Mün landing , TWR needet?

[Sereneti]

Hi
I plan to make a small Mün vehicle (only LKO to mün and back)

What TWR is needet to  orbit and  land on the mün?

 

[Warzouz]

It depends on what you're easy with. you can land on Mun with TWR=1.2 It'll be touchy, but doable.

If you want easy landing have a TWR>2.

Check Kerbal-X

[FungusForge]

As long as its TWR is something more than 0.2 (in Kerbin gravity mind you) the lander should be fine. As for getting there, as long as you're in orbit TWR doesn't matter terribly, it just makes the maneuvers faster to have a high TWR.

[Sereneti]

so its "good" to archive a TWR (ore a overal acceleration from 2m/s² ) ...

50 minutes ago, FungusForge said:

As long as its TWR is something more than 0.2 (in Kerbin gravity mind you) the lander should be fine. As for getting there, as long as you're in orbit TWR doesn't matter terribly, it just makes the maneuvers faster to have a high TWR.

if the twr is to low, its not possible  to got into an orbit, and the flight is only a flyby.. :/

Thank you

(i want to build a "proffesional" drone, not more " it works" drone, with a engine, "that weight more then the whole vessel"...)

Edited May 11, 2016 by Sereneti

[RizzoTheRat]

The higher the TWR the easier the landing is in general.  With a very low TWR you need to adjust your ships angle to maintain a low (or zero) sink rate as you shed horizontal velocity, and it's tricky to work out where you'll land because it can take a long time to shed that speed and occasionally mountains get in the way.  With a high TWR you just burn retrograde to get an impact point a bit beyond where you want to land, and then suicide burn to stop before you hit the surface, but a high TWR means you're carrying more engine and therefore need more fuel so a heavier ship, bigger launcher, etc.  I'm with Warzouz that 2 is probably about as low as you want to go.

My first munlander of this career didn't need a separate transfer stage and had a Mun TWR of over 5 when it left Kerbin SOI so I'm guessing it would have been 8-10 when it landed.

[GoSlash27]

1G acceleration on the Mun's surface is 1.63 m/sec². Figure 2Gs local for a good balance between weight and engine mass, That's 3.26 m/sec²; about 1/3 G on Kerbin.

Best,
-Slashy

[Wemb]

The answer is not much , but it's probably more than you think - the reason being that while the mun has no atmosphere a very slow ascent with a TWR of near to one will not necessarily be less efficient than using  a higher power engine since because there is no atmosphere, you don't need to do it quickly - BUT  landing definitely is time bound - you need enough time to reduce horizontal speed and reduce vertical speed at the same time (in most scenarios), and you also need to time to spare and time to recover from unexpected mountains.

For all I know (not much) this could be one of the reasons why it was beneficial for the real Lunar Module  to use a separate lander and ascent stages (though there were plenty of other reasons, mostly weight and reliability issues, I bet, that were probably more important)

Wemb

[wumpus]

2 hours ago, Wemb said:

For all I know (not much) this could be one of the reasons why it was beneficial for the real Lunar Module  to use a separate lander and ascent stages (though there were plenty of other reasons, mostly weight and reliability issues, I bet, that were probably more important)

Wemb

What I've heard is that destroying the accent stage of the lunar lander was the most likely way of Apollo astonauts dying.  Another issue is that the descent engine had to be extremely throttleable (almost unheard of for rocket engines).  My guess is that they were willing to make the descent engine single use (and not care if it breaks on landing) but needed to make the ascent engine stronger and lighter.  I was expecting a solid rocket (for reliability) but instead they used a pressure-fed liquid rocket (although using a hypergolic pressure fed system might be more reliable than solid igniters) with an ISP of 322 (which may have precluded using a solid).  In any even none of these are issues with KSP (you can simply jettison side booster tanks on the Mun).

