3 ms·

Why not? Aren't rocket-fueled thrusters much heavier due to the weight of the fuel alone?

by james4k 14y ago

mschuster91 14y ago
The four engines of an Airbus A380 provide 311 kN of thrust each, so roughly 1.2 MN or 100 tons of thrust, where the aircraft itself weighs ~500 tons (MTOW-payload). Biggest weight point is the fuel with 320 tons... I wonder how much fuel is needed for the same energy output as the A380 engines.
WhaleBiologist 14y ago
Pretty much. More mass to lift = more fuel = more mass = more fuel etc., plus burning fuel isn't the most efficient way of getting kinetic energy into the thing you are lifting. I would say it is unsuitable for atmospheric engines because of the way it runs. Earth to Mars in 30 days will need you to be travelling at almost 90 km/second. The engine has to fire for 3 days straight because the delta v you need is retardedly high, so it pulses the engine once a minute so that the passengers don't die and the ship doesn't break up due to huge g-forces. Lifting something into orbit needs to be done fast, because most of your energy is spent counteracting gravity, not air resistance. A slow pulsing engine would not be enough to get you up.
- omegant 14y ago
  Three days of hammering impulse is not going to be fun!
  - mschuster91 14y ago
    I guess you won't feel the pulsing that much...
danielbarla 14y ago
There's a trade off between how efficient a thruster is, and how much power it produces over a fixed amount of time. (Good reading is the specific impulse article on Wikipedia: http://en.wikipedia.org/wiki/Specific_Impulse http://en.wikipedia.org/wiki/Specific_Impulse ) At any given point, our technology allows us to create highly efficient but relatively "weak" engines (e.g. ion thrusters), or "powerful" but inefficient ones (e.g. rockets). Actually, both use Newton's second law to generate thrust, i.e. some kind of material has to exit the engine in the opposite direction, in order to get things moving. So, even ion drives and similar designs carry some material for the purpose of being ejected, it's usually just on a completely different scale to rockets. Energy also needs to come from somewhere to facilitate this, with rockets, it's the chemical energy from the fuel itself. With ion drives, it is usually some form of electricity, generated with a solar panel or nuclear power plant, etc. So in reality, even the efficient drives also have to carry some way to power themselves. To get more thrust, they need to have more power. If you run the maths, by the time they have enough power to overcome Earth's gravity, they need to have an entire solar farm worth of panels (which are heavy), or a rather large nuclear power station on board (which is also quite heavy). So, unless we find ways to make more powerful sources of energy, or even more ridiculously efficient drives, the thrust coming from these will not be able to overcome near-Earth gravity.

Why not? Aren't rocket-fueled thrusters much heavier due to the weight of the fuel alone?

by james4k 14y ago

Why not? Aren't rocket-fueled thrusters much heavier due to the weight of the fuel alone?

The four engines of an Airbus A380 provide 311 kN of thrust each, so roughly 1.2 MN or 100 tons of thrust, where the aircraft itself weighs ~500 tons (MTOW-payload). Biggest weight point is the fuel with 320 tons... I wonder how much fuel is needed for the same energy output as the A380 engines.

WhaleBiologist 14y ago

Pretty much. More mass to lift = more fuel = more mass = more fuel etc., plus burning fuel isn't the most efficient way of getting kinetic energy into the thing you are lifting. I would say it is unsuitable for atmospheric engines because of the way it runs. Earth to Mars in 30 days will need you to be travelling at almost 90 km/second. The engine has to fire for 3 days straight because the delta v you need is retardedly high, so it pulses the engine once a minute so that the passengers don't die and the ship doesn't break up due to huge g-forces. Lifting something into orbit needs to be done fast, because most of your energy is spent counteracting gravity, not air resistance. A slow pulsing engine would not be enough to get you up.

omegant 14y ago
Three days of hammering impulse is not going to be fun!
- mschuster91 14y ago
  I guess you won't feel the pulsing that much...

danielbarla 14y ago

There's a trade off between how efficient a thruster is, and how much power it produces over a fixed amount of time. (Good reading is the specific impulse article on Wikipedia: http://en.wikipedia.org/wiki/Specific_Impulse http://en.wikipedia.org/wiki/Specific_Impulse ) At any given point, our technology allows us to create highly efficient but relatively "weak" engines (e.g. ion thrusters), or "powerful" but inefficient ones (e.g. rockets). Actually, both use Newton's second law to generate thrust, i.e. some kind of material has to exit the engine in the opposite direction, in order to get things moving. So, even ion drives and similar designs carry some material for the purpose of being ejected, it's usually just on a completely different scale to rockets. Energy also needs to come from somewhere to facilitate this, with rockets, it's the chemical energy from the fuel itself. With ion drives, it is usually some form of electricity, generated with a solar panel or nuclear power plant, etc. So in reality, even the efficient drives also have to carry some way to power themselves. To get more thrust, they need to have more power. If you run the maths, by the time they have enough power to overcome Earth's gravity, they need to have an entire solar farm worth of panels (which are heavy), or a rather large nuclear power station on board (which is also quite heavy). So, unless we find ways to make more powerful sources of energy, or even more ridiculously efficient drives, the thrust coming from these will not be able to overcome near-Earth gravity.