2
$\begingroup$

The Orion spacecraft which is to be used for the Artemis missions will be rated for 6 astronauts. Why so many? Given the "political" assignment is to put "a woman and another man to the Moon" why don't we just build a spacecraft for a single astronaut, fly it twice and later develop a new spacecraft when more people need to fly?

The best reason that comes to my mind is that the spacecraft development cycle is too long (too expensive). Thus it may actually be cheaper today to build one big spacecraft then to develop 2 different ones. If there are plans to send more humans to the Moon in the future we will develop a single spacecraft and use it for all missions even if the capsule is half empty for the initial missions. Is this true?

$\endgroup$
7
  • 3
    $\begingroup$ @kozuch - suggest removing the 3D printer part of the question as off topic to crew size and stack exchange being about single question+one best answer. If you want you can ask it as a separate question but the short answer is 'what does 3D printing solve over conventional fabrication?'. 3D printed engines are thing because of the very complicated design of cooling channels and continuously varying thickness that do not apply to a capsules. $\endgroup$ – GremlinWranger Jan 16 at 13:38
  • 4
    $\begingroup$ Especially the last paragraph sounds more like a rant than an honest question. $\endgroup$ – user2705196 Jan 16 at 14:30
  • 1
    $\begingroup$ I think your discussion of 3D printing was attracting down votes to an otherwise interesting question with good answers. I've deleted it and added an upvote. You can still find it in the edit history; why not ask it as a new question? I think it will also have good and interesting answers! $\endgroup$ – uhoh Jan 16 at 22:22
  • 1
    $\begingroup$ Claiming that NASA just wants to land two people on the Moon only for a publicity stunt is wrong. They don't want to repeat Apollo, the world already knows they can do it. Instead, as NASA has stated innumberable times, they want to go back to the Moon, "this time, sustainably and to stay". If you want to be needlessly pessimistic, you can assume the true goal of the program is to funnel NASA money into big aerospace's pockets, not launch people. $\endgroup$ – Dragongeek Jan 29 at 0:18
  • 2
    $\begingroup$ I remember being asked why I thought the LBJ space center was in Houston. My fourteen year old brain starting thinking about orbital angles and redundant communication when the asker told me that it was to win LBJs support for the program by bringing money to the state. The asker? The TOUR GUIDE at the LBJ Space Center. $\endgroup$ – Chris B. Behrens Jan 29 at 0:36
10
$\begingroup$

The Orion capsule can carry 6 because that is what the NASA requirements it was designed to satisfy asked for.

Unbelievably these requirements date from 2004. Orion then was supposed to be a multi-purpose vehicle taking crew to the ISS as well as lunar and planetary destinations.

~16 years later it still hasn't flown with a crew.

https://en.m.wikipedia.org/wiki/Crew_Exploration_Vehicle

$\endgroup$
2
  • $\begingroup$ "16 years later it still hasn't flown with a crew." The technological legacy they have to keep is just terrible. This is the long development cycle I am talking about. Look at technology advancements in 16 years (computing, materials etc.). Sure they can swap the computers for newer ones but what about materials and manufacturing? Not so much. $\endgroup$ – Kozuch Jan 16 at 14:00
  • 3
    $\begingroup$ It's a sad result of NASA's post-shuttle flailing about. See space.stackexchange.com/q/31746/6944 $\endgroup$ – Organic Marble Jan 16 at 14:04
6
$\begingroup$

Leaving aside the very valid political elements of the Orion design choices.

As noted in the question, many costs/weights in manned space flight are fixed per craft regardless of crew size. A computer,antenna or sensor does not know how many crew there are and weigh the same. Some are also impact by square cube law, so doubling the internal space of a capsule does not double the mass of the hull surface area to contain it.

The actual consumables mass for adding additional humans is also relatively low element of overall mass, only a couple of kg per person per day and easy to turn into a double length mission for half the crew, much more easily than doubling endurance of a single seater.

The rocket to carry the capsule also do not directly scale, so building an engine twice as big does not necessarily make it twice as expensive (though having enough engines to survive a failure is nice), and many launch/mission costs are fixed or nearly so regardless of size at least up to the point where building/moving the thing around on the ground is a problem.

In practice there seems to be a sweet spot for rocket design smaller than Apollo but larger than Titan/Gemini where materials humans have access to intersect with square cube law to make reasonably efficient rockets per kg to orbit. And if you stick a single person capsule on that size rocket it starts to look a bit wasteful, and easier to scale up the capsule than scale down the rest of existing rocket infrastructure.

All of this means that making a smaller manned rocket is not necessarily much cheaper to design.

Redundancy/safety is also much easier to add to a multi crew vehicle, for example one spare set of life support fixtures/plumbing etc in a six person capsule is a much lower% of total mass than in a single person capsule. The biggest redundancy boost is in the crew themselves. The 2 person Apollo crew size was locked in very early in the progam, in large part based on the increased safety in having another set if hands in event of injuries/illness.

In addition much of the cost of a crewed rocket is in man rating. A new vehicle regardless of crew size would need to fly at least once empty, plus do a number of abort tests in various shapes (plus lots of man hours in contingency planning/modeling) so flying two single crew moon missions might involve building and destroying a dozen in tests before being human rated.

So even if not ideal for a given lunar mission a 6 person capsule that has flexibility for other missions may turn out cheaper in the long term assuming sensible choices.

It is of course possible to overdo it, with feature creep being the death of many many tech projects, so possibly making return to moon a single seater would have meant it flew years ago.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.