Why are true-LOR used for modern plans instead of EOR?

[Jestersage]

While I understand why partial LOR is used for Saturn-V, I am wondering why all modern plans seems to go with True LOR instead of EOR.

From what I gather, only the Constellation plan using Earth Departure Stage use EOR. The rest, from Energia Lunar (aka energia LOK), to Shuttle-C, to SLS, and even modern Roscosmo utilize a complete LOR, where both Capsule and Lunar lander is sent to Lunar orbit and rendezvous there.

[Bill Phil]

Well it reduces the size of the necessary vehicle, or conversely increases the capability of the mission given a specific launch vehicle.

For example, if an Apollo mission used two Saturn V launches the lander could've been quite larger. Or perhaps the second launch carries a single stage lander while the first launch carries the crewed CSM and the LM for the crew to land and return to LLO. In such an architecture the crew could spend far more time on the surface and potentially do more science and experiments.

Shuttle-C wouldn't have been able to do what Saturn V did in one launch. It wouldn't be able to launch as much mass to TLI. Indeed, neither can SLS, it's not as powerful as the Saturn V.

[Starman4308]

I'm assuming you mean "why LOR-LOR instead of EOR-LOR*".

*LOR-LOR as in "rendezvous with the lander at the Moon, transfer crew, the lander separately lands, and rendezvous again in lunar orbit". EOR-LOR would be "rendezvous with the lander in LEO, go to the Moon, separate and perform the landing, etc". Both are "LOR" in the Apollo sense of using a separate lander.

A big part of this is probably the use of cryogenic upper stages. Every hour your vessels spend in LEO performing rendezvous and docking is an hour that you are slowly bleeding out cryogenic propellants. Easier to just send each part off separately, and use storable* propellants on the purpose-built lunar vehicles for rendezvous and docking.

*Or, at least, cryogenics in low- or zero-boiloff tanks.

[Jestersage]

1 hour ago, Starman4308 said:

I'm assuming you mean "why LOR-LOR instead of EOR-LOR*".

*LOR-LOR as in "rendezvous with the lander at the Moon, transfer crew, the lander separately lands, and rendezvous again in lunar orbit". EOR-LOR would be "rendezvous with the lander in LEO, go to the Moon, separate and perform the landing, etc". Both are "LOR" in the Apollo sense of using a separate lander.

A big part of this is probably the use of cryogenic upper stages. Every hour your vessels spend in LEO performing rendezvous and docking is an hour that you are slowly bleeding out cryogenic propellants. Easier to just send each part off separately, and use storable* propellants on the purpose-built lunar vehicles for rendezvous and docking.

*Or, at least, cryogenics in low- or zero-boiloff tanks.

Yeah, that's what I meant -- I forgot EOR may also implies assemble a lander for direct landing.

And thanks for the answer (the boil off is one of those stock-ksp-does-not-teach stuff)

Edited February 3, 2019 by Jestersage

[DDE]

I don’t think cryo is a showstopper. If need be, you can rendezvous quickly enough - look at the recent radically accelerated ISS rendezvous plans. Existing facilities for some of those launchers could support multiple simultaneous or near-simultaneous launches - Energiya had three pads, for example.

Ultimately, I think there was a distinct understatement of difficulties of EOR - namely docking huge cryogen-laden stages or misshapen pieces of the craft. It would get even worse when you had to execute zero-g propellant transfer (good luck, Elon), which many such plans required.

Ultimately, it’s easier to eat that elefant in small slices, which represent payloads delivered to LLO.

Edited February 4, 2019 by DDE

[magnemoe]

48 minutes ago, DDE said:

I don’t think cryo is a showstopper. If need be, you can rendezvous quickly enough - look at the recent radically accelerated ISS rendezvous plans. Existing facilities for some of those launchers could support multiple simultaneous or near-simultaneous launches - Energiya had three pads, for example.

Ultimately, I think there was a distinct understatement of difficulties of EOR - namely docking huge cryogen-laden stages or misshapen pieces of the craft. It would get even worse when you had to execute zero-g propellant transfer (good luck, Elon), which many such plans required.

Ultimately, it’s easier to eat that elefant in small slices, which represent payloads delivered to LLO.

Problem is if you get hold on the second launch you fail. Just launch the lander to Moon orbit, then launch crew, you want them off as fast as you can but a week delay will not kill the mission. 
 

[DDE]

1 hour ago, magnemoe said:

Problem is if you get hold on the second launch you fail. Just launch the lander to Moon orbit, then launch crew, you want them off as fast as you can but a week delay will not kill the mission. 

Well, LLO and mascon-induced drift are no jokes either. But overall, week-long holds seem unlikely when you are on a Politically Important Mission.

[sevenperforce]

ACES would conceivably be a major enabler in additional rendezvous options with its ICE and independent maneuvering capabilities.

[Racescort666]

10 hours ago, DDE said:

I don’t think cryo is a showstopper.

This paper from ULA indicates that getting LH2 boiloff down to <2.5%/day on centaur is reasonable with minimal modification. Also, as @sevenperforce indicates, ACES will also have a significant affect on heating as it allows settled coast which further reduces boiloff.

Edited February 4, 2019 by Racescort666
forgot hyperlink

[YNM]

I suppose there's an interest to actually being able to deliver permanent payloads to the surface of the Moon. An LOR mission would be a better test for such procedures, plus you might leave behind some good tanks around the Moon.

An EOR mission means that the payload will not have an integrated propulsion module. A partial LOR a la Saturn V would usually have a small footprint and lesser complexity such that it won't be a good measure.

Edited February 10, 2019 by YNM