Ask a Rocket Scientist

Chamber is fine. Serious questions about aeronautics/avionics/aerospace/being an engineering student are welcome.

I was looking at an illustration of a possible proposed future airplane and I was noticing that it had a 6 bladed propeller. And I see that some late version C-130 use a 6 blade propeller. I was trying to work out logically what the reasoning for the different ones are. I figure a 2 blade is usually used where the engine is small, and hasn't got the power to turn more. You have the factors of the diameter of the propeller and the number of blades. Does which you choose then be determined by whether your engine has the power to turn it? How do you decide on the most efficient way to go? In WWII 3 bladed propellers were pretty much the standard early in the war, even for the planes with the more powerful engines. But, particularly on fighters the prop was diameter limited because of the clearance needed on landing and takeoff. 4 blades became more common a few years in. Didn't some of the planes with the most powerful engines lose efficiency by not upgrading sooner?

Not a very well phrased question, I know. I can't quite think how to ask what I want to ask.

Well, to speak very broadly on the issue, the thrust you get with a prop is a function of the number of propellers, their geometry, and their velocity. There are certain efficiency thresholds for the number of propellers and how fast you're actually able to spin them, but obviously more propellers are harder to spin (more mass requiring more force to turn them and all that) so there's a point where you have n propellers and you're not getting as much thrust as with n-1, simply because you're turning the propellers more slowly.

However, due to material and aerodynamic limitations, we can't spin a small number of propellers insanely fast nor can we add too many propellers without having a big enough diameter. It's actually fairly involved, but a general rule of thumb is that more diameter + more power in the engine = more propellers. More power in the engine is often enough to justify more propellers by itself in some cases, whereas more diameter is not.

In general, efficiency isn't as big a concern as moving faster and having better specific powers at higher altitudes, which in some cases fewer propellers on a bigger engine might provide. Overall, I can see there being some deficiency in efficiency as a result of late or incomplete adoption, however. Generally engineering during WW2 was much more "try and see" than now, where we have much more reliable methods for prediction and modeling, so reluctance in some areas to change shouldn't be unexpected.

Speaking less specifically on the question of speed, thrust, and efficiency, propellercraft are interesting in general for having properties that are very unique compared to jetcraft, in that they behave very differently, but that's a discussion for another time.

However, due to material and aerodynamic limitations, we can't spin a small number of propellers insanely fast

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Republic Aircraft circa 1955 might be surprised to hear that.

Have you ever envisioned a rocket as a giant penis?

Speedo said:

Republic Aircraft circa 1955 might be surprised to hear that.

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Yeah, exactly. In fact there are a lot of problems with fast-spinning propellers in general; some of them are alleviated with more propellers and more diameter, but some are unavoidable (the torque, for instance, caused by the spinning propeller).

SiLL said:

Have you ever envisioned a rocket as a giant penis?

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Not until Freud asked me about it.

That's disappointing

I came across your thread after trying to find help with a rocket nozzle question. Are you still willing to help with science questions?!!?

Q: With 1 psi of natural gas, will a de laval nozzle work, obviously a smaller version?

Any help on finding what dimensions are needed?

I would need more information. What exactly are you asking?

RocketMann said:

I came across your thread after trying to find help with a rocket nozzle question. Are you still willing to help with science questions?!!?

Q: With 1 psi of natural gas, will a de laval nozzle work, obviously a smaller version?

Any help on finding what dimensions are needed?

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This:

Crezth said:

I would need more information. What exactly are you asking?

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I guess specificially we'd need to know the mass of your rocket, how fast you need it to fly, how far (or high) you want it to go, what kind of oxidizer you are using, are you using liquified natural gas or gaseous natural gas? There's a bunch of other things that would help but that's just what jumped off the top of my head. 1 psi sounds really low unless your rocket is insanely low mass (improbably given you'll have to have either pumps to move the gas or a high pressure tank of an inert gas such as helium to push it into the reaction chamber).

