I'd like to see some math on the "lower performance" bit. The prop mass fraction is 21% better with kerolox, but Raptor's insane pressure and FFSC more than make up for it.
Quote from: envy887 on 04/12/2017 02:59 pmI'd like to see some math on the "lower performance" bit. The prop mass fraction is 21% better with kerolox, but Raptor's insane pressure and FFSC more than make up for it.Methane has higher ISP, but RP-1 has higher ISP Density. Given the same volume, there is simply much more energy in RP-1. Given the same mass, there is more energy in the CH4.
Quote from: envy887 on 04/12/2017 02:59 pmI'd like to see some math on the "lower performance" bit. The prop mass fraction is 21% better with kerolox, but Raptor's insane pressure and FFSC more than make up for it.Methane has higher ISP, but RP-1 has higher ISP Density. Given the same volume, there is simply much more energy in RP-1. Given the same mass, there is more energy in the CH4.Math can help you compare, but math is not going to change the laws of chemistry.
You know, that makes me think of the possibility of going rather in the opposite direction, swapping out LOX for nytrox (say, 50%/-80°C), and ditching the tank pressurization system Density is better than regular LOX (although not quite as good as subcooled), but beyond ditching (historically problematic) pressurization system's mass...
This theoretical LV is intended to serve the heaviest payloads, which otherwise can't be launched by FH at all, thus they go to some competitor instead of SpaceX.If the center core is single-raptor, expending it may be acceptable.
You know, that makes me think of the possibility of going rather in the opposite direction,
Quote from: Rei on 04/13/2017 11:24 amYou know, that makes me think of the possibility of going rather in the opposite direction, Bad idea and not feasiblea. Lose too much performanceb. does't solve the tank pressurization issue. RP-1 still needs to be pressurizedc. Tank pressure is less than 50 psid. Loading the He slower or GOX autogenous pressurization are better ideas to solve the perceived problem
Nytrox wins by 322 m/s. And by simplicity, and by chemical compatibility, and propellant storability, and ease of ignition, higher thrust, and everything else listed before.
... for a rocket of a fixed volume, switching to a higher ISP / lower density propellant is generally not a way to get better performance.
Quote from: Rei on 04/13/2017 03:08 pmNytrox wins by 322 m/s. And by simplicity, and by chemical compatibility, and propellant storability, and ease of ignition, higher thrust, and everything else listed before.No, does not win. Nytrox is not going to be able to use a stock Merlin. And you are ignoring tank mods for mixture ratio changes.
And you cannot add in 500kg for pressurant system and also ignore the much much heavier Nytrox tanks.
It is not simple
and there is no real difference in the ease of ignition (there never has been an issue with LOX RP-1 ignition)
More thrust? nonsense
you have no data to support that claim from a stock Merlin.
1. Quite to the contrary, self-pressurizing stages are a hallmark of simplicity in rocketry, which is why they're so often used in spacecraft, where reliability takes top priority.2. Which is why spacecraft tend to use "easy to ignite" RP-1 rather than hypergolics, right?The Merlin burns LOX/RP-1 in liquid phase. Nitrox burns gas phase. Gas phase means easier and faster mixing. Eliminating uneven combustion stemming from uneven mixing is one of the biggest difficulties in rocket engine design programs. Gas mixtures tend to ignite without chugging and burn steadily. Liquids make you earn it.3. I'm going to assume that you didn't actually mean what you wrote, because do not believe that you wouldn't be aware that higher propellant densities are associated with higher thrust for a given flow rate, all else being equal. A turbopump sweeps out a fixed volume per revolution and is designed to operate at a given RPM. The denser the propellant, the more energy is in that volume (again, all else being equal).
We don't live in a world where you just load a rocket with an arbitrary propellant that it wasn't designed for; there's always reengineering, to various degrees. However, when it comes to engineering tradeoffs,
Nor is there a competitor to lose it to.
Quote from: Jim on 04/13/2017 01:26 pmNor is there a competitor to lose it to.ULA thinking: "There is no competition (now), so why bother trying to do better?"
There is no competition because there is no payload or launch vehicle in this class
on, if SpaceX wants FH to compete with SLS. There's a host of other issues, obviously, but right now FH simply can't launch Orion BLEO due to lack of performance.And Lunar Dragon mission, which has at least 2 paying customers - that was definitely a case of extra LV performance opening up a new market. More performance might open up the lunar orbital (vs. flyby) market.
Let's run the rocket equation for, say, for simplicity's sake, F9 s2, no fairing. Because I can't be bothered to do the full staging setup