Another issue for Mun landing is finding a flat area to land on (when I started with .2n, the Mun was a lot flater and easier.  I make a point of suggesting landing on Minmus first regardless of where career mode sends you).  A TWR of 1.2 might be theoretically possible, but don't count on being able to change your landing zone.  This might work if you have an unmanned rover find an area for you and attempt to land right on top of it, but make your plans beforehand.  Also make sure there is absolutely nothing higher than the landing zone on the approach, or you will crash into it on an ideal landing course (I couldn't manage this on the flats of Minmus... I hit the slopes going down to it).  Apollo sent plenty of missions to map the moon and further examine every landing site.  Keep that in mind.

[Fearless Son]

6 minutes ago, wumpus said:

Another issue for Mun landing is finding a flat area to land on (when I started with .2n, the Mun was a lot flater and easier.  I make a point of suggesting landing on Minmus first regardless of where career mode sends you).  A TWR of 1.2 might be theoretically possible, but don't count on being able to change your landing zone.  This might work if you have an unmanned rover find an area for you and attempt to land right on top of it, but make your plans beforehand.  Also make sure there is absolutely nothing higher than the landing zone on the approach, or you will crash into it on an ideal landing course (I couldn't manage this on the flats of Minmus... I hit the slopes going down to it).  Apollo sent plenty of missions to map the moon and further examine every landing site.  Keep that in mind.

My suggestion to many players new to the Mun is to aim for the large, dark craters visible on its distant surface.  Even though they are relatively flat, they have plenty of smaller craters inside them.  Ideally you want to aim to land not in one of those nearer craters, but next to one.  The slope of a crater can play hell with the landing, especially if your angle is bad, and the low gravity can make a craft tip easily if its center of mass is even moderately high unless the landing surface is extremely level.  

[Norcalplanner]

In my guide (link in sig) I recommend an initial TWR between 3 and 5 for the body you're landing on. For the Mun specifically, a Rockomax 16 and a Terrier makes a great power section for a manned lander. This combination has a wide base, low part count, and lots of Delta V for biome hopping.

[moogoob]

I've heard that a higher TWR is actually more efficient when doing suicide burns, though I suppose that there's likely a point at which a high engine mass might start to make that less of a useful option.

[Warzouz]

58 minutes ago, moogoob said:

I've heard that a higher TWR is actually more efficient when doing suicide burns, though I suppose that there's likely a point at which a high engine mass might start to make that less of a useful option.

Well, it's efficient to do a suicide burn with a high TWR lander. But again, efficient in what ? Delta-V, mass, cost ?

For example, I packed a 7500m/s delta-V into a 750kg probe Mun-capable (TWR was 1.3 to 1.5), it was very mass efficient, but I probably lost a lot of delta-V due to gravity loss because of that low TWR.

Edited May 12, 2016 by Warzouz

[moogoob]

7 minutes ago, Warzouz said:

Well, it's efficient to do a suicide burn with a high TWR lander. But again, efficient in what ? Delta-V, mass, cost ?

For example, I packed a 7500m/s delta-V into a 750kg probe Mun-capable (TWR was 1.3 to 1.5), it was very mass efficient, but I probably lost a lot of delta-V due to gravity loss because of that low TWR.

Good point - your lander may not have been so efficient, but you probably saved tons of funds in terms of launcher cost/size/mass due to the rocket equation.

[Alshain]

Suicide burn is not the best landing though.  It's just a more efficient way to burn in a very inefficient landing method.  To line up for the fall straight down so you can do a suicide burn, you have to expend an awful lot of fuel.

 

Slash has a tutorial on the best way to land here:

Eventually you get to a point where you don't need to do all the maneuver node stuff, you will be able to 'eyeball' it pretty well.  Back when I learned to do it, I never thought to use a maneuver node so I've not really ever done it exactly as he does, but it's still a good tutorial.

Edited May 12, 2016 by Alshain

[Warzouz]

 

14 minutes ago, Alshain said:

Suicide burn is not the best landing though.  It's just a more efficient way to burn in a very inefficient landing method.  To line up for the fall straight down so you can do a suicide burn, you have to expend an awful lot of fuel.