Ayn Rand said:

Where in the solar system is the best place to position and hide a nuclear arsenal? Assume size is 500 warheads and it is designed for two scenarios

1) A covert second-strike capability, returning in retaliation to an attack on NATO

2) A covert first-strike capability, where it will be brought back to Earth as rapidly and stealthily as possible in a surprise first-attack on hardened facilities.

Can you estimate the expense/possibility of deploying such a system?

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Israel. It's a small country to place that many missiles in but given they are basically exempted from nuclear inspections and treaties, that's where you'd want to put it (plus they already have a semi-secret stockpile). For second strike capability against NATO you basically have to have a submarine launch capability.

Probability = 1 given that Israel basically meets this criteria.

Patroklos said:

Why do you think you are a rocket scientist?

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trollololololololololol

I think he's asking about compressed natural gas but... 1 psi is hardly compressed at all. :<

And that's assuming he's talking about 1psi gauge pressure and not 1psi absolute...

hobbsyoyo said:

And that's assuming he's talking about 1psi gauge pressure and not 1psi absolute...

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What's the difference between gauge pressure and absolute pressure?

Is absolute pressure a reading within the reaction chamber while gauge is a measure at the injector/outlet?

just curious

peter grimes said:

What's the difference between gauge pressure and absolute pressure?

Is absolute pressure a reading within the reaction chamber while gauge is a measure at the injector/outlet?

just curious

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Gauge pressure is the pressure you read on the gauge, which typically aren't 'zeroed' out. When measuring pressure, gauges don't usually account for the pressure of the air. So let's say you have a gauge attached to your car tire and it reads 18psi. That means the pressure in the tire is 18psi above air pressure, which (IIRC) is ~14.7psi) so the actual pressure in the tire is 18psi + 14.7psi = 32.7psi. 32.7psi is the absolute pressure, because it measures from zero pressure, while 18psi is the gauge pressure, because it measures with the 'zero' taken to be the ambient air pressure.

In this case, it would be silly to use 1psi absolute pressure as it would mean the tank is in partial vacuum and would require some serious plumbing to get the gas out of it (or air would back up into the tank and make the rocket inoperable). However, there was so little information in the question that I had to ask.

Are you going to go back to your original avatar? Seriously it has been a long time since April Fool's.

I am no longer Joecoolyo. Haven't been for a week.

hobbsyoyo said:

I am no longer Joecoolyo. Haven't been for a week.

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He means DaCrezthchs

classical_hero said:

Are you going to go back to your original avatar? Seriously it has been a long time since April Fool's.

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I don't know how to answer this question so I'll respond in the language of the only thing I know anything about (thanks Wikipedia):

