting off 1/2" from the height of the fins. The simm really doesn't change much, maybe 8' or so more altitude. I like long, sleek fins, it should look better.

I have another partial day at work tomorrow so ........... If anybody has any thoughts on this (good or bad) please let me know as I should be cutting and sanding by 9:00am or 10:00am tomorrow morning. I would have preferred to do this on my miter saw before the fins were installed but I will make do.

 

[ckreef]

The Business end all "Buttoned Up."

Internal fin fillets done.
Aft centering ring installed.
BT bottom stiffener installed.
Motor retainer installed.

I then went and cut and sanded the fins down to 2 1/4" high. This pic is a comparison of the old fins to the new fins. The 2 pics are not the exact same size resolution but easy to see the difference. I like long sleek fins so I like the new version better.

Getting close to finishing :grin:

 

[bill_s]

Fins don't large at all to me. Edit: too late, the shape is better though.

You may need to look at delay time adjustment for this due to the deployment velocities. I have no idea why AT doesn't have a shorter delay version on the F39 since otherwise that's the one for larger rockets. Also paint adds a lot of weight, several ounces. The weight system is nice but may not be needed. If those rod velocities are at 3' (I can't run Open Rocket) might want to consider just using a longer rod to allow the F24-4W etc. I've had success with launches at 30 fps and below with a finny, weight in ends rocket but will need more experience to say if this one will do well then. Alignment of the thrust and masses is also a big factor. Also could look at the SU E30 and F32-4 if there are trees around to lose even more in.

 

[ckreef]

You may need to look at delay time adjustment for this due to the deployment velocities. I have no idea why AT doesn't have a shorter delay version on the F39 since otherwise that's the one for larger rockets. Also paint adds a lot of weight, several ounces. The weight system is nice but may not be needed. If those rod velocities are at 3' (I can't run Open Rocket) might want to consider just using a longer rod to allow the F24-4W etc. I've had success with launches at 30 fps and below with a finny, weight in ends rocket but will need more experience to say if this one will do well then.

Yes I agree the F39-6 really needs the delay cut down to a F39-4. I have already been looking into that and will probably drill out 2 seconds from the delay.

The weight system in concept I like but I mucked up the execution. Either way if I want to fly a E28-4 to only 250' I need to be hauling some weight. Right now to get only 250' on the E28-4 the velocity off rod is 32.5 ft/s. A little on the low side but I figured it is doable. That is with 6 weight pallets. Once painted I just need to use less weight pellets to achieve that goal. (I guess that is the beauty of a configurable weight system )

 

[MasonH]

This is a very well thought out build!!

 

[ckreef]

It seems the only logical 24/40 RMS to maiden LowBotomy on would be a F39. So tonight I ordered some F39 reloads, and a Altimeter 2 (to confirm the OpenRocket data). I also got locally some DupliColor paint. I should be able to maiden her the weekend of the 23rd-24th (weather permitting).

Depending on how she flies I will slowly tweek the launches both up and down and if stable enough go down to a E28. Actual flights will tell.

A few finishing pics to follow along with flight report and data (pics and video too).