Brief:

My "Scale" project is going to be the Hermes A-3A Missile. Most people have never seen this rocket which makes it a perfect candidate for this particular contest. The original Hermes A-3 project called for a liquid-fueled rocket-powered surface-to-surface missile that could carry a 1000lb warhead to 150 miles with an accuracy of 200ft. The Hermes A-3 also had a top speed of Mach 3.1. Only eight of these missiles were ever built and tested so you probably won’t find one in a museum. To date I have only seen two pictures of this rocket but hope to have more before the end of this build. I have requested information on this missile from both the Smithsonian National Air and Space Museum as well as the White Sands Missile Range Museum. A friend of mine built one of these a few years back in all carbon fiber and flew it at a BALLS launch. I believe the body diameter of his was around 10 inches which would have made it a quarter scale version. On its second launch it had some recovery problems and subsequently crashed. I don’t know what the current state of it is.

Construction:

The scale of this project was based on what I had laying around. I had a Scotglas 5:1 54mm ogive nosecone, a 3 inch long piece of 54mm filament would G10 body tube, a PML 4:1 54mm ogive nosecone, a 54mm to 29mm centering ring,, some PML 29mm motor tube, and some .062" thick G10. Viola! The scale of this project is roughly 17.58 to 1.

The first thing I did was get the original dimensions off of the following web site: http://www.designation-systems.net/dusrm/app1/ssm-a-16.html. I then printed out the picture of the Hermes A-3A and used a ruler and a scale factor determined by measuring the diameter and overall height in order to fill in the dimensions that were not provided on the previous web page. After getting all of the necessary dimensions I entered them into a drawing program that I have called, FASTCAD. Once the basic structure was laid out I then scaled down the drawing so that the outside body dimension was 2.275 inches or the same as the G10 filament wound body tube that I was going to use.

I then took all of the components I had and entered them in to the RockSim software from Apogee Components to calculate the center of pressure. From this I was also able to figure about how much weight I was going to need to add to the nose cone to ensure a stable flight. I based the additional nose weight on the largest motor that I would put in this which would be a small 29mm H. The additional nose weight came out to about 120 grams.

I first cut the fins out of the .062 inch thick G10 plate to the dimensions I had. This was done with a simple hack saw. I then took all four fins and held them in a vice and sanded them as one unit so that they would all be the same size.

I then took the PML 54mm ogive nosecone which would be the tailcone section and cut it down so that it would match the dimensions that I had determined to be of scale. I ended up making it a little over 6 inches in exposed length with a shoulder length of 0.8 inches. After sanding down the edges I took a Dremel and sanded the inside of the tailcone so that the 29mm motor tube could easily slide in.

Next, I cut the slits in the tailcone for the fins. I again used the Dremel tool with a cutting disc. I generally do all of my fin slots this way. Getting the slots to be equally spaced and straight was a little more challenging since it was on a boat tail and you can’t just use a door jam or a piece of angle to draw the lines. What I ended up doing to solve this problem was cut two of the slots along the seams of the tail cone. These were easily visible and pretty straight. I then measured and marked equal distances between the two slots that I already cut out and drew the lines to get the additional fin positions.

I then cut down the 29mm motor tube to a length of 7 inches so that when the motor tube was flush with the bottom of the tail cone and the centering ring was placed on the top of the shoulder section, everything would be flush at both ends.

I then took the body tube and set it on top of the shoulder and the centering ring so that it would be held perfectly in the center. I then proceeded to epoxy each of the fins in place. Using a piece of paper that I had printed out that had a few concentric circle and four lines representing the positioning of the fins I was able to ensure that each fin was indeed straight and 90 degrees apart.

Once the fins were secure I removed the body tube and centering ring. I then applied epoxy fillets to the inside of the tube to better secure the fins. I also attached a ¼" wide by 24" long piece of Kevlar® to the outside of the motor tube using some two-part, five minute epoxy. The Kevlar® will be the bottom part of the shock cord. The Kevlar® is also fire resistant and you don’t have to worry about it breaking. I plan to use either tubular nylon or elastic for the top part of the shock cord. I then filled the rest of the tail cone with some two-part expanding foam.

Next, I cut a notch in the centering ring so that the centering ring would slide more easily arond the motor tube. Once the foam had cured I used the two-part, five-minute epoxy to secure the centering ring on to the tail cone section. I then used the 5-minute two-part epoxy to secure the 3 inch long body tube to the tail cone section.

I also added a fillet of some 3M Scotch-Weld 1838 B/A green two-part epoxy to the fins where they meet the tail cone. I used this specific epoxy here since it has very good bonding properties and works very good when attaching G10 to plastic.

As you can see there isn’t a lot of room for the recovery components if I were to do a standard bulkhead in the nose cone so I decided to use as much of the inside of the nosecone to hold the parachute and recovery straps as I could. I accomplished this by placing a small bulkhead with an attached u-bolt up near the top of the nosecone. The bulkhead was placed approximately 3.25 inches down from the tip. Due to the design of this missile and its center of pressure location I had to add a considerable amount of nose weight, approximately 120 grams. For this I used some lead shot in a mix of epoxy and secured everything all at one time. Placing the bulkhead at the front of the nosecone does two things, it allows for a lot of space for the recovery system and also helps out with the balance of the rocket by moving as much weight forward as possible.

Finishing:

Now it’s ready for some body work and ultimately some paint. I am still waiting to figure out which paint scheme I am going to use but the basic scheme will be white with checkered black on the nose cone and the fins. I may add a specific round number to the tail fins if I am able to acquire good enough photos. Right now I have a photo for round number 4.

The Hermes A-3A has finally received its first coat of primer as well as some red putty body filler to get rid of the body seams. It’s ready to fly right now but I generally have my rockets painted before they fly.

I build all of my rockets to fly off of rails so I ended up purchasing some surface mount aluminum rail guides from Giant Leap rocketry. I have a limited amount of space to secure these so I just decided to go with one. A single rail guide is about 2 inches long so I think it will keep everything pretty straight and should stay on.

Flight:

The first motor I used was a single use Roadrunner F45-8. The flight was just perfect. The delay went right at apogee and the rocket flew straight as an arrow. I ended up using an Estes nylon parachute that now accompanies such kits as the D-Region Tomahawk and the Interceptor E. It brought the Hermes down very nicely and there was no damage.

The second flight was also nearly perfect and the Hermes was catapulted into the air using a Roadrunner F60-10. This time the delay was a little long but still a great flight. There was no sign of any damage.

The last flight I did was with the Roadrunner G80-10 and like the F60-10 the delay was a little long. Out of the three motors that I have flown this rocket on I like the F60-10 the most. It seems to be the best bang for the buck so to speak. Once again there was no damage and it was another spectacular flight

Recovery:

For recovery I used a new Estes Nylon chute out of an Estes D-Region Tomahawk that I upgraded to a 29mm motor mount. The Tomahawk ended up being a little heavy for this particular parachute. The Hermes rocket is a perfect weight for this parachute and with three flights seems to work great.

Summary:

Overall this was another fun project to design, build and fly. It is nice to show people a rocket they may have never seen or ever heard of. I don’t think I would change any part of the design.

Overall PROs:

• Rugged design!
• Futuristic look!
• Simple construction.

Overall CONs:

• No motor retension (Difficult to do in a boat tail design)
• A little heavy due to its shape.
• Time consuming to mark and cut out boat tail for fins.
• A little pricey with the fiberglass nose cone and filament wound G10 body tube.