Tuesday, January 29, 2013

Thrusters in Gandum Unicorn.



I don't know if they're just writing down random numbers, but the thrust of these MS from Gundam Unicorn is just crazy, insane. Look at this one here, its thrusters have 142,600 kg of thrust but the whole thing only weighs 23,700 kg. Well first of all it can pull off 6Gs of acceleration linear or making turns because they are the same thing in space. But just look at the size of his thrusters, only two on the back and that pulls off 6Gs? In comparison modern fighter jets usually have a thrust to weight ratio of around 1. Giant heavy lifting rockets also only have slightly higher thrust to weight ratios that's why you see them ascend very slowly, they can go up to 3 at higher altitudes when they've discarded some stages and boosters I think.

At first I thought it meant the NT-D form where it would have a total of 8 thrusters. But actually no, it's just in normal mode.
Guess what, it has unlimited thrust and unlimited output in NT-D mode. I don't know how the pilot survives it, even for New Type.


I also thought it would be funny to look at some comparisons with our technology.

Here on the left is a proposed manned spacecraft that can travel to Mars and back. It's not really a fair comparison because this thing has months of burn time and is a much larger vessel, I think it is over 100 meters long. Not heavy though, because most of it is just lightweight metal structure and radiators, only the crew area on the bottom and the reactor on the top are somewhat solid.
It uses a thermonuclear fission reactor to generate electricity which then powers four sets of ion thrusters. The output power is extremely low, so it will take months for it to just leave Earth orbit. Currently we cannot make a high power electric rocket.
As you can see, it is covered in massive radiators, most of them are used to cool the reactor because sadly, that's how they work in real life. Taking second place are the huge propellant tanks, and this is already the most efficient propulsion system, much more efficient than chemical rockets.
And then you have the nuclear reactor on the top, which is a great design because you can just put a small block of lead behind it and able to shield a large part of the ship.

In comparison, let's look at our Gundam.
  • It is very solid, though not very heavy at just 20 tonnes, modern tanks weigh over 70 tonnes and they are a lot smaller. That's some super light weight construction right there.
  • The Gundam has no radiators. I still put some in over the red parts when it goes into NT-D, I guess they could be radiators. Maybe it carries coolants with it? But that's a lot of liquids to be carrying around in a humanoid combat robot.
  • RX-0 apparently has a 3,480 kW reactor. The Mars transit Vehicle has I think something like 10 kW. Output power is also directly proportional to the amount of cooling required. So if you take into account of the power difference of about 35,000%, that's how much more radiator you would need in reality. So a real world reactor of such output would need radiators so so huge it might as well be a solar sail.
  • Sometimes they still run out of propellant but that is still a lot of propellant they've managed to somehow fit into the humanoid shape. If you look at a rocket today, it's almost ALL fuel and it burns for about 5 minutes at a thrust to weight ratio of let's say, 2. The Gundam is almost all payload, but it can burn for like 10 minutes at 6 thrust to weight ratio.
And look at these poor guys here. Their thrusters can barely give them a weight loss. What's the point? It can;t even make you jump much higher because they weight almost 70 tones but the thruster only gives 9 tones, makes them just over 10% lighter.

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At first I thought they're just messing around writing down random numbers here. I mean how can the thruster output range from 8,800 kg to 142,600 kg when they seem to be exactly the same size? That's a difference of 16 times.But if you look at technologies today, say, cars. The amount of power provided by car engines can differ by that much. If you compare a Formula 1 which has a very light 100 kg engine but with about 1000 hp of power. And if you look at a regular small car that also has a 100kg engine. It may have a power of well under 100hp. And if you are comparing to another type of engine, say turbo jets, it can be 10x higher than that. There are also a number of restrictions placed on F1 race cars that aims to limit the amount of technology and resources you can put into these engines. Where as there obviously won't be such restrictions for a prototype Gundam.
The only problem here seems to be why would the military use the worst possible engine in their MS?

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