Two-launch scheme for manned Lunar mission

[Teilnehmer]

I mean two-launch schemes when a reentry-capable manned spacecraft is launched separately from the Lunar lander with a subsequent Lunar orbit rendezvous (or maybe an Earth orbit rendezvous). Why were they rejected by both NASA and Soviets? What are downsides of those schemes in comparison to Apollo/N1-L3-style ones?

The upside is an almost 2 times lighter rocket needed. This could be tempting, especially for the Soviets who experienced lots of problems with N-1.

Edited April 27, 2018 by Teilnehmer

[YNM]

Needs two launch site.

Needs two crews (ground, control etc).

 

Such option is only viable once fast launches is possible (ie. Now), or when you're too scared of failing (ie. when they fixed Hubble the last time).

[Teilnehmer]

Why does it need two launch sites?

An unmanned lander can be launched from the same launch site months before the crewed module. Not having any cryogenic fuels on board, the lander can wait for a rendezvous in a Lunar orbit for an arbitrary time.

Edited April 27, 2018 by Teilnehmer

[Tullius]

But then the lander needs to be able to survive for a few months in lunar orbit, i.e. have solar panels etc. On the other hand, Apollo did it with only batteries and fuel cells.

[Teilnehmer]

Good point.

Anyway, building a super-heavy rocket seems a bit more complicated task than adding some solar panels.

[YNM]

32 minutes ago, Teilnehmer said:

Why does it need two launch sites?

Gemini and Agena Target Vehicle.

5 minutes ago, Teilnehmer said:

Anyway, building a super-heavy rocket seems a bit more complicated task than adding some solar panels.

Say that in the 50s, everyone laughs at you.

34 minutes ago, Teilnehmer said:

Not having any cryogenic fuels on board, the lander can wait for a rendezvous in a Lunar orbit for an arbitrary time.

... And you're left with a cracked fuel tank. Hypergolic stuff are nasty.

[Tullius]

6 minutes ago, Teilnehmer said:

Anyway, building a super-heavy rocket seems a bit more complicated task than adding some solar panels.

Saturn V was at the time the largest American rocket by quite some margin. Choosing a two-launch scheme would have meant building a half-size Saturn V, which would have still been the largest American rocket. Only problem now is that you need to build and launch twice as many rockets.

[sevenperforce]

Unless you're already flying with a pretty high launch cadence, a two-launch scheme is extremely sensitive to timing and requires twice as many orbital rendezvouses. In the Apollo era, we hadn't yet gotten REALLY good at orbital rendezvous quite yet.

Another issue with meeting up in lunar orbit is that you need two different lunar orbit insertion burns, one for each vehicle. Which means both vehicles (e.g., your unmanned lander and your manned return vehicle) need to have fuel tank capacity specifically for braking into lunar orbit, which makes both of them bigger than they need to be. Apollo used the return vehicle's (oversized) engine and tankage for the LOI burn, meaning the lander only had to perform the landing, while Constellation planned to use the lander's engine and tankage for the LOI burn, meaning the return vehicle only had to perform the return.

Plus, as @Tullius said, you still need a big rocket but now you need twice as many. Which means either you need VERY fast pad turnaround, or you need multiple pads.

There are several possible architectures for a flags-and-footprints moon mission:

• Direct ascent. Big rocket goes to Earth orbit, goes to lunar orbit, goes to lunar surface, and comes back, dropping stages all the way. See Nova.
• Earth orbit rendezvous. Small rockets construct a big rocket in Earth orbit, which then goes to lunar orbit, goes to lunar surface, and comes back, dropping stages all the way.
• Lunar orbit rendezvous. Big rocket goes to Earth orbit, goes to lunar orbit, and then a lander breaks off, goes to the lunar surface, comes back to lunar orbit, and the original vehicle comes back to Earth. See Apollo.
• Earth-lunar orbit rendezvous. Small rockets construct a big rocket in Earth orbit, which goes to lunar orbit and then does everything that the lunar orbit rendezvous would have done. See Constellation.
• Joint lunar orbit rendezvous. Two separate rockets go separately to Earth orbit and then to lunar orbit, meet up, and then do everything that the lunar orbit rendezvous would have done. This is possible now via distributed launch.

[Bill Phil]

5 hours ago, Tullius said:

But then the lander needs to be able to survive for a few months in lunar orbit, i.e. have solar panels etc. On the other hand, Apollo did it with only batteries and fuel cells.

The biggest problem with surviving in lunar orbit for months is the instability of most lunar orbits. You can put a spacecraft in hibernation for quite a while. Of course you still need a relatively big rocket.

Fuel cells are arguably more complex than solar panels. 

