MRDC-1 / Inspired by the cover of 'Model Rocket Design and Construction'

[brianc]

The MRDC-1 is a multi-configuration rocket inspired by the cover
of Tim Van Milligan's Model Rocket Design and Construction.

The book cover features a prominant rocket, but unlike some of the
others pictured, this one is not mentioned in the text. I had posted
a question earlier asking if anyone had details or seen one of these
built. I also PM'd Tim, but didn't get a reply.

So, with just the cover illustration in hand, I set off to design
my own incarnation of this rocket!

 

[brianc]

I decided to construct this as a two-stager that could also be
flown as a single stage. Of course, the sustainer could easily
be flown by itself too. Three rockets in one!

The sustainer is a simple 3FNC design using a BT-60 paper towel tube,
an Estes plastic NC and a 18mm MMT.

The Booster is a 24mm MMT with an 18mm ejection transfer tube. All
the centering rings were made from shoe box cardboard. The boattail
and shoulder transitions are 67 lb. cardstock. The outer shell is BT-70.

The staging coupler is a toilet paper tube, resized to fit inside the
BT-60 of the sustainer and cut to length.

The upper stage is 37cm long (~14.5") and the booster is 32cm (~12.5").
If I get ambitious (and there's some interest), I might put together
some dimensioned drawings.

 

[brianc]

The fins are constructed from 1/8" basswood. I drew and photocopied
patterns, which were temporarily spot glued to the wood for cutting.
The grain follows the leading edges of the main fins. The grain runs
lengthwise (parallel to the BT), for the strakes on the booster. This
required building the booster fins in two parts.

Here's a shot of the fin patterns with a dry-fit of the airframe.

 

[brianc]

I don't recall the first time I saw this coupling technique, but it
had to be in the mid-1970's...

The upper section of the ejection transfer tube actually becomes
a portion of the sustainer motor mount, wrapping the bottom of the
motor and hook. A standard staging coupler provides additional
structural strength to hold the upper stage on the booster.

The centering rings are mounted up in the sustainer BT to allow
for the coupler. The motor extends beyond the end of the BT, much
like a Tulanko Tail.

 

[brianc]

Here's a closer view of the transfer tube and upper stage MMT.

 

[brianc]

Here's a look inside the staging coupler. You can get a better
feel for how the pieces go together. About 1/2 of the exposed
motor fits into the transfer tube, while the coupler inserts
inside the sustainer BT.

The key is to use bleeder holes just below bottom edge of the
motor. These holes will vent the cold air from the tube allowing
the hot ejection gases and particles to ignite the upper stage
without pushing the tubes apart.

You can't see the vents on the inner tube, but the coupler holes
are at 5 and 11 o'clock.

 

[brianc]

This is the staging coupler (without the shoulder transition wrap)
holding the sustainer. The 3/16" vent hole is clearly visible.

Vent holes will also need to be drilled into the transition.

 

[brianc]

Here's a look at the business end of the booster, before installing the
boat tail. The green centering ring will be left exposed making a lip
for gluing the boat tail. This will also provide a place for the masking
tape motor retention.

 

[brianc]

And here's the final assembly, ready for sealer and primer! Notice the
launch lugs on both the booster and sustainer. The sustainer lug would
be used when launching the upper stage alone.

Also noticable in this shot is how I offset the fins. Tim's cover illustration
has the fins inline. Offsetting them allows the booster fins to be a bit
more efficient.

I needed to add some nose weight to the sustainer to pass a swing test. Then
had to double it to sucessfully swing with the booster. Surprisingly,
the dry weight for the booster and upper stage came out to be 55g each.
Hopefully the paint won't add much more to this.

 

[Reed Goodwin]

Neato! That looks really awesome. Congratulations.
Reed

 

[MetMan]

brianc,

Neat project. How do you plan to recover the sustainer?

MetMan

 

[brianc]

Originally posted by MetMan
How do you plan to recover the sustainer?

I'm planning to install a tri-fold anchor and some elastic.
I'll do some flight tests to see if a parachute would be necessary,
but I figure a good sized streamer will be enough. A chute
will probably be needed when flying both stages as in the
single stage configuration.

The booster appears to fall horizontally (simple hand tossing
in the back yard) so my original plan for a streamer in the
coupler compartment may not be necessary.

