Saturday, July 6, 2019

Rocket science...

My place of work, Marshall Space Flight Center, is hosting a series of employee events to mark the Apollo 11 50th anniversary on July 16. One of these will be a launch involving the Estes 1/200th scale Saturn V and a custom SLS (also built by Estes) of the same scale. My job is to work with Chuck and the Public Affairs people to get everything tested and ready so that our center director can press the fire button. I'm rather enjoying it, especially given that the test flights are coming up early next week. Have to make sure we have the right motors and the birds fly straight - Redstone Arsenal insists we keep the peak altitudes low, and an unstable flight would pose a risk to the onlookers. So we test.

Estes custom SLS and Saturn V rtf - 1/200th scale (Click to enlarge).
But a few calculations are needed before the test.

As mentioned in a previous post, the only recommended motor for the Estes Saturn V is the C6-3. However, the liftoff weight of the model is 172 grams (6.1 ounces), well above the maximum recommended weight of 113 grams (4 ounces). This is not a good situation, and the Internet has many accounts of the model achieving a very low altitude and ejecting the parachute well past apogee. But there are other 18mm C and D motors available - do they provide a more acceptable performance?

Reproducing the rocket in Rocksim or OpenRocket would have taken a lot of time, so I resorted to using an app I have on my iPhone (RocketCalc) with inputs from my measurements (weight, maximum diameter) and (which also has an app). I also used an online descent rate calculator to check the descent rates to make sure they were slow enough to avoid damaging the model. The results for the Saturn V are below - note that the C6-3 gives a thrust-to-weight that is way too low (should be around 5) and that the burnout velocity is under 100 feet per second. Maximum altitude is only 200 feet. Easy to see why this motor produces flights that are nail-biters. The next motor, the Quest C12-4, produces a decent thrust-to-weight, burnout speed, and altitude, but the delay is about 1.8 seconds too long (4 versus 2.2 seconds to apogee). This means the parachute will deploy after the rocket has fallen 52 feet from its peak, where its speed is a fast 58 feet per second - quite a stress on the shock cord and attach points! The best motor choice is the Aerotech D10-3W, but the projected 651 foot altitude blows right through the ceiling imposed by the Arsenal, so it's out. The Quest D16-4 also has some good numbers, but the delay is too long, by the same amount as for the C12. The irony here is that the C6-3 is the best choice as far as delay, but the other numbers suck bad. Also, the reports out there in the wild mostly state that ejection was well past apogee, which flies in the face of these calculations.

Performance calculations for the Estes 1/200th scale Saturn V (Click to enlarge).
Which is why we are going to test - with a C12-4. I'm hoping she stays together.

I repeated the calculations for the Estes-built SLS, first calculating an effective diameter from the sum of the frontal areas of a BT-60 and 2 BT-20s (49 mm). It's lighter than the Saturn, and here the C6-3 is the right choice. The thrust-to-weight is a little low, but the altitude is comparable to that of the Saturn on a C12 and the delay is good.

Performance calculations for the Estes custom 1/200th scale SLS (Click to enlarge).
And while I was at it, I ran the numbers for my repaired Cox Saturn 1B, which is the pig of the three. Even though it won't fly at the MSFC event, I will probably launch it during the testing of the other two models. Since I won't be restricted in altitude, the D10-3W is the obvious choice here.

Performance calculations for the Cox 1/125th scale Saturn 1B (Click to enlarge).
Motor choices made - here's hoping for good testing weather!


  1. Hi Bill,
    What's the story behind the "Custom" built Estes SLS? Is this a possible future kit?

  2. As far as I know, it’s a one-off. MSFC asked Estes to produce a 1/200th scale flying model to go along with the Saturn, and Estes kindly obliged. We received it in late June, and the plastic parts (nozzles, srb supports, Orion capsule and tower) are all 3D printed. I suppose they could turn it into a kit - it flies well on a C6-3.