3 ms·

I already stated my ignorance, so - I'll believe whatever claims are made here. But - my mind desperately tries to fight that and tells me about a _huuuge_ mac

I already stated my ignorance, so - I'll believe whatever claims are made here. But - my mind desperately tries to fight that and tells me about a _huuuge_ machine that needs to be maintained, and consumes fuel as well. And getting to 30km straight up in a rocket shape instead of going up in a loooong slope powered by someone else seems expensive, in terms of energy used.

marvin 15y ago
It's not about energy, it's about momentum and energy/weight ratio. Due to the energy density of chemical fuels, the vast majority of a space rocket's fuel is actually used to carry the _rest_ of the rocket's fuel to the altitude at which it will be burned. I'm not good with rocket physics, but I'm pretty sure that gaining a few kilometers of altitude and 1000 km/h of kinetic energy will have a large impact on the total amount of fuel/propellant required to reach orbit. Is someone in here good enough with rocket physics to comment on this? Of course, it would be even better if they could go to business-jet altitudes or above (15000 meters instead of the 9000 quoted in the article), and I am guessing from the number and size of the engines that this is what they are actually aiming at. But this is just idle speculation on my part.
bulte-rs 15y ago
I clicked reply during the time you wrote your own reply I guess. The fact that going up a slope with a jet-engine is preferred is all due to costs. Jet-engines are just cheaper to operate (and easier to supply with fuel). Perhaps more energy is required in total; but this energy is released in a much cheaper way.
- dalke 15y ago
  Bear in mind that jets carry only fuel and use air for the oxidizer while rockets have to carry fuel and oxidizer with them.
sp332 15y ago
If you have to travel through air, you might as well use a wing to get some lift. That way the air is useful, instead of just slowing down your rocket.
mikeash 15y ago
747s are cheap to operate compared to rockets. You can buy a ticket on one for less than a thousand bucks. Commercial airlines collectively operate them by the thousand. Airplanes are really cheap in both money and fuel compared to rockets. The technology is more mature, the energies involved are much less, and involves much more finesse, as you gain lift from manipulating the air rather than simply smashing your way through it. A system which can substitute an airplane for a rocket for a decent chunk of the ascent can pull off a big win. Rockets also suffer from painful exponential effects. If you add weight, you need to add fuel. But adding fuel adds weight, so you need to add more fuel. That adds more weight, etc. Removing a small portion of the trip can have big consequences for weight and fuel. Fun fact: the Saturn V first stage "only" got the rocket to about 1/3rd of its final altitude and speed despite making up 3/4ths of the rocket's total mass. Airplanes actually experience the same exponential effect, but because the fraction of the total mass spent on fuel is so small, it's much less significant.

marvin 15y ago

It's not about energy, it's about momentum and energy/weight ratio. Due to the energy density of chemical fuels, the vast majority of a space rocket's fuel is actually used to carry the _rest_ of the rocket's fuel to the altitude at which it will be burned. I'm not good with rocket physics, but I'm pretty sure that gaining a few kilometers of altitude and 1000 km/h of kinetic energy will have a large impact on the total amount of fuel/propellant required to reach orbit. Is someone in here good enough with rocket physics to comment on this? Of course, it would be even better if they could go to business-jet altitudes or above (15000 meters instead of the 9000 quoted in the article), and I am guessing from the number and size of the engines that this is what they are actually aiming at. But this is just idle speculation on my part.

bulte-rs 15y ago

I clicked reply during the time you wrote your own reply I guess. The fact that going up a slope with a jet-engine is preferred is all due to costs. Jet-engines are just cheaper to operate (and easier to supply with fuel). Perhaps more energy is required in total; but this energy is released in a much cheaper way.

dalke 15y ago
Bear in mind that jets carry only fuel and use air for the oxidizer while rockets have to carry fuel and oxidizer with them.

sp332 15y ago

If you have to travel through air, you might as well use a wing to get some lift. That way the air is useful, instead of just slowing down your rocket.

mikeash 15y ago

747s are cheap to operate compared to rockets. You can buy a ticket on one for less than a thousand bucks. Commercial airlines collectively operate them by the thousand. Airplanes are really cheap in both money and fuel compared to rockets. The technology is more mature, the energies involved are much less, and involves much more finesse, as you gain lift from manipulating the air rather than simply smashing your way through it. A system which can substitute an airplane for a rocket for a decent chunk of the ascent can pull off a big win. Rockets also suffer from painful exponential effects. If you add weight, you need to add fuel. But adding fuel adds weight, so you need to add more fuel. That adds more weight, etc. Removing a small portion of the trip can have big consequences for weight and fuel. Fun fact: the Saturn V first stage "only" got the rocket to about 1/3rd of its final altitude and speed despite making up 3/4ths of the rocket's total mass. Airplanes actually experience the same exponential effect, but because the fraction of the total mass spent on fuel is so small, it's much less significant.