Re:SpaceX test flight of Starship SN-5 [video]
by bryanlarsen on Wednesday August 05, 2020 @12:33PM
(#24062682)
There are two main metrics of rockets and rocket engines. Specific Impulse (ISP), and the Thrust to Weight Ratio (TWR).
ISP is essentially a measure of the fuel efficiency. And since there are no gas stations in space, the further you can go per kg of fuel, the further you can go, period.[1]
TWR is the measure of useful power.
TWR is the key measure for the first stage because a ton of power is required to escape Earth's gravity and atmosphere. For subsequent stages ISP is the main metric.
The Space Shuttle Main engines have an excellent ISP but because hydrogen needs massive tanks, it has a crappy TWR. Which is why the shuttle had a couple of strap on solids with a high TWR.
But instead of specialized engines, SpaceX designs engines that are good at everything. The ISP isn't as high as an advanced hydrogen engine and the TWR isn't as good as a solid, but both numbers are quite respectable and they bring other advantages such as the ability to relight and to be reused without refurbishment, they don't need use ablative cooling.
1: Although SpaceX is relying on in flight refueling for it's Moon and Mars plans. Nobody's done it before, but that doesn't stop SpaceX.
ISP is essentially a measure of the fuel efficiency. And since there are no gas stations in space, the further you can go per kg of fuel, the further you can go, period.[1]
TWR is the measure of useful power.
TWR is the key measure for the first stage because a ton of power is required to escape Earth's gravity and atmosphere. For subsequent stages ISP is the main metric.
The Space Shuttle Main engines have an excellent ISP but because hydrogen needs massive tanks, it has a crappy TWR. Which is why the shuttle had a couple of strap on solids with a high TWR.
But instead of specialized engines, SpaceX designs engines that are good at everything. The ISP isn't as high as an advanced hydrogen engine and the TWR isn't as good as a solid, but both numbers are quite respectable and they bring other advantages such as the ability to relight and to be reused without refurbishment, they don't need use ablative cooling.
1: Although SpaceX is relying on in flight refueling for it's Moon and Mars plans. Nobody's done it before, but that doesn't stop SpaceX.