Another Boy Scout rocket...

I just finished the Estes "Space Exploration" Boy Scout rocket, which I bought in a bulk pack from the Boy Scout store (Scout Shop) many moons ago. It was a great deal - 6 rockets for $30, including shipping; unfortunately, this product has been discontinued, with the only offerings being standard Estes fare. The reason is pretty clear - Estes has stopped producing the Ricochet, and the Space Exploration rocket was nothing more than a Ricochet with different stickers. So it went bye-bye with the Ricochet.

The kit parts are good Estes quality - laser-cut fins, nicely fitting plastic adapter and nose cone, and decently strong body tubes. I would rate this build a definite skill level 2, as you have to line up the three components making up the fin. This is more tedious than hard, but it does require that you pay some attention to what you are doing. Finishing was straight forward - 4 coats of Brodak sanding sealer on the fins, then a couple of coats of primer followed by the paint. I screwed up the paint scheme by relying on my failing memory, for if I had bothered to take another look at the box art, I would have seen that one fin was red and the others blue. My memory had all four fins blue, and that's how she was painted. The stickers went on easily, but appear shiny against the paint - you can tell there are stickers on the model. Why can't Estes just give us waterslide decals, which look so much better? I know - stickers are easier for young ones to handle, but they all too often look crappy to us old geezers.

Boy Scout "Space Exploration" rocket (Click to enlarge).

Anyway, the rocket turned out OK, as you can tell from the pic. A bit of trivia for you non-Scouts - the circular logo beneath the American flag is a replica of the Boy Scout Space Exploration merit badge. I thought it was kinda neat that they managed to incorporate that into the kit.

Now it's back to assembly line - I have to apply the second primer coat to the lower part of the Artemis (my 2nd Geezer TARC rocket), build an egglofter for the NRC Eggloft Duration event, and finish assembling my Apogee tower. Gonna be a busy weekend...

Time to do some TARC math!

Now that the Reliant is finished, it's time to figure out the size of the parachutes needed to land the booster and payload sections safely. To do so, you need to measure the weights of both these sections, which in my case are 178 and 318 grams, respectively. The booster is easy, as one can choose the descent rate and size the parachute accordingly. A "safe" descent rate - one that avoids damage to the fins - is usually around 15 feet per second, so we go to one of several handy-dandy parachute descent rate calculators available on the Internet - I use either Model Rocket Parachute Descent Rate Calculator or Descent Rate Calculator - Rocket Reviews or an app I have on my iPhone. All 3 give similar results, so the choice does not really matter.

But, wait... Why can't we just use Open Rocket, as that's the design program? It gives descent rates, doesn't it?

Yes, but Open Rocket cannot handle the case where the payload section comes down separately from the other section. So what it will give you is a descent rate based on the sum of the areas of all your parachutes and the total weight of the rocket. That's not going to work this year. So we are going to have to do things outside of the simulator. If you use Rocksim, you can trick it into handling this year's TARC circumstances by creating a 2 stage rocket and designating the payload section as a motorless upper stage; you can then attach a recovery device to each stage and set the deployment circumstances. However, most folks don't have Rocksim.

Now I'm ready to do the compute the booster chute size, so I bring up the RocketReviews calculator and put in the booster weight. I prefer to use Top Flight thin mil rip stop nylon parachutes, which are hexagonal in shape, so I choose "hexagonal" from the Parachute Shape drop down menu. Just for grins, I put in the altitude goal of 856 feet, and finally make a guess at the right parachute size before I click the "Submit" button:

Click to enlarge.

It appears that my initial guess of 15 inches was too small - the calculator is recommending a parachute size of 18 inches. Recalculating with this value yields

Click to enlarge.

This is pretty close to my desired descent rate of 15 feet per second, so I'll run with it. My booster parachute will be an 18" Top Flight, bright orange in color.

