Odd rocket shapes

I think the classical design is usually chosen because it is simple, not because it is the best. It's easy to get round tubes; it's easy to make round centering rings; it's easy to make round nosecones.

It's also possible that StarShipOne is doing the best because they have a billionaire for financial backing. Just a guess.

The classical design is the simplest for doing what the majority of our rockets do. They fly straight up, with only fin guidance, and float back to earth using a parachute.

The X-Prize contestants have a very specific goal, and less money than would be necessary to produce a Saturn V.

If our model rocket goal was to fly up with less g-force than is deadly to a human and return with such precision and safety to ensure no danger, our rockets would look different too. But, for the most part, 3FNC works quite well to give me the rush of speed, flame and sound that I so dearly desire.

The people that do build odd designs do it for the challenge mostly. What "good" a design is, is dependant upon the mission goal.

steve

The X-Prize folks are able to combine traditional aircraft with rockets. Using wings to provide lift gets them a lot of altitude with less fuel.

Doing something like that with models presents some problems. For instance, to replicate the flight profile of SpaceShip One, you'd need a model airplane with R/C guidance to lift the rocket to altitude, then you'd have to drop the rocket, ignite it, guide it from horizontal to vertical, and safely recover it. Not only is this tricky, but I believe there are rules about launching rockets from model planes, not to mention launching rockets horizontally.

Parasite staging is a better approximation of that kind of flight. Just mount a small rocket onto the side of a larger booster rocket. When the booster burns out, ignite the smaller rocket and it separates from the booster. No horizontal flight or guidance is needed. It's cool, and is great for photographic payloads but is probably not as efficient as traditional inline staging.

The standard round tube is by far the easiest body shape to make simultaneously light, strong, and easily. Circular cross-section nosecones and transitions are also easily made on a lathe. Flat fins are easy to make. Once you start trying to do more sophisticated shapes the problem of how to make them becomes quite challenging.

The optimum shape for a single-stage water rocket is very much like the top part of an exclaimation point. But it isn't easily fabricated by hobbyists out of the necessary materials. In model rocketry you can do some interesting things by starting with thin sheets of flexible material and then coating or impregnating them with stuff to make them stiff. But wrapping fiberglass into complex shapes gets, pardon the pun, rather sticky if you also want to preserve high strength to weight ratios.

Last year I built a rocket out of Quantum Tube without using any glue. The fins were tube fins, except that I used 3/4 sections of tube and then flexed them to form a kind of semi-squared set of tube fins. This unusual shape allows the fin assembly to squeeze down on the body tube(which was still round). It only works because of the physical properties of QT, although it could be duplicated with paper tubes.

But basically, weird shapes are just too hard to make. Rocketeers only use them when, and because, they WANT to use them for some effect. (Generally, because "it looks cool!")

The 3/4fnc design is actually pretty close to theoretical optimum anyway for a rocket that just goes up and down.

Hello Jonathan,

I agree and think this is one of the more subtle "cool" aspects of the X-Prize contest... I suppose another reason for design divergence is probably due to what the contestants have ready access to for materials, what fabrication capabilities they have available, and their background. Not sure how well StarShipOne will upscale but that's for later. I don't know about you but I grew up wanting to be like the Jetsons--puttering around in the little family spaceship. With X-Prize, there seems to be the more tangible progress towards that dream than the humungous national space programs of the world.

In the model rocket world, NASA Houston Rocket Club hosts an annual Wacky Wockets event going to odd rocs over all sorts. It could be an RSO's nightmare but the field is plenty adequate for what's usually entered. This year's event got rained out.

Regards, Andy

True. And the nature of model rocketry is that they are expendable and easily lost. They should be designed to be constructed quickly and cheaply. A reusable launch vehicle is something completely different.

I have a growing interest in water rockets. Every time is see model rocket motors at retails stores, I look at the sticker price, shake my head and pull myself away. Your shape description is a little vague, but I am more interested in your optimality conditions for water rocket shape. Please explain. I'm also interested in optimal trust of water rockets, neglecting for the moment the mechanism for throttling, but recognizing that the driving pressure decreases as the water is expelled... I'm also wondering about the combination of water rockets with model rocket motors and how that effects performance and legal usage.