 

Slash has a tutorial on the best way to land here:

Eventually you get to a point where you don't need to do all the maneuver node stuff, you will be able to 'eyeball' it pretty well.  Back when I learned to do it, I never thought to use a maneuver node so I've not really ever done it exactly as he does, but it's still a good tutorial.

@Goslash27 method is less efficient than suicide burn. It's a compromise between a suicide burn and a "flat landing/zero descent" used for low TWR landers. Slash is ambiguous about terms. A "reverse gravity turn" IS a "suicide burn". The technique he describes is not a reverse gravity turn. In a gravity turn, after giving your ship a bump, it is suppose to follow prograde on its own.

His technique is also very nice for precise landing. I used it and it worked very well. But it's less delta-V efficient. For example, for letting the target always be at the impact point you have to burn partially radial. This is not efficient but convenient.

[Alshain]

3 minutes ago, Warzouz said:

 

@Goslash27 method is less efficient than suicide burn. It's a compromise between a suicide burn and a "flat landing/zero descent" used for low TWR landers. Slash is ambiguous about terms. A "reverse gravity turn" IS a "suicide burn". The technique he describes is not a reverse gravity turn. In a gravity turn, after giving your ship a bump, it is suppose to follow prograde on its own.

His technique is also very nice for precise landing. I used it and it worked very well. But it's less delta-V efficient. For example, for letting the target always be at the impact point you have to burn partially radial. This is not efficient but convenient.

Well I agree with the name being a misnomer (I've mentioned that before) but it is definitely more fuel efficient than a suicide burn.

Edited May 12, 2016 by Alshain

[Warzouz]

57 minutes ago, Alshain said:

Well I agree with the name being a misnomer (I've mentioned that before) but it is definitely more fuel efficient than a suicide burn.

Well I'd like to see some real data. That would mean the most efficient way to takeoff is to fix your start point on your take-off location (even you burn radial, and keep you insertion node half of your total dV to reach orbit. Which is very far from what is explained here.

The Slash method, when you don't aim for a target is basically a suicide burn with a very high deorbit burn which brings you to a very non horizontal trajectory. On the opposite, you want to set your landing point as shallow as possible to only fight horizontal speed, until the last moment. That reduces gravity loss. Shash explain how to aim which include radial burns which is again losses (you fight gravity more to maintain your altitude for a longer time).

But if you have precise data, I very would like to compare both methods.

 

 

[Alshain]

1 minute ago, Warzouz said:

Well I'd like to see some real data. That would mean the most efficient way to takeoff is to fix your start point on your take-off location (even you burn radial, and keep you insertion node half of your total dV to reach orbit. Which is very far from what is explained here.

The Slash method, when you don't aim for a target is basically a suicide burn with a very high deorbit burn which brings you to a very non horizontal trajectory. On the opposite, you want to set your landing point as shallow as possible to only fight horizontal speed, until the last moment. That reduces gravity loss. Shash explain how to aim which include radial burns which is again losses (you fight gravity more to maintain your altitude for a longer time).

But if you have precise data, I very would like to compare both methods.

Making your orbit perpendicular to the planet in order to line up a suicide burn costs a heck of a lot of fuel. You are welcome to try both methods and gather the data you need, I would certainly be interested to see it.  I'm not spending the time to do it myself, I already know which is better.

[Plusck]

4 minutes ago, Alshain said:

Making your orbit perpendicular to the planet in order to line up a suicide burn costs a heck of a lot of fuel. You are welcome to try both methods and gather the data you need, I would certainly be interested to see it.  I'm not spending the time to do it myself, I already know which is better.

I think you are mistaken about the meaning of "suicide burn" as it is used generally here and elsewhere. I think I included a post about this towards the end of Slashy's thread...

Suicide burn does NOT mean killing horizontal velocty then falling to the surface. Suicide burn means you start burning the minimum time from impact on an angled approach. The more acutely angled that approach, the more efficient, and the less suicidal it becomes (since you could add radial to escape death if you are too late).