Now for the harder part - the two identically-sized parachutes attached to the payload section carrying the eggs and altimeter. To calculate this, we must first figure out the required descent rate to land the payload section 43 to 46 seconds after liftoff, which means we have to go to our simulations to get the time of ejection, and subtract that from the duration goal to get the length of time the parachute is deployed. Open Rocket tells me that it will take about 7 seconds to reach 856 feet, so I will subtract that value from the midpoint of the duration goal - 44.5 seconds - to get the parachute time. The descent rate - in feet per second - is simply the altitude (856 feet) divided by the parachute time (37.5 seconds), or

So we need 2 parachutes that will lower the payload section at 22.8 feet per second. Now I go back to the descent rate calculator and put in the weight of the payload section, keeping everything else the same:

Click to enlarge.

Turns out that 18" is pretty close, with a descent rate of 22.6 feet per second. If I fiddle a bit, I end up with a parachute size of 17.8".

However, I need two parachutes, not one, so how do I do that? What I need to do is divide the area of the above by 2 to get the area of each identical chute. Since area goes as the square of the diameter, this means I must divide the diameter by the square root of 2 to get the size of my chutes. Doing so gives me a diameter of 12.6". I can check my work by doing a comparison:

which is close enough for government work.

Trouble is, there are no 12.6" commercially available rip stop nylon parachutes. The closest match is the 12" diameter. So what happens if I use that? Well, 2 12" parachutes have a scaled area (I'm leaving out the factor of pi) of 288, which translates to a diameter of square root of 288, or 17.0". I can now plug this value in into the descent rate calculator, which gives:

Click to enlarge.

This shaves about 1 second off the time, but it is still within the allowed duration range. Quite acceptable, especially given that I really don't want to have to learn to make nylon parachutes. Therefore, Reliant's booster will be recovered by an 18" parachute and the payload section will be brought down by two 12" chutes.

This concludes today's TARC exercise.

3 done.....

The wet summer continues, but I have managed to find brief periods of lower humidity (between 45 and 50%) to get some final painting done. First up is the clone of the Centuri Stellar Starlifter, painted bright blue and white - the "challenging" decor mentioned in the instructions:

Centuri Stellar Starlifter (Click to enlarge).

The only thing challenging about this build - a snap with Semroc/eRockets laser-cut fins - was the application of the blue Testors enamel. It blistered, necessitating a major sanding and re-coat. That one applied OK, but it reminded me how much I hate dealing with enamel paints. I have very poor luck with them. The orange horizontal stripe decals also turned out to be a bit translucent when applied to the blue body tube, which turned the orange into a brown hue. I'll leave them in place for now, but am seriously thinking about replacing these decals with a couple of pieces of orange pinstripe tape.

My 1st Geezer TARC rocket, the Reliant, received its final coats of paint and markings. The name I ginned up in a graphics program and used the Cricut Maker to cut it out of black vinyl. The black roll pattern on the payload section is made of strips of black chalkboard paper, also cut by the Cricut. It turned out pretty nice, and I was surprised that the model came in over 50 grams under weight, which may force my motor selection to a F30J, rather than a F32 (Open Rocket says the Reliant will go about 300 feet too high). The F30J yields a margin of about 50 feet, which is kinda small - I will have to import the design into Rocksim and see what altitude it gives me.

Reliant (Click to enlarge).

I also played some more with applying decals to OpenRocket designs, and was able to replicate the Reliant fairly well, even down to the fin art. The only downside is that Open Rocket insists on putting the decal on both sides of the fin, which isn't the case for the actual model. Still, I am pleased with the result, and am liking Open Rocket more and more for conventional designs. The only thing I wish it had is a drag coefficient override, like that in Rocksim. It would be nice to put in a numerical value, rather than messing with the drag by altering the finish of the rocket (smooth paint, unfinished, etc). That's clunky, and too coarse for serious work.

Virtual Reliant rendered by Open Rocket's photo studio (Click to enlarge).

Finally, I completed my first competition build in decades, the 1/4A parachute duration rocket, which comes in at 7 grams without the chute and motor. That's probably about 3 grams too heavy, but 1) I suck at parachute packing and needed the extra room of an 18 mm tube, rather than going with the straight minimum 13 mm diameter structure, and 2) I could not resist spritzing on a light coat of paint, which added about 0.5 grams. However, it is an improvement over my competition birds of yore, and I am looking forward to seeing this little puppy fly at Pegasus.

1/4 A Parachute Duration rocket (Click to enlarge).