Please delete

[Jestersage]

Please delete

Edited August 5, 2019 by Jestersage

[dire]

Are you trying to softland somewhere or are you trying to build a skycrane? A picture or craft file might help.

Here are some lander/rovers I have on Minmus refueling right now that I eventually plan to softland on Duna. Two of them are 30 tons dry, about 60 tons full wet mass.

My rules of thumb for landers:

Parachutes are cheap, easy and can be used to keep you oriented.If you are landing somewhere with an atmosphere, add some chutes.

Wheels or landing struts are a must. If you want your big, expensive lander to survive, don't try to put it down on its engine bells unless you never want to use that engine afterwards.
Have RCS at the top of the craft and not less than one big 2.5 meter reaction wheel per 30 tons. 2 plus command reaction are better.

If you want to keep your lander, use a pilot or turn off radio blackout. You are absolutely going to lose connection to KSC right when you want to start that suicide burn.

Take the suicide burn delta-v and bring at least 50% more than that. Double is better, triple if you aren't familiar with the landing.

(v1.5) Use Flight Engineer to keep an eye on your altitude over ground and paint a big red target on your landing spot. Later versions let you see your radar altitude.

(Experimental) Am experimenting with separatrons around center of mass for that final landing kick. 

Worth mentioning the obvious: Is your lander center of mass over your center of thrust when you are landing? Are your RCS thrus centered around at least one axis of your center of mass? You will need more reaction wheels the less true those things are.

Also worth mentioning: You say "Average center of mass." That's probably not your actual CoM when you're landing. In the editor set your fuel tanks to what you expect in the last ten seconds of touchdown and then look at your actual CoM with that fuel.

Edited August 5, 2019 by dire

[Jestersage]

9 hours ago, dire said:

Are you trying to softland somewhere or are you trying to build a skycrane? A picture or craft file might help.

Here are some lander/rovers I have on Minmus refueling right now that I eventually plan to softland on Duna. Two of them are 30 tons dry, about 60 tons full wet mass.

My rules of thumb for landers:

Parachutes are cheap, easy and can be used to keep you oriented.If you are landing somewhere with an atmosphere, add some chutes.

Wheels or landing struts are a must. If you want your big, expensive lander to survive, don't try to put it down on its engine bells unless you never want to use that engine afterwards.
Have RCS at the top of the craft and not less than one big 2.5 meter reaction wheel per 30 tons. 2 plus command reaction are better.

If you want to keep your lander, use a pilot or turn off radio blackout. You are absolutely going to lose connection to KSC right when you want to start that suicide burn.

Take the suicide burn delta-v and bring at least 50% more than that. Double is better, triple if you aren't familiar with the landing.

(v1.5) Use Flight Engineer to keep an eye on your altitude over ground and paint a big red target on your landing spot. Later versions let you see your radar altitude.

(Experimental) Am experimenting with separatrons around center of mass for that final landing kick. 

Worth mentioning the obvious: Is your lander center of mass over your center of thrust when you are landing? Are your RCS thrus centered around at least one axis of your center of mass? You will need more reaction wheels the less true those things are.

Also worth mentioning: You say "Average center of mass." That's probably not your actual CoM when you're landing. In the editor set your fuel tanks to what you expect in the last ten seconds of touchdown and then look at your actual CoM with that fuel.

• For secrecy purpose I will not show the picture yet until it is good. However, it will be the typical Altair/Apollo type, with a descent stafe at the bottom. Unlike Apollo/Altair type, there is no ascent stage (it is more of a base)
• Due to Mun use, parachute solution is not applicable
• regarding RCS and Reaction wheel: I am trying to minimize the usage of Reaction wheel if possible. Hence, "Short of ... reaction wheel" in the original post.
• It has more than enough delta-V, as I typically do.

So what I need to know is:

1. What does the RCS build aid means in terms of thrust and torque? Is it better to have higher torque? Lower torque? Higher Thrust? lower thrust?
2. Assume a spherical/cubical lander, with RCS as a ring around the CoM. Does shifting the RCS outward help or not?
3. Instead of RW, what is the rule of thumb for no/minimal RW builds? As stated, currently it only have a set of 4 quads around the equator/CoM of the entire lander.

[snip]

Edited August 5, 2019 by Snark
Redacted by moderator

[dire]

1. Thrust for RCS means the same thing as thrust for rockets. Take the thrust and divide by mass and you get meters per second squared acceleration. Torque is like thrust, but for rotating and spinning your craft. More of both is better up to the point where you can control your craft.

2. Yes, shifting the RCS outwards gives you more leverage, increasing your torque. It doesn't increase your thrust.

3. I don't know.The solution to your problem is reaction wheels. Since you're keeping the build a secret, I don't know what you are making available to yourself to compensate. Perhaps another solution might be to build a 3-ton lander instead of a 22-ton lander.

Since you're not a beginner you should be able to find articles like https://en.wikipedia.org/wiki/Torque, read them and understand at least the general idea of the physics KSP is simulating. Especially if you're using phrases like, "Assume a spherical cow."

Sorry I can't be more helpful to you, but perhaps if a beginner reads your question they'll find my answers helpful.

[Jestersage]

17 hours ago, dire said:

3. I don't know.The solution to your problem is reaction wheels. Since you're keeping the build a secret, I don't know what you are making available to yourself to compensate. Perhaps another solution might be to build a 3-ton lander instead of a 22-ton lander.

Really? no Reaction wheel = 3 ton?

No, that is just poor advice. "Reaction wheel only" is, quite frankly, the worst answer one can give, considering one of the master craft builder, Raptor9, did it entirely with a similar setup (also 20 something ton with only 4 RCS quads) And I had made an Apollo LEM with asymaterical CoM and it works fine, and that was a 5 ton lander.

And you can always add RCS thrusters, 1 set on top, and 1 set on bottom; Reaction wheel is benefical for low-part count build, but it is not the only solution.

I figure it out afterward by comparing the values in the torque. The answer is, for the amount of altitude adjustment, you want more torque in RCS BuildAid.

Edited August 5, 2019 by Jestersage

[dire]

[snip]

 

Edit: Oh, I did another landing while we were talking. Full training wheels, landed on the engine bell, not enough reaction wheels, not enough LFO to get more than 2 m/s dV for most of the descent. Because sometimes you gotta refuel an interplanetary engine stage and you don't want to make ten trips with your shuttle. The green tins down the middle are Snacks, not fuel; 40+ kerbals from the other two landers will chill on the Behemoth for eight or nine months on their way to Duna and they will be hungry.

Edited August 5, 2019 by Snark
Redacted by moderator

[Snark]

Some content has been removed and/or redacted due to personal remarks.  Let's remember that we're all friends here, and avoid personal criticism.

Thank you for your understanding.