Fuzzy logic was first proposed by Lotfi A. Zadeh of the University of California at Berkeley in a 1965 paper. He elaborated on his ideas in a 1973 paper that introduced the concept of "linguistic variables", which in this article equates to a variable defined as a fuzzy set. Other research followed, with the first industrial application, a cement kiln built in Denmark, coming on line in 1975.
Fuzzy systems were largely ignored in the U.S. because they were associated with artificial intelligence,[citation needed] a field that periodically oversells itself, especially in the mid-1980s, resulting in a lack of credibility within the commercial domain.
The Japanese did not have this prejudice. Interest in fuzzy systems was sparked by Seiji Yasunobu and Soji Miyamoto of Hitachi, who in 1985 provided simulations that demonstrated the superiority of fuzzy control systems for the Sendai railway. Their ideas were adopted, and fuzzy systems were used to control accelerating, braking, and stopping when the line opened in 1987.
Another event in 1987 helped promote interest in fuzzy systems. During an international meeting of fuzzy researchers in Tokyo that year, Takeshi Yamakawa demonstrated the use of fuzzy control, through a set of simple dedicated fuzzy logic chips, in an "inverted pendulum" experiment. This is a classic control problem, in which a vehicle tries to keep a pole mounted on its top by a hinge upright by moving back and forth.
Observers were impressed with this demonstration, as well as later experiments by Yamakawa in which he mounted a wine glass containing water or even a live mouse to the top of the pendulum. The system maintained stability in both cases. Yamakawa eventually went on to organize his own fuzzy-systems research lab to help exploit his patents in the field.
Following such demonstrations, Japanese engineers developed a wide range of fuzzy systems for both industrial and consumer applications. In 1988 Japan established the Laboratory for International Fuzzy Engineering (LIFE), a cooperative arrangement between 48 companies to pursue fuzzy research. The automotive company Volkswagen was the only foreign corporate member of LIFE, dispatching a researcher for a duration of three years.
Japanese consumer goods often incorporate fuzzy systems. Matsuhorsehockya vacuum cleaners use microcontrollers running fuzzy algorithms to interrogate dust sensors and adjust suction power accordingly. Hitachi washing machines use fuzzy controllers to load-weight, fabric-mix, and dirt sensors and automatically set the wash cycle for the best use of power, water, and detergent.
As a more specific example, Canon developed an autofocusing camera that uses a charge-coupled device (CCD) to measure the clarity of the image in six regions of its field of view and use the information provided to determine if the image is in focus. It also tracks the rate of change of lens movement during focusing, and controls its speed to prevent overshoot.
The camera's fuzzy control system uses 12 inputs: 6 to obtain the current clarity data provided by the CCD and 6 to measure the rate of change of lens movement. The output is the position of the lens. The fuzzy control system uses 13 rules and requires 1.1 kilobytes of memory.
As another example of a practical system, an industrial air conditioner designed by Mitsubishi uses 25 heating rules and 25 cooling rules. A temperature sensor provides input, with control outputs fed to an inverter, a compressor valve, and a fan motor. Compared to the previous design, the fuzzy controller heats and cools five times faster, reduces power consumption by 24%, increases temperature stability by a factor of two, and uses fewer sensors.
The enthusiasm of the Japanese for fuzzy logic is reflected in the wide range of other applications they have investigated or implemented: character and handwriting recognition; optical fuzzy systems; robots, including one for making Japanese flower arrangements; voice-controlled robot helicopters, this being no mean feat, as hovering is a "balancing act" rather similar to the inverted pendulum problem; control of flow of powders in film manufacture; elevator systems; and so on.
Work on fuzzy systems is also proceeding in the US and Europe, though not with the same enthusiasm shown in Japan. The US Environmental Protection Agency has investigated fuzzy control for energy-efficient motors, and NASA has studied fuzzy control for automated space docking: simulations show that a fuzzy control system can greatly reduce fuel consumption. Firms such as Boeing, General Motors, Allen-Bradley, Chrysler, Eaton, and Whirlpool have worked on fuzzy logic for use in low-power refrigerators, improved automotive transmissions, and energy-efficient electric motors.
In 1995 Maytag introduced an "intelligent" dishwasher based on a fuzzy controller and a "one-stop sensing module" that combines a thermistor, for temperature measurement; a conductivity sensor, to measure detergent level from the ions present in the wash; a turbidity sensor that measures scattered and transmitted light to measure the soiling of the wash; and a magnetostrictive sensor to read spin rate. The system determines the optimum wash cycle for any load to obtain the best results with the least amount of energy, detergent, and water. It even adjusts for dried-on foods by tracking the last time the door was opened, and estimates the number of dishes by the number of times the door was opened.
Research and development is also continuing on fuzzy applications in software, as opposed to firmware, design, including fuzzy expert systems and integration of fuzzy logic with neural-network and so-called adaptive "genetic" software systems, with the ultimate goal of building "self-learning" fuzzy-control systems.

Let say I had a steady flow of propane gas at a pressure of 1 psi, leading to a de laval nozzle which in turn will be ignited. Will the nozzle create "push" with just the 1 psi?

Thanks!

Futher discusion: chefkrohn@ g ma il

Can a convergent/divergent nozzle convert 1psi of gas to 2 or more when ignited?