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Doug-1B - Saturn inspired L2 project

Above: The rocket is loaded on the pad prior to its maiden flight, and my level 2 cert flight (LDRS, 2006).
 This rocket was my Level 2 project, and is modular.  It shares components with my upscale Avenger project.

It was inspired by and is loosely based on some of the Saturn variants that never got off the drawing board.  Besides the well-known Saturn V and Saturn 1 variants, there were several other designs considered that used a mixture of Saturn components as well as parts from other rockets.  For example, the Saturn A-1 had a Saturn 1 first stage boosting a Titan 1 as the second stage.



Other goals for this design, besides being modular and being my level 2 vehicle, were as follows:
  1. Since it was a cert flight, I wanted it to be fairly draggy to keep it low and slow.
  2. I wanted it to be single deploy using electronics with motor backup.
  3. The electronics bay needed to include provisions for a second, backup device, be it a timer, magnetic apogee detector or a second altimeter.
  4. I wanted it very stable.
Given these goals, I started out with the idea of having a rocket that had one or more transitions to progressively larger diameter sections.  This would make it draggy and would also make it more stable by having much of the lateral area concentrated in the aft end.  In turn, less fin area is needed which allows the fins to be placed in such a way as to maximize their strength and minimize their chances of breaking.


It turns out that eight  29mm LOC motor tubes, interspersed with eight 1/8" thick plywood fins, fit very nicely around a 54mm PML motor tube.  This gives the aft end of the rocket a diameter of 4.68", right at twice the 2.34" forward section.

The central 54mm motor tube would hold an adapter with a 38mm J350.  Later flights could use a 54mm central motor, but the maiden - and cert - flight would be 38mm powered. 

After the Level 2 certification was gained, it later flew with a 38mm I motor and two air-started 29mm F motors in the outboards.  Only one of the outboards lit, but it is so stable, there was no apparent steering - it appeared to keep going straight up with just one outboard burning.



The fin can is built with mostly wood glue.  Only small dabs of epoxy were used to affix the motor retention hardware (threaded inserts).

The central 54mm tube extends forward into the 3" coupler.  Special rings were cut to mate them.  One benefit of this setup is that more than 1 caliber of coupler inserts into the next section which helps guard against the rocket folding up at that failure point.

The u-bolt is joined to the recovery system.  By separating here, and having the fin can pull the chute out of the next forward section, the chances of a zipper are minimized.  Hence the term zipperless construction.


Here, the rocket is loaded on the pad for its second flight, with the two 29mm motors in the outboards, and yours truly grimacing in the sun

The nosecone got dinged up a little on the cert flight, so I repainted it, this time red.

It is supposed to be conical and thus representative of the Apollo capsule, but I wasn't very exacting when I turned it on the lathe, so it's not very Apollo-like

The electronic ejection charges go in the base of the forward (silver) transition, firing aft.  For the two flights thus far, only one charge was used with the motor providing the backup charge. 

The chute and recovery harness reside in the middle, 3" diameter tube. 

Since the motor charge comes from the fin can, aft of the chute section, the result is that charges are firing into the chute bay from both ends - heat protection must be packed into both ends of the middle section. 

Plus, the recovery harness is rigged to drop the forward section first thus letting the fin can hit last, descending more slowly by itself the last few feet, helping to guard against a fin break which could disqualify the flight.  (This explains why the nosecone got dinged on the first flight )  Doing it this way requires much more shock cord.  Thus, between double chute protectors and an extra long shock cord, things are very tight in the chute compartment.  I must have repacked it 4 or 5 times prior to the cert flight, trying to get everything packed in there without jamming anything that might prevent a successful deployment and recovery.


In the pic at right, the rocket lifts off on its second flight (Sept 23, 2007).  The outboards have not yet been ignited.  As I recall, the main motor was an Aerotech I161.

The electronics all fit in the forward, 2.34" diameter tube.  Vent holes are positioned ahead of the transition but well aft of the nosecone to give minimal disturbance to the barometric altimeter.

While only one of the two outboards lit on this flight, it was still considered a success.  It was the first time I'd ever air-started a motor, and the rocket was recovered in excellent condition, except for the nosecone, which as since been painted black

Somewhere, there's a launch pic from my L2 flight, but I never was able to find it on the websites of the photographers who were at that LDRS.  But this one is pretty kewl

8-26-2010


After some web searching, I found a liftoff shot of my L2 flight at LDRS25.  It's from a distance so it's a little blurry.  But you can definitely see the flame from the J350.  It looks like it still has 2 feet of rail to climb before it's clear. 



9-1-2010


38 to 54mm motor adaptor and motor retention
The Doug-1B features a 54mm central motor tube surrounded by eight 29mm tubes.  A 38 to 54mm adaptor was built to allow using my set of Aerotech 38mm motor cases.  The adaptor was built using a 38mm LOC motor tube and some custom cut rings from BMS.

After installing the two centering rings, I added the aft (left below) ring which is a little oversize and acts as the stop for the adaptor.  All the rings were made with light ply. 

After getting the rings installed, I began worrying if the stop ring was sturdy enough.  The last thing anyone wants is to see their motor blast thru the rocket and cause a wreck.  It may make for some exciting entertainment for the spectators, but it's not pleasant for the rocket's owner  

So I added some fiberglass around the tube as a fillet of sorts, trying to improve the bond between the tube and ring.  Still not satisfied, I drilled several holes and connected the two rings with dowels.  At that point, I was satisfied the assembly wouldn't come apart under thrust from the motor. 

If I got to do it again, I'd simply stack the two rings together moving the one ring aft against the stop ring. 



The motor retention scheme for this rocket begins with two threaded inserts mounted in the voids between the 29mm outboard tubes.

These were fabricated as follows.  I mounted the inserts in some good quality plywood, securing them with a little epoxy.  Then, I cut the wood into small triangles somewhat larger than the voids.  Next, I carefully milled them to size using first my belt sander and then hand sanding with a 29mm tube wrapped with medium grit paper, all while repeateldly test fitting them until I was satisified I had a good fit.  They were then epoxied into place taking care to keep any epoxy from getting into the threads.


Here's the adaptor installed in the rocket along with a 38mm motor.  Notches have been cut into the stop ring to clear the two threaded inserts.  Further cuts were made to clear three of the 29mm tubes.  If I ever fly this rocket with more outboards, I'll add more cuts as needed.



The motor retainer is a piece of thin brass plate from the hobby store (or hardware store).  I cut a large hole to clear the motor exhaust and drilled two holes for screws.



Cap screws are used to secure the retainer.  One nice thing about them is that the allen wrench doesn't slip out easily.  I much prefer that over using a screw driver.  A nut on the screw functions as an ersatz washer with the intent of making it difficult for the hole to be stretched over the screw head.


The picture shows more clearly the bends in the brass to contour it around the motor's end and down to the adaptor.  A key point is that this retainer secures both the motor and the motor adaptor.


The ends of the plate are folded over thus making it double thick at the ends.  Again, this is to guard against the holes tearing out.




9-19-2010