[Norcalplanner]

53 minutes ago, Plusck said:

I think you are mistaken about the meaning of "suicide burn" as it is used generally here and elsewhere. I think I included a post about this towards the end of Slashy's thread...

Suicide burn does NOT mean killing horizontal velocty then falling to the surface. Suicide burn means you start burning the minimum time from impact on an angled approach. The more acutely angled that approach, the more efficient, and the less suicidal it becomes (since you could add radial to escape death if you are too late).

This is what I do, whatever label happens to be applied to it. It's nearly as efficient as a true horizontal landing, generally only requiring small adjustments of +5 or +10 degrees of radial compared to the true retrograde surface vector. Furthermore, by having the craft on a suborbital trajectory, you can access helpful information from mods like MechJeb, Trajectories, or Better Burn Time which show you where youre going to land, time to impact, and a suicide burn timer.

[Warzouz]

1 hour ago, Alshain said:

Making your orbit perpendicular to the planet in order to line up a suicide burn costs a heck of a lot of fuel. You are welcome to try both methods and gather the data you need, I would certainly be interested to see it.  I'm not spending the time to do it myself, I already know which is better.

That's not a "suicide burn". What you describe is truly totally inefficient though. In a suicid burn your "perpendicular" orbit is nearly no existent (well it's your safety margin...)

To do a suicide burn use KER or use this method :

• Do a light deorbit burn (for Mun, around 20 to 50 m/s from 20km)
• Where your trajectory touch the ground, create a node and pull it retrograde until it flips
• The burn time gives you a gross approximation** on when you need to start burning retrograde.
• Start burning retrograde full when the time to node is equal to you burn time and keep retrograde.
• When you start burning, remove the node, you don't need it, you must only focus on your surface velocity.

If your burn is timed correctly you should get a near 0 speed not far from the ground. If you are too high, that mean you started burning to early (and waste fuel to gravity loss, because you fall vertically in the end). But that's better than too late which crashing into the ground.

** I noticed that with a higher TWR you can burn later.

But again, for more precise suicide burn, KER gives you nice data as well as @Snark mod (better burn time). If you use those, you can readjust your timing and temporarily stop burning.

Suicide burn is not possible with too low TWR because you vertical speed may become uncontrollable

[GoSlash27]

9 hours ago, moogoob said:

I've heard that a higher TWR is actually more efficient when doing suicide burns, though I suppose that there's likely a point at which a high engine mass might start to make that less of a useful option.

Moogoob,

It is, and it also isn't. It's more efficient in the sense that infinite t/w is ideal for maneuvers with an absolute minimum of DV waste. *But* considering that t/w costs mass by way of extra engine mass and quickly outstrips the operator's ability to control it with precision and efficiency... my experience and calculations show that the cheapest and most mass- efficient designs will be under 2:1 t/w local.

 Ultimately you have to use enough t/w to operate safely with your preferred technique, but I really don't recommend "moar boosters" beyond that point.

Best,
-Slashy

 

 

[Warzouz]

@Alshain,

After rereading you posts and mine, I must correct my point of view. What you described and what I describe are BOTH suicide burns.

BUT, to do a efficient suicide burn, you need to burn most of your fuel as low as possible from the ground.

• You suggested to cancel your orbital speed then fall down vertically* : this is very inefficient, I agree with you
• I suggest to do the lightest reasonable** deorbit burn, then fall down (mostly horizontal)

* This is even not a good option for new players because it's quite difficult to escape from a vertical fall when you discover that your landing spot is not great (high slope). If you're more horizontal, a light radial burn will quickly offer you another spot for a low dV.

** If you're too shallow you have more risk to hit ground features before your targeted landing spot. The most efficient deorbit burn would be when you PE touch ground on the other side of the body you want to land on (180°). But doing that, your nearly sure to crash into something before.

I usually target ground around 120° from my deorbit burn. If terrain is harder, like Bop, it's more like 60°.