falcon 9 to GEO

[magnemoe]

Falcon 9 has good payload faction going to LKO but a pretty weak one going to geostationary orbit.

My guess is that this is because they uses only two stages and the second stage is overkill for this.

If you added an small 3rd stage if should be possible to get the same payload faction as for ariane and other rockets going to GEO.

Downsides would be increased complexity and cost with another stage. you might also need an larger fairing.

[SargeRho]

The reason is that GTO requires much more deltaV than LEO.

[Streetwind]

The reason that the GEO payload fraction is so low is because the second stage is a kerosene stage with an overpowered engine. The Merlin 1D Vacuum is very efficient, for a kerosene engine, but it is still a far cry from cryogenic hydrolox. It is also far bigger and stronger than it needs to be. SpaceX simply doesn't have an alternative (yet).

As such, it's simply a matter of losing dV to the second stage's own weight and inefficiency. That means you can't taxi as much mass through space as other upper stages can.

That's actually a reason why NASA launches so many deep space missions on the Atlas V: the Centaur cryogenic upper stage is America's best (and quite possibly the best in the world) earth departure stage. No other gives you that much payload to earth escape per dollar invested. Part of that is the fact that it has an engine with 465 Isp. Compare that to the Merlin Vacuum at 342. Even if SpaceX were to offer much lower prices, the Falcon 9 likely cannot fly these kinds of missions at all. NASA is considering the Falcon Heavy for some missions, largely because a light payload on that giant rocket can get enough dV even when relying on kerosene.

I'm actually wondering what SpaceX's plans for upper stages are. It's surprisingly quiet on that front. We keep hearing juicy bits about the Raptor methane engine, and methane can give easily give 10% more Isp than kerosene without being any more difficult to handle, but the Raptor is waaaaay too strong. Heck, it would be too strong for the F9 as a singular first stage engine. Unless SpaceX's plan is to retire the F9 in favor of something bigger soon - and with the Falcon Heavy relying on F9 cores, that's extremely unlikely - they'll eventually need a new engine of some description that is not called Raptor.

Maybe we'll see a methane-based Merlin 2 generation eventually.

Edited August 13, 2014 by Streetwind

[Agamemnon]

the reason being that it uses quite crappy upper stage engine - quite low specific impulse for an upper stage - for m1d vac it is only 340 s... and the more energetic orbit (gto, geo, escape) you are targeting the better isp is required or you are getting quite harsh payload penalties...

ariane 5 or atlas 5 uses hydrolox upper stage engines with specific impulse around 440-450 s (btw - russian staged combustion kerolox upper stage engines have isp close to 360 s)

so... using better upper stage engine would help (eg. ariane, atlas, delta)... using 3rd stage would help too (eg. proton)

[Agamemnon]

The Merlin 1D Vacuum is very efficient, for a kerosene engine

it is effiecient for a gas generator kerolox...

not so much for a kerolox in general - eg. there is a staged combustion kerolox engine (routinely used today) optimised for high thrust (not for isp) and for use on first stages that has nearly the same isp as merlin 1d vac optimised for high isp (the engine is rd-180 with vac isp 338 s)

there is an isp optimized kerolox engine used in soyuz 2-1b upper stage with isp 359 s (rd-0124)

[Sky_walker]

Part of that is the fact that it has an engine with 465 Isp.

It's ~450, not 465.

465 is an Isp of RL60, engine that never been flown on any rocket.

Edited August 13, 2014 by Sky_walker

[K^2]

Take pure equatorial launch. That's 465m/s of surface velocity, and lets take 1.5km/s for atmo/gravity losses. You need about 8km/s to launch to a 400km LEO. So the total delta-V is about 9km/s. Direct launch to GTO is about 10.3 km/s. So that's 11.3km/s of delta-V.

LEO Falcon 9 v1.1 mass ratio is 38.5. With delta-V of 9km/s, that gives us effective I_SP of 251s.

GTO mass ratio of v1.1 is 104.3. With delta-V of 11.3 km/s, that gives us effective I_SP of 248s.

Given that I've used less precision in my math above, both of these numbers are equal. Falcon 9 is equally effective in reaching LEO and GTO.

[magnemoe]

The reason that the GEO payload fraction is so low is because the second stage is a kerosene stage with an overpowered engine. The Merlin 1D Vacuum is very efficient, for a kerosene engine, but it is still a far cry from cryogenic hydrolox. It is also far bigger and stronger than it needs to be. SpaceX simply doesn't have an alternative (yet).

As such, it's simply a matter of losing dV to the second stage's own weight and inefficiency. That means you can't taxi as much mass through space as other upper stages can.

That's actually a reason why NASA launches so many deep space missions on the Atlas V: the Centaur cryogenic upper stage is America's best (and quite possibly the best in the world) earth departure stage. No other gives you that much payload to earth escape per dollar invested. Part of that is the fact that it has an engine with 465 Isp. Compare that to the Merlin Vacuum at 342. Even if SpaceX were to offer much lower prices, the Falcon 9 likely cannot fly these kinds of missions at all. NASA is considering the Falcon Heavy for some missions, largely because a light payload on that giant rocket can get enough dV even when relying on kerosene.

I'm actually wondering what SpaceX's plans for upper stages are. It's surprisingly quiet on that front. We keep hearing juicy bits about the Raptor methane engine, and methane can give easily give 10% more Isp than kerosene without being any more difficult to handle, but the Raptor is waaaaay too strong. Heck, it would be too strong for the F9 as a singular first stage engine. Unless SpaceX's plan is to retire the F9 in favor of something bigger soon - and with the Falcon Heavy relying on F9 cores, that's extremely unlikely - they'll eventually need a new engine of some description that is not called Raptor.

Maybe we'll see a methane-based Merlin 2 generation eventually.

yes the part about taking the large second stage up to GEO was my point and why i thought about a 3rd stage but I also see how H2 with it high ISP is better for upper stages but is harder to handle.

[Streetwind]

It's ~450, not 465.

465 is an Isp of RL60, engine that never been flown on any rocket.

Ooops, you're correct. I knew the RL-10 is being used on the Centaur, and I knew there exists the RL-10-B variant with >460 Isp. However, it turns out that the Centaur indeed only has a 451 Isp variant, whereas the RL-10-B flies on the Delta Cryogenic Second Stage instead.

The Centaur is still cheaper per kg to earth escape than the DCSS though. There was an interesting presentation somewhere recently where various upper stages were compared in that regard.