TRU has an add for a Super Soaker 7 foot tall Hydrorocket. (Limited quantities no rain checks???) It only claims to go over 100 Ft. Has anyone tried one of these yet?

Alan

There are water rocket web sites and at least one mailing list with tons o' info.

Heavy duty math. Which is why I've never finished my math model...

The primary implied assumption in the shape I stated was single-stage, i.e., no boosted darts etc. It comes down to minimizing drag and velocity loss in water flow while maximizing mass ratio. And a bunch of other various factors all pulling in different design directions. The main design problem with water rockets is that the heavy part of the propellant is on the bottom when you want it towards the top or at least more neutral. The original toy water rocket was an elliptical shape--the best design is simply to elongate this on the bottom half. But it's a bear to actually make. The long cylindrical shape works ok and allows far easier thrust tailoring.

Water rockets are excellent teaching tools for grades junior high through college. (The correct first approximation mathematical model is a complicated set of simultaneous differential equations derived from fluid flow, thermodynamics and aerodynamics-- usually third year college stuff.) And they work well in pure experimental mode without the math--junior high demonstration of optimization trade-offs and data analysis. But they just don't have the sex appeal of noise, fire and smoke.

Indeed, there is so much out there that I don't have enough time to sort through it and find what interests me.

I could probably help you with that...

That is also a little vague. What about optimal mass or ballistic coefficient of the expended water rocket? Must structural mass be minimized or just used well?

You know it is easy to build a two chamber water rocket with the water held in the forward chamber. This would improve stability and permit smaller lower drag fins.

I don't understand how the long cylindrical shape allows for easier thrust tailoring, nor what trust tailoring you may want to achieve.

Nor the performance, but the fuel is cheap, easy to get, and not oppressively regulated.

Alan

Well, SpaceShipOne isn't really a rocket, it's an airplane carring enough rocket fuel to meet the 100 km requirement. It's maneuvered as an airplane both on the way up and in the latter portions of coming back down. Now, it's weird looking for an airplane, too, but mainly as a result of trying to keep the stabilizer and aft fuselage out of the exhaust plume. Listening to Rutan at Oshkosh in the late 90's, I got the impression that he had been planning something that was basically a pure "rocket". He described a carrier vehicle that would enter an immelman turn and release a ballistic vehicle right as it passed through vertical. The vehicle would climb to 100 km, then descend to a passively controlled reentry. He described it as a giant "badminton birdie" that would simply orient itself aerodynamically just as model rockets do. Even the current design, when the tail assembly is in the "feathered" position, I believe requires no control input for a normal reentry. RE

yup. its a space plane :-)

much cooler if you ask me :-)

Screw effeciency. if its not cool fun and inspiring whats the point.

Darn Google, can't sort out just the latest posts!

I wanted to create a complete mathematical model and then run extensive sims to optimize over all the variables. Can't do that with an incomplete model!

More complex, heavier, and rather hard to do with advanced shapes. It's one of the questions I originally wanted to explore, whether or not it was a good trade-off.

Simple sliding disc with a hole in it. High initial thrust as the water below it gets expelled through a large nozzle, then lower, longer thrust as the water above it has to pass through the smaller hole. Can't beat it for simplicity. Wish I could claim it as my idea... A look at the typical velocity plot for a water rocket flight makes it obvious why thrust tailoring is the biggest single factor that can increase peak altitude. Feather shot from a cannon...

But now I'm years behind the technology leaders in water rocketry. Oh well.

Well, it's probably actually much more efficient, since you're not using up fuel just to hold it up in the air. I think Rutan chose the easiest method, the V2-like entries need huge fuel loads and motors. RE

Not YET, but when the government finds out that they are fueled with one of the most corrosive chemicals on earth, and that this fuel is responsible for hundreds if not thousands of deaths annually due to inhalation, regulation is just around the corner.