In November 2015, MAC Performance introduced their kit of the ever-popular Arcas sounding rocket. However, without the boat tail, it looked more like sustainer of a 2-stage version which gave me the idea of using it as a basis for modeling the "Boosted Arcas."
The prototype was 4.5" O.D. and I decided to model this using 3" tubing (3.15" OD), making this a 70% scale model.
I didn't end up using the MAC Performance Arcas kit since I already had the parts, but their kit is very close and would make a good option for the sustainer. I did decide to build the sustainer with their 3" canvas phenolic tubing and the interstage with their 3/16" phenolic plate.
After selection of the body tube size (standard 3.0" tubing), the next job was to make a dimensioned drawing of the rocket to be built. This turned out to be harder than just scaling dimensions from a complete set of plans because I couldn't find one.
Eventually I found drawing with most dimensions marked and was able to take a shot at the others by scaling them from the drawing itself (see the Historical Info section below) and produced my overall drawing.
Once again the interstage was an open truss structure. (I seem to keep making these lately, first for the Aerobee and then for the Iris). This time I decided to use phenolic for the plates, rather than aluminum, since I was building use MAC Performance canvas phenolic tubing.
There are two main parts of this rocket: the sustainer and the booster + interstage. The sustainer is straight-forward, except for arrangements for sustainer motor ignition and coupling to the interstage. The booster is also straight-forward, making the interstage the most interesting part.
The prototype interstage is quite long for the diameter. My 70% model interstage is 6¾" overall with 6" rods plus an aft and forward end plate. Each plate is made from two 3/16" thick canvas phenolic plate cut differently and then bonded together.
Above you can see the five rings cut out. (The smaller-diameter ring on the bottom left fits into the electronics bay in the booster, so doesn't add to the interstage length.) The top row is the forward end and the lower row is the aft end.
In order to keep them aligned during glue-up, I drilled 1/8" holes in all pieces, so a dowel could keep them in registration. Above you can see the two assemblies curing.
In this rocket, the struts are arranged between the fins rather than in line with them, so I decided to use the threaded rod ends as pins for coupling to the sustainer aft end. To make this worked, I used #8 threaded rod and turned down the ends to 1/8" diameter pins. (I don't have a metal lathe so I recruited a friend to help; thanks Tom Rouse!)
Above left you can see one of the rods in the lathe chuck and above right you can see the four rods with ¼" of the end turned down and slightly chamfered. #8 threaded rod is marginal for clean 1/8" diameter pins, but I wanted to use ¼" tubing and these rods are a good fit inside it.
These pins socket into the aft end of the sustainer and still can be bolted at both ends to hold the interstage together.
The booster is just a short rocket. The only real complication is that the prototype narrowed in the middle, so to simulate that I used a 3" coupler tube full length with sections of body tube at the ends.
The sustainer is all MAC Performance canvas phenolic, but I needed a full-length coupler so I used some Giant Leap phenolic I had laying around. (See my Body Tubes article for more info.)
Above you can see the airframe parts for the booster. The coupler has a short piece of airframe at the forward end and a longer piece for the fin can. The MMT is a 10" long 38mm tube, leaving adequate room for recovery.
Of course, since the body of the booster is a coupler, I needed a coupler to fit inside a coupler for the avionics bay at the forward end. I made a video on how to Make a Coupler to demonstrate the process.
Above you can see the coupler section and the small avionics bay to accomodate a Missile Works RRC2+. This is the smallest unit I had on hand and I wanted to keep the bay short to leave room for the recovery system.
This avionics bay is actually part of the interstage, which acts as the "nose cone" for the booster. It won't go very high and doesn't weigh much so single deployment and a small 'chute will be sufficient.
The sustainer is a straight-forward rocket rocket. Note that the Boosted Arcas didn't have a boat tail, presumably to better couple with the interstage. So there isn't even the normal Arcas aft-end complexity.
The information below comes from NASA Contractor Report CR1529 Part 2, "A study of 30km to 200km meteorological rocket sounding systems." This was particularly interesting because it puts the different sounding rockets into context:
The key finding was the drawing on page 312 of the report (328 of the PDF). Also interesting were the specs before and after the drawing.
Above left is a photo of the Boosted Arcas in flight and above right is the dimension drawing from the NASA report.
There are various versions of the boosted Arcas in limited use today as indicatd in table 6-20. The Arcas is used as a second stage in this configurations and the payload capacity is as follows:
|Payload Weight||10-30 lb|
|Payload Diameter||4.25 in|
|Payload Lenght, Nominal||26 in|
|Payload Volume, Nominal||305 in|
The performance of the boosted Arcas vehicles depends upon the booster used. A brief description of each of the boosted Arcas configurations is presented in paragraphs which follow.
The booster is an Atlantic Research 0,8-KS-2700 rocket motor. Power for ignition of both stages is provided by a ground source, a motion switch firing the sustainer igniter after positive ignition. An optional sustainer igniter incorporating a 2-second delay is also available. Fin assemblies are preset to provide a roll rate of approximately 25 rps at burnout.
The vehicle is launched from the standard ARCAS rocket launcher with the breech door open. Assembly and launch preparation requires no special handling equipment and can be accomplished by a two-man crew.
|Nominal performance rating||0.8-KS-2700|
|Principal diameter||10.2 cm (4.0 in)|
|Overall length||71.6 cm (28.2 in)|
|Recommend firing current||3.5-5 amp|
|MARC 2B1 ARCAS|
|Nominal performance rating||29-KS-324|
|Principal Diameter||11.4 cm (4.5 in)|
|Overall length||154.2 cm (60.7 in)|
|Recommended firing current||5 amp|
Weights (less payload)
|Gross launch weight||46.3 kg (102.0 lb)|
|First-stage burnout||41.3 kg (91.0 lb)|
|Second-stage ignition||30.1 kg (66.4 lb)|
|Second-stage burnout||10.6 kg (23.3 lb)|
The entire report can be downloaded from the NASA archive: NASA Contractor Report CR1529 Part 2.
Marcus Harmon scanned a double-page drawing from Small Sounding Rockets (now out of print), which has some more detail (although for a different arrangement than I'm planning to model).
This gives a better view of the booster, showing the changes in diameter and that there is a small cone on the forward end.