[unspecified]

The landing is all based on how much fuel is left after the primary mission is complete: some missions require more fuel to get to their destination orbits, others require less.

This launch was an ISS resupply, which is on the low end of fuel requirements. So the first stage can get all the way back to its launch site.

Other launches are in the middle of the fuel range: they just follow their ballistic path to the barge, and do not have the extra fuel to get turned all the way back around for a return to the launch site.

A few launches are at the very high end: the upcoming Echostar mission has such a high fuel requirement that SpaceX isn't even attempting a landing: they're just dumping the whole stage into the ocean like we've been doing for 60 years.

SpaceX is saying that this upcoming Echostar launch will be their last "expendable launch", ever.

[wiremine]

Very helpful. Thx!

> A few launches are at the very high end: the upcoming Echostar mission has such a high fuel requirement that SpaceX isn't even attempting a landing: they're just dumping the whole stage into the ocean like we've been doing for 60 years... SpaceX is saying that this upcoming Echostar launch will be their last "expendable launch", ever.

Can I infer that they won't be doing launches that require high-end fuel usage? Are they just giving up on those types of use cases, or do they have another solution for those moving forward?

[greglindahl]

The future plan for SpaceX is to use Falcon Heavy for heavier payloads. When they sold the launch to Echostar I'm not so sure that they knew exactly where the booster-recovery line was for F9 FT. F9 block 5 is expected to be able to launch a bit more with booster recovery.

Check http://www.spacex.com/about/capabilities, you'll see both recovery-compatible launch mass and full performance without recovery are.

[zackmorris]

Something to keep in mind is that for space exploration, all that really matters is achieving low earth orbit (LEO). After that, ion engines/solar sails/space tethers, all manner or propulsion can get us out to further reaches. But it takes dedicated engineering to build craft powerful enough to get through the atmosphere with only a small fraction of mass going to engine and airframe.

The cost of fuel is generally pretty negligible compared to the cost of spacecraft. Even 1,000,000 pounds of kerosene only costs about $500,000 ($3 per gallon at 7 pounds per gallon). I'm having trouble finding current prices on liquid oxygen but it looks like about 15 cents per pound, so a 2.56:1 ox:fuel ratio gives $384,000 for 2,560,000 pounds of oxygen. (500,000 + 384,000)/(1,000,000 + 2,560,000) = 25 cents per pound of fuel+oxidizer (honestly I have to saw wow here, as this is much lower than I expected). If a shuttle launch cost $1 billion, less than $1 million went to fuel at 2017 prices (assuming that more expensive liquid hydrogen and less expensive booster solid fuel roughly match the cost of ox-kerosene). Another way of saying this is that the design tradeoffs of the shuttle cost 100 to 1000 times more than other designs might have (mainly due to reusing the orbiter rather than the first stages). A lot of people knew this in the late 70s and warned about it, but due to political reasons NASA went with the more expensive shuttle system and now here we are.

My guess is that from here on, SpaceX will do incremental launches, using a rocket to carry a stage to LEO and then assemble those stages in orbit into new rockets to go further out, landing the first stages to reuse them. This is scalable all the way out to Mars and further, which is pretty remarkable and obvious in hindsight. It wasn't until I considered how one would land a stage on Mars with such a thin atmosphere that I realized why SpaceX has been so committed to retrorocket landing.

https://www.nyserda.ny.gov/Researchers-and-Policymakers/Ener...

https://en.wikipedia.org/wiki/Oxygen#Industrial_production

http://www.astronautix.com/l/loxkerosene.html

http://coolcosmos.ipac.caltech.edu/ask/268-How-much-did-the-...

https://en.wikipedia.org/wiki/Hydrogen_economy#Current_hydro...

[greglindahl]

Why are you guessing? This is SpaceX's announced plan.