A few things to know for this week. First: we’re going to be flying again this weekend on the 23rd at the Tripoli MN launch up in North Branch. I know many of you on this list are rocketeers, so I hope to see you there! Definitely come by and talk to us! We’ll be the ones with the very drab, very plain rocket. Nothing interesting this time ’round except for the wireless communication, we’re afraid. Later on, we’ll try to convince them to let us do some interesting things, but I don’t want to spoil the subteam update.
One more thing: remember how last year, we were asking for “safety approval” to use money from the College of Science and Engineering, and they decided since we weren’t using their money, we weren’t their jurisdiction? Well, now we want to use there money, so we’re going to request approval again. We’ll see how that goes, but if we come out of it intact, we might have some additional sources of funding to go into cool things!
Subteam Spotlight: Flight
In the Flight team, now that we’ve successfully constructed a testbed for avionics in our modified Torrent Madcow rocket, our biggest project is the development of what we call an “asymmetric thrust rocket.” For your visual pleasure, I have inserted pictures from our recent low power asymmetric experimentation below.
That video above is NOT how we’re going to be doing it. We just wanted to see how some rockets would react when we designed them quite wrong. The method we decided on is much less haywire.
Basically, when we attempt to fire a liquid propellant rocket engine, we run into a major issue. When the rocket engines are small (which they must be to avoid scaring the University), their thrust to weight ratio is poor. Fortunately for us, we can reach about a two to one thrust to weight ratio for the entire rocket.
If we had active guidance and thrust vectoring which allowed us to stay upright, we wouldn’t have an issue. Unfortunately, we don’t have thrust vectoring or active guidance, so we must rely on passive, aerodynamic stability to keep ourselves pointed upwards. For general rocketry, the accepted speed off the rail is 45 ft/s, or 14 m/s. Unfortunately, for your standard six foot rail, this means a thrust to weight ratio of about six. Now we have an issue. We can either build a very long launch rail, or we can attempt to boost thrust at the very beginning of our flight. Since building a longer launch rail is easy (in theory), we can do more interesting work and allow for more flexible flights by increasing thrust at the beginning.
Typically, there are two methods to increase that thrust (if we want to still retain our liquid propellant motor. The first is staging, where a first stage solid motor would boost our rocket up to speed before dropping away to allow our second stage liquid motor to push the rocket the rest of the way. The problem? Starting the liquid propellant rocket engine in the air. If it doesn’t start, that’s a flying tube full of fuel and oxidizer waiting to crash into the ground. So staging is tough.
The second option is clustering or essentially using boosters. Instead of a single liquid motor, we would have additional solid propellant motors around the sides. Now the problem lies in lighting all of the motors at once. Probable? Yes. Guaranteed? Not at all. Therefore, best practices will angle the motors into the center of mass so that if one motor doesn’t ignite, it won’t throw the rocket too far off course. Problem? The increased mass and size of the rockets means a larger diameter rocket means more thrust is needed means a larger diameter rocket, etc etc.
Note: this method doesn’t work. We tried it. Don’t do it at home.
So we decided we would experimentally pursue a third option: an in-line solid rocket motor as a booster which is angled slightly off center to allow its thrust plume to clear the rocket body. If we place the motor’s center of mass at the same location as the rocket’s center of mass (and the burn duration is short), then we can place the booster at any arbitrary angle without worrying about it throwing the rocket too far off balance (so long as it cuts out before aerodynamic effects become dominant).
This way, we can hold down the rocket, ensure ignition of the liquid propellant rocket motor, and only then ignite the solid booster. Because there’s only one solid to light, we don’t have to worry about igniting everything at once, and the whole assembly is lighter, easier to fly, and safer, despite its odd look.
This week we will be purchasing the Loc-Precision Aura kit to modify to allow for the addition of an angled motor. The Aura is designed to be extra rugged and minimalist, meaning we can add extra weight and still allow the rocket to take off This means (at least according to Open Rocket) that we can still take off safely even if one of the motors fails to light. We have selected the 143-G-33 and 53-F-70 motors as our most likely candidates for flight. The G33 gives a nice, long, 4 second burn while the F70 gives a short, sharp kick off the rail, just like our eventual liquid propellant rocket would be using.
With any luck, this’ll be an exciting thing to try out. We hope to fly it during the summer.
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