Author Topic: Mars Crew Landers - Two Stage or Single Stage?  (Read 17785 times)

Offline Robotbeat

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Re: Mars Crew Landers - Two Stage or Single Stage?
« Reply #60 on: 01/10/2017 02:35 AM »
A first step to estimating a spartan ascent vehicle dry mass is to look at the dry mass of the Apollo LM including both stages. That had enough delta-v to get into LMO (ignoring T/W for the moment). Probably can fit 3 people in it, in a pinch.

The Apollo LM completely dry had a dry mass of about 4t. I think if you combined it into a single stage with a small pumpfed motor and no legs, you could probably do it in 2 tons with modern materials. 1 ton if you make the astronauts go just in their spacesuits* protected by a windshield (no pressurized cabin). Low enough to land it with a modified Skycrane. :)


*This is why they don't let me make these decisions. :D
« Last Edit: 01/10/2017 02:36 AM by Robotbeat »
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Offline Robotbeat

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Re: Mars Crew Landers - Two Stage or Single Stage?
« Reply #61 on: 01/10/2017 02:40 AM »
Problem with your analysis is you assume same dry mass for both methane and hydrogen. Very poor assumption.

Not really.  The same design for hydrogen could store either oxygen or methane easily (but the inverse not so much) and insulate it well; the engine to burn the fuel is the real issue.  Also it is now known water is fairly common on Mars; both hydrogen and methane could be manufactured from it.  Since both propellants can be obtained using water, it is reasonably possible either could be utilized.
Methane is far denser so the propellant tank can be much smaller. Hydrogen tanks are huge and need better insulation. Pressure vessel mass is proportional to volume. Rocket tanks are pressure vessels. Also, rocket engine thrust, for a given turbopump capacity, is roughly proportional to bulk density so again, you're hurt by engine mass.

And larger objects require longer braces which are more massive.

Therefore methane will almost certainly give you a lower dry mass.

So, actually, YES really. :)
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Russel

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Re: Mars Crew Landers - Two Stage or Single Stage?
« Reply #62 on: 01/16/2017 01:22 AM »
I simply don't believe that a bare bones ascent vehicle has to weigh anywhere near that of the LEM. There's so many things (batteries for instance) that are a lot lighter now. Not to mention basic structure.

Offline Russel

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Re: Mars Crew Landers - Two Stage or Single Stage?
« Reply #63 on: 01/16/2017 01:31 AM »
I got a lot of flak a while back for suggesting the crew just use their space suits (no pressurised capsule). Two things changed for me. One was the realisation that there are good reasons for a capsule (when things go wrong). The other is that a capsule doesn't have to add too much mass. Indeed it can double as part of the structure. It only has to be rated to half an atmosphere. So I got used to thinking in terms of a capsule and designing around it.

Offline Robotbeat

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Re: Mars Crew Landers - Two Stage or Single Stage?
« Reply #64 on: 01/16/2017 03:05 AM »
The Apollo LM was just a third of an atmosphere. I honestly think we'd be hard pressed to improve on the Apollo LM dry mass for the main cabin unless we reduced its capabilities. Safety folk would probably want twice the pressure as the LM nowadays, for instance. You had people literally grinding off chunks of it with a file just to save a few grams.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Russel

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Re: Mars Crew Landers - Two Stage or Single Stage?
« Reply #65 on: 01/24/2017 02:14 AM »
If you've got the design detail on the LM. All the components. Equipment. Batteries. Structural design. Material type and thickness. Then I'd be happy to go through it with you. I don't have these details. All I have is rough estimates of what it would take to build a pressurised space using modern materials - hundreds of Kg. And some pretty good ideas on things like batteries, power conversion, etc..etc.
« Last Edit: 01/24/2017 03:19 AM by Russel »