 

[brianc]

Here's a shot of the finished product!

Can't wait to fly!

 

[Leo]

Very nice.

Be interesting to find out how it flew so let us know when the time comes

 

[brianc]

(Sorry no pix, the batteries died & my daughter "borrowed" half
of the rechargables I had ready to go)....

Got some flight tests in today!

First, I flew the sustainer alone on a B6-4. We had a bit of wind
that pushed the rocket downrange slightly, but this was still a great
first flight. One part down.

Next, I taped the booster and sustainer together and flew on a C11-3.
Another nice 'low and slow' test complete.

Finally, I loaded up an A8-3 and a D12-0. The countdown proceeded
and the flame shot out the bottom. The rocket climbed about 12" up
the rod and stuck! but just for a moment. When it released, it
cleared the rod with insufficent velocity to remain stable. The rocket flopped
to the ground and thrashed for a bit. Then the ejection charge lit the A8-3,
the sustainer separated and began it's dirt dance. The NC popped
and the chute draped over the shock cord, hiding in shame. No damage either.

Post flight analysis shows the top lug binding on the rod. This was totally
unexpected, since the previous lift-off was fine. It was on a different
rod however. Speculation is the second rod may have been bent just
enough and/or the wind blew just right to cause the binding. I also did
not clean the rod of debris before loading up, which may have contributed.

Personally, I think there's probably a drop of Future or paint inside the lug.
It'll be corrected.

Despite the dance in the dirt, I feel confident in this design. The first
two flights proved the stability in each configuration. The second stage motor
ignited as it should, so the stuffer tube and vents are working.

Next time, I'mconfident everything will go well. And I promise to recharge the
camera batteries before heading out to the field!

 

[brianc]

Whoo hoo!

Got in a successful 2-stage flight today!

Loaded with a D12-0 and A8-3. The boost was quick and the
sustainer took off to about 1000 feet. Should've been higher, but we
had 10-15 mph winds on the field. The rocket weathercocked some
as it cleared the rod.

The booster dropped off and fell mostly horizontial, just like my
'hand tossing' test in the backyard.

Apogee and ejection occured very close together. I used a 12" TopFlight
nylon chute on the sustainer which carried it down range maybe 75 yards.

Overall a great flight!

I'n goona have to apologize in advance for the poor quality of these
pics. I'm not sure exactly what happened, but I'm sure it can be
attributed to operator error.

This is just clearing the rod.

 

[brianc]

This is just after the sustainer lit. All smoke! But you can see how
it's still weathercocking into the wind.

 

[richalex2010]

On your "dirt dance"

I HATE it when that kind of thing happens - like half of the power on a cluster not lighting so that it gets MAYBE a foot up the rod, the 'chute staing in the tube, and no motor mount when it lands... You know - insignifigant little things

Anyways, it looks pretty cool! Maybe I could make one for BT-50 to BT-5 or BT-20 one, with 18mm to 13mm motors? I think that that might work...
Did you put toothpicks on the ends of the booster fins? I didn't see them, and the drawing shows them...

 

[brianc]

Originally posted by richalex2010
Did you put toothpicks on the ends of the booster fins? I didn't see them, and
the drawing shows them...

No, I haven't added them yet. Still not sure if I will. If I do, I'll probably add
lengths of weed trimmer fillament a'la <a href="https://rocketryforum.com/showthread.php?s=&threadid=10425&perpage=20&pagenumber=4#post133004">the FarScape</a>.

 

[huxley]

it can be attributed to operator error.

Cool project! So did that "operator error" thing get worked out!? I'd love to see more pics!

Pat

 

[brianc]

Originally posted by huxley
Cool project! So did that "operator error" thing get worked out!? I'd love to see more pics!

Yeah, the camera operation is under control.

Unfortunately, the MRDC-1 is on the rebuild stack. On it's last flight, the wind picked
up at just the wrong moment and the sustainer landed on a road. A dirt road. In the
middle of nowhere. An obscure and almost never travelled dirt road. Where, of course, it
was promptly squashed by a passing vehicle.

I have all the parts, I just need to rebuild the MMT, put the fins on a new BT-60 and paint.

 

[JeffS]

A very creative build! You seem to recycle alot of household paper products. That's really cool.

Jeff

 

[kelltym88]

Very nice job. I'll have to try that type of staging one of these days.