Hacker News Reader

Comment by Robotbeat | Hacker News Reader

SpaceX are using methane for BFR point to point, not kerosene. Burns cleaner, and they're planning on synthesizing it eventually like they need to do for Mars.

Although I suspect that long-term, if you're synthesizing your fuel, you're better off using hydrogen instead of methane. Structural mass is generally higher if you use hydrogen, but there is a performance boost especially at higher velocities. And you avoid the inevitable energy loss from the Sabatier Reaction. And generally better for the climate than methane, particularly if you're running fuel-rich.

View on HN

Hydrolox will always be a more efficient fuel, but it's a nightmare to store and manage, and destructive to engine components, fuel lines, storage tanks, and, well, everything it touches.

It may be tough to have a reusable hydrolox rocket that's efficient from a total cost of ownership perspective.

From a conservation of mass standpoint, synthesizing methane would be better for the climate. The stored fuel represents temporarily sequestered carbon. The exhaust, whether the engine runs rich or lean, only represents carbon that was in the atmosphere before the Sabatier reaction was run. However, the total size of the launch vehicle market renders these speculations superfluous, for now.

The problem isn't just carbon mass (i.e. CO2 in the atmosphere), but the fact that methane is a much stronger greenhouse gas than CO2 per unit carbon.

The total size of the launch market today is infinitesimal (~100 launches per year and vast majority less than a tenth the payload capacity of BFR). But BFR is being built to change that by orders of magnitude. Long-haul point-to-point transport of people means potentially hundreds of thousands to millions of flights per year. Long road to get there, though, and the primary obstacle may just be the incredible airline safety record (to make it close, you probably need to include lifetime saved by making the trips shorter).

It's not a stronger greenhouse gas than CO2 when it's sitting in a tank, and it's not a stronger greenhouse gas when it's emitted as hot, reactive exhaust into the atmosphere, where it is immediately reduced to CO2, CO, H2, H, H2O, HO, and so on.

But, like I said, premature and superfluous, even though we should still do our accounting carefully, and not rely on "OMG methane!" as the guiding principle of our thinking.

Reply on news.ycombinator.com