Estimating torpedo energy and resistance

[marcelo_malara]

New thread.

[marcelo_malara]

The Long Lance had a 64 kgf engine or 627 N. It was 2700 kg. This comes to 0.23 m/s acceleration. This is a low value so it might be a torque value rather than a energy value.

Diameter area was 0.37 m2. Top speed is 27 m/s. It could run 555 seconds at this speed.

Moving 27 m/s over a 0.37m2 area is 10 tons of water per second. That takes 7.3 megawatts to do. Over the duration this is 4 gigajoules.

It had approximately one ton of fuel. Fuel is 10 mwh a ton normally or 36 gigajoules. 90% of this is filler so all together the energy density of the fuel is close to what it would need to reach its range by pushing water out of the way. All fuels have similar energy density, regardless of petrol, gas, oxygen etc.

So it has more or less exactly enough energy to travel at top speed to its stated maximum range, pushing water out of the way, and taking about a minute to accelerate to top speed which is about a tenth of its entire trip. As a result it's going to be at top speed most of the time and the fuel is the constraining factor.

Hi!

I do not understand what a "64 kgf engine" means. Is this the propeller thrust?

According to http://www.navweaps.com/Weapons/WTJAP_WWII.php, I did the following reasoning.

The torpedo had a max engine power of 520 hp for 50 knots.

520 hp * 75 kgm/s = 39 000 kgm/s

Using a propulsive efficiency of 50% (EHP/SHP) I assume that 19 500 kgm/s are actually used on the actual pushing of the torpedo, and for 50 kt it would give a propeller thrust of 780 kg:

780 kg * 25 m/s = 19 500 kgm/s

Now lets take a trajectory of 100 m, 4 seconds. The work done by the torpedo:

780 kg * 100 m = 78 000 kgm

[Mustang]

It is not constrained by the engine, that is just there for example. The only factor is the fuel. If you had unlimited fuel you would accelerate forever, that's the real issue.

The only boundary is mechanical breakdown and other factors, there's not an equation limiting speed.

[marcelo_malara]

It is not constrained by the engine, that is just there for example. The only factor is the fuel. If you had unlimited fuel you would accelerate forever, that's the real issue.

The only boundary is mechanical breakdown and other factors, there's not an equation limiting speed.

This´s not so. The drag/resistance actuating in the opposite direction will increase with speed, till the point that thrust equals resistance, a sort of horizontal free fall terminal speed. You can not go beyond that point if you do not increase thrust.

[Mustang]

It is not constrained by the engine, that is just there for example. The only factor is the fuel. If you had unlimited fuel you would accelerate forever, that's the real issue.

The only boundary is mechanical breakdown and other factors, there's not an equation limiting speed.

This´s not so. The drag/resistance actuating in the opposite direction will increase with speed, till the point that thrust equals resistance, a sort of horizontal free fall terminal speed. You can not go beyond that point if you do not increase thrust.

Free fall applies if a force is constant. If it is an engine the force is not constant. Imagine a rocket exploding, 100% of the fuel is consumed instantly and there's no limit to speed.

This "rocket exploding" problem limits speed. The mechanical ability of the engine to transform fuel into work constrains speed, and this isn't a fixed value, theoretically you could consume fuel at an extreme rate and overburden the machines.

There isn't a fixed point where this happens either, the engineering limits set on engine wear are arbitrary and there's no actual difference between going 100 seconds at 1% power and destroying the engine in 1 second.

If you use the official engine power stats the max speed is usually extremely low, if you use fuel consumption you realize the value is arbitrary.

[wadinga]

Hi All,

Marcelo is 100% correct

The drag/resistance actuating in the opposite direction will increase with speed, till the point that thrust equals resistance, a sort of horizontal free fall terminal speed. You can not go beyond that point if you do not increase thrust.

Drag: Square Law Rules

Imagine a rocket exploding, 100% of the fuel is consumed instantly and there's no limit to speed.

Each fragment has a different drag factor to the entire rocket, determining its speed after initial acceleration, and anyway the energy in the fuel is dissipated in a spherical expansion.

Anybody driving a vehicle before modern fuel injection (say an early Mustang) will know you can flood the carburetor by jumping on the throttle supplying too much fuel and engine power goes down. It certainly did on my 350cc single pot Ariel motorcycle.

Modern super torpedo designs interfere with drag by changing boundary layer characteristics using air bubbles emitted through the casing to massively lower drag. Such technology was not available in the 1940s.

If you had unlimited fuel you would accelerate forever,

Even Star Trek's photon torpedoes don't have unlimited fuel, but in space there is indeed no drag.

All the best

wadinga