|Estes custom SLS and Saturn V rtf - 1/200th scale (Click to enlarge).|
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 Thrustcurve.org (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).|
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).|
|Performance calculations for the Cox 1/125th scale Saturn 1B (Click to enlarge).|