[Canopus]

4 minutes ago, Bill Phil said:

The biggest problem with surviving in lunar orbit for months is the instability of most lunar orbits. You can put a spacecraft in hibernation for quite a while. Of course you still need a relatively big rocket.

Fuel cells are arguably more complex than solar panels. 

This is exactly the reason why many Lunar landing architectures since the retirement of STS make use of Lunar langrange points or other stable lunar orbits. Stuff like the Exploration gateway and LOP/G. They basically trade station keeping delta v for additional Delta v needed to land from higher orbits, but at the same time allow for longer wait times between launches.

[kerbiloid]

In 1965 there already were 2 launchpads for UR-500. Later - 2 launchpads for N-1 (and UR-700 if it were built). So, not such a problem with twin launch.

Edited April 27, 2018 by kerbiloid

[YNM]

6 hours ago, kerbiloid said:

In 1965 there already were 2 launchpads for UR-500. Later - 2 launchpads for N-1 (and UR-700 if it were built). So, not such a problem with twin launch.

Don't think they can hold two N-1 in their hangar...

[magnemoe]

15 hours ago, Tullius said:

Saturn V was at the time the largest American rocket by quite some margin. Choosing a two-launch scheme would have meant building a half-size Saturn V, which would have still been the largest American rocket. Only problem now is that you need to build and launch twice as many rockets.

This, note that we are discussing this now because of falcon heavy and new glen. who makes an dual launch moon mission plausible.
And it would be the way I would design an moon mission today.
Back in the 60s making one rocket was smarter. 

[mikegarrison]

But once you change focus from the lander it goes "on rails" and nothing bad can happen to it, so this shouldn't be a problem. I mean, if it works in KSP it must work IRL, right?

[YNM]

12 hours ago, Bill Phil said:

The biggest problem with surviving in lunar orbit for months is the instability of most lunar orbits.

Ah, right.

We didn't even knew we were able to land on the Moon, so much so we needed an impactor sent first !

 

But yeah. Let's not forget we were talking of the 50s, 60s and 70s. Brave, yes; Knowledgeable, not really.

[kerbiloid]

4 hours ago, YNM said:

Don't think they can hold two N-1 in their hangar...

Looks not so big.

Spoiler

What if build another hangar?

[Teilnehmer]

Not so big? :O

[kerbiloid]

2 hours ago, Teilnehmer said:

Not so big? :O

Spoiler

1969, L=105 m, D =17 m, M = 3000 t.
Dry mass = 208 t.

***

Spoiler

1964, L=166 m, D =12 m, M = 18000 t.

***

http://www.depo-portal.ru/zheleznodorozhnaya-tsisterna.php

Spoiler

Dry mass = 48 t. Cargo mass = 120 t.
So empty N-1 on transporter weigths like two such cysterns.

Empty Proton weights like a main battle tank.

 

Nothing special.
Once upon a time in Soviet Russia:

Spoiler

 

Edited April 28, 2018 by kerbiloid

[wumpus]

On 4/27/2018 at 3:33 AM, YNM said:

Needs two launch site.

Needs two crews (ground, control etc).

 

Such option is only viable once fast launches is possible (ie. Now), or when you're too scared of failing (ie. when they fixed Hubble the last time).

NASA had at least two launch sites and the VAB has the room to build four rockets (don't think it has ever been used to full capacity, and half the bays go to pads that don't exist).

Gemini (and similar Soviet missions) required two rapid launches (although one was typically, but not always, unmanned).  This was a relatively proven technology by the time Saturn V was being designed.

For Earth rendezvous I'm guessing it made more sense to fire a J-2 [hyrdrolox] to the Moon (after one or two extremely low orbits) than to wait for the second rocket and spend hours docking and then firing a hypergolic rocket to the Moon.  Also it isn't clear how to break the payload up into roughly even bits: the only obvious item to lose is the lander.

Lunar rendezvous sounds just too dangerous (but both rockets could get to the Moon by hydrolox).  I know there is at least one NASA official that kept insisting that direct landing was the way to go long after the missions were a success.  I'd hate to have to perform an Apollo 13 style rescue with not only a crippled SM, but have the LM in another rocket (that can't get back on it's own, and only one astronaut aboard).

[Nibb31]

At the time Apollo was conceived, some people thought that it would be impossible to achieve the level of precision that was required to rendezvous in orbit. It was something that hadn't been proven and therefore couldn't be relied on for the baseline mission profile. Therefore Saturn was to be a precursor to Nova, which was pretty much the "Plan B" if it turned out that orbital rendez-vous was impractical.

Gemini proved that RV and docking was possible, which opened up the EOR and LOR profiles, but the Saturn V was well under way by then. It did allow them to abandon Nova.

Edited May 1, 2018 by Nibb31