Moho Lander

[Red Shirt]

Looking at a dv chart it appears only 870 dv is needed to land from 50k on Moho. So, 870 + 870 = 1740 dv required to land and ascend. That's what the chart says. Today I started from 100k with 2.3k dv and a TWR of 1.3. Even starting much higher, there was over 1k dv left at landing. TWR is now around 2. Ran out of fuel by 40 dv before being able to circularize at 20k during ascent. Craft is lander can with 10 Oscar B tanks and 6 Spider engines. DV and TWR from MJ in VAB. Any clues why 1k wasn't enough? Getting ready to leave for Thanksgiving meal so no picture.

By the way, Used Oscar B's, 5 on each side turn on side for a low and wide profile. FL-T would work the same but bulkier in appearance. Edited November 26, 2015 by Red Shirt

[Snark]

Depends a lot on your ascent profile-- are you burning efficiently? i.e. crank it over to nearly horizontal immediately on takeoff, so that you're skimming the terrain?

Your thrust seems really low. Six Spiders will give only 12 kN of thrust. A lander can + 10 Oscar B + 6 Spider = 3.03 tons, even without additional paraphernalia. Moho surface gravity is 2.7 m/s2, so which makes your weight at least 8.2 kN on Moho.

Therefore, your TWR on Moho is no higher than 12 / 8.2 = 1.47, assuming full tanks on takeoff.

That seems pretty low, you could be stacking up quite a lot of gravity loss on ascent. Edited November 26, 2015 by Snark

[Red Shirt]

I'll try a far more horizontal ascent. I was at about 45 at 12k.

[COLOR="silver"][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

[quote name='Snark']Depends a lot on your ascent profile-- are you burning efficiently? i.e. crank it over to nearly horizontal immediately on takeoff, so that you're skimming the terrain?

Your thrust seems really low. Six Spiders will give only 12 kN of thrust. A lander can + 10 Oscar B + 6 Spider = 3.03 tons, even without additional paraphernalia. Moho surface gravity is 2.7 m/s2, so which makes your weight at least 8.2 kN on Moho.

Therefore, your TWR on Moho is no higher than 12 / 8.2 = 1.47, assuming full tanks on takeoff.

That seems pretty low, you could be stacking up quite a lot of gravity loss on ascent.[/QUOTE]

Tanks were less than half full so TWR was higher.

[Snark]

[quote name='Red Shirt']I'll try a far more horizontal ascent. I was at about 45 at 12k.
Tanks were less than half full so TWR was higher.[/QUOTE]

That should help a lot. You always want the lowest, most horizontal ascent possible when there's no atmosphere (the only reason you don't do that on Kerbin is because of all that pesky air). Not only does that make optimal use of Oberth effect, but on a craft with a relatively low TWR like yours it will minimize gravity losses.

I say "low" because even if your TWR is around 2 (Moho-relative), that's still low enough for significant gravity loss if you're flying up rather than sideways. For example, if you were to take off straight up on Moho with a local TWR of 2, you'd pile up over 250 m/s of gravity loss by the time you reach 12 km altitