# Tag Info

11

Believe it or not, we could have done this 50 years ago, if government funding hadn't been pulled from a project at the last minute. Frustratingly, after years of work by scientists, engineers and technicians, the Boeing X-20 Dyna-Soar project was cancelled just after work had started on the actual spacecraft. Here's an artist's impression of the X-20: The ...

10

In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and ...

9

Now, I wonder if an air-launched spacecraft in the next 20 years realistically could really fly us to the moon - that is, could they reach escape velocities? Air launch to LEO: Done now Air launch to lunar orbit - yes, but at 20%-25% of LEO payload Air launch to Moon and back to LEO: Yes, but with about 5% of LEO payload It is easy to overlook some ...

6

Your fans are too small. If you hold 1 and make it go max speed you will barely feel the force it applies. Instead consider a drive-train going into the water and turning a screw. Because water is much more viscous and dense you get much better acceleration.

6

There are propellers with different aerofoil sections. For example, the propeller attached to the Rolls Royce engine kept in my graduate college had symmetric aerofoil sections roughly about 25 % and smoothly varying to bottom flat airfoil sections at the rest. The aerofoil profile selections are based on the performance requirement, structural rigidity, ...

5

The short answer is these terms are defined by standards organizations based on specific test methods. These methods specify the engine condition, environmental factors, fuel chemistry, inlet and exhaust systems configurations, and the correction factors applied to the measurements. The specific tests and standards used vary by country and by industry. So ...

5

No, it would just create a torque couple and rotate the boat anticlockwise. Let's say the small propeller has an output volume, q grams/s, and the distance between inlet pipe and outlet is 5cm. The thrust and suction of each end of pipe. $$F = ρ q (v2 - v1)$$ say density of water is =1, and V1 is initially zero, for simplicity, even though it wouldn't ...

5

Trains do sometimes use different means for propulsion: Rack railways use cogs, and Funiculars use ropes I think the reason these are uncommon is that the available traction of smooth tracks is more than sufficient for all but really steep slopes (>10% ). Train engines may be limited in both torque and power when it comes to moving a heavy train up a steep ...

4

You are looking to build a sewer robot, so you maybe want to look at other sewer robots. I have no diret experience with designing or handling sewer robots, but I can talk about the ones I saw at trade fairs. Some builds have sets of 3 or 4 wheels with a mechanism that pushes the weels against the pipe walls, or - more typically - sets of 2 wheels. In the ...

4

First, I'll say that from personal experience, locating and implementing treads on a robot that size can be VERY difficult. I was building a robot that was supposed to be 8"x8"x20" (WxHxL), including the treads, and we had a lot of trouble finding usable treads that would be small enough but still hold up to some more intense use than Lego or K'nex robots. ...

4

Paradoxically, maybe the home-sized magnetic levitation could be a narrow area, where superconducting solutions could be cheap. I explain, why. Although room-temperature superconductivity is a dream, room superconductivity is not. :-) There are soon superconducting materials which can be cooled by liquid air, and they aren't even costly. Liquid air is ...

4

Start your mental model by assuming a rocket flight path. A velocity/altitude vs time chart for the Space Shuttle: (source: aerospaceweb.org) Jet engines have better $Isp$ than rockets. Let's put jet engines on our rocket. The Falcon 9 puts out about 1.1m lbs of thrust, so we can use a GE-90 to add 120,000 lb, doubling the acceleration at sea level. ...

4

The answer is that in order to survive and function, the rocket nozzles need to be cooled. How that's done varies with the application, but includes: Liquid Cooling Jacket: The liquid propergols and/or fuels ar circulated through a jacket around the nozzle before being directed to the combustion chamber. This is how the Saturn V F-1 engines and the Space ...

3

Your air propellers did push your boat, but the delivered thrust was not enough! As ratchet freak an Dan said, the density of the medium in which the propeller is working is of great importance, as the delivered thrust $T$ is roughly calculated by: $$T = c_{th} \frac{\rho}{2}v^2A_{prop}$$ nevermind the factor $c_{th}$ but you easily see that  T \sim \...

3

You mean BRAKE horsepower. It is the same as shaft horsepower. It is called brake horsepower (abbreviated BHP) because it is measured at the output shaft of the engine while it is running on a test stand with a mechanical device called a brake. There are other definitions of the power output of an engine which rely not on measurements but on calculations ...

3

Look up the surface gravity of the moon. It'll be in units of meters per second squared. Look at the definition of the Newton. Use those two things to determine the thrust needed to keep 1kg of mass aloft Look up the specifications for current ion thrusters. Pay attention to mass vs. thrust ratio, or maximum acceleration. Do some math. I think you'll find ...

2

Submarine propulsor designers are least concerned about hub vortex cavitation as they are primarily concerned about cavitation inception, or the onset of cavitation, which first occurs in the tip vortex regions, where the velocities are highest. Once cavitation occurs you are no longer operating in stealth mode. However, the hub vortex does waste a ...

2

When fluid flow increases in velocity (because of smaller area of blade root) the pressure decreases and can be low enough to produce cavitation. The spheres produce turbulence changing the flow characteristics, keeping the pressure up. Hub tip pressure drops as flow fills in after the hub and causes cavitation. The flat hub blades disrupt the flow and ...

2

It would be slower. Compressing the air takes energy - that will slow the propeller, and the plane as a whole. Imagine a water pump, which is pumping water out of a pipe. Now, if you put your finger over the end of the pipe, the water speeds up, but the pump impeller will slow down, and, as a result, the total mass-flow rate goes down. The speed of the ...

2

As a reference, consider the Meredith effect, which applies (applied) in the construction of the P-51 Mustang radiator. From the linked Wiki site: The Meredith effect occurs when air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid such as ethylene glycol. Typically the fluid is a coolant carrying waste ...

2

To answer the other part of your question, the temperature is that in the chamber, but note that that figure assumes perfect combustion, which may not be the optimum point for specific impulse in your geometry. Many rockets run fuel rich as a reducing atmosphere in the hot bits is easier to cope with then an oxidising one. The largest thermal flux is ...

2

In the 50's the DARPA designed such a craft but due to aerodynamic instability it was a complete failure and abandoned. The problem you have is maintaining 4 separate intakes which have a greater volumetric capacity than each propeller can handle at maximum power in coldest conditions. You cannot combine the intakes and expect stability, it's just not ...

2

At those speeds, several effects become important. We consider air-breathing engines in what follows. First of all, aerodynamic heating of the airframe becomes important. In broad terms, to prevent structural failure, the maximum temperature of the hottest portions of the airframe must be held below red heat. Going faster than mach 3.3 pushes this limit. ...

2

Thanks for a nicely presented first question. First you need a model for the equation of state for the tank. Given the initial conditions, develop formulas that yield static pressure, temperature, work, etc as a function of the mass remaining in the tank. (You need at least one fully defined state point for the tank, I'm assuming it's the initial condition). ...

1

In your force/momentum balance equations you always can include the pressure of the gas. The only time you can ignore it, is if the difference in $PA$ is small compared to other forces. For example in a diffuser at the entrance you have a lower pressure and lower area than at the exit. This results in a significantly lower $PA$ at the entrance, more than ...

1

the amount of energy per second you would have to dump into the air passing through the "heating chamber" in order to make the turboprop engine work would have to be at least as great as that produced by the usual combustion of jet fuel in the heating chamber. Your homework is to do a quick energy balance on the system (using the two heat sources- jet fuel ...

1

This is done in a jet engine, and partly, by the cowl around the front fan on a jet airliner engine, but on a large propeller, you could only do it slightly without slowing the airflow through the propeller. The extra weight would outweigh the benefits.

1

Two challenges (not necessarily "problems" so long as the device is designed to deal with them) are: Vibration. If the pulse frequency matches a resonant frequency of the turbine, the vibrations may reach a high enough amplitude to break something, or significantly reduce the life of the components. The waveform of the pulse will probably contain a large ...

1

It will vary between 0 and maximum depending on the demands of the drive system - ie the engine output is shared whatever the wheels need and the remainder to the generator to charge the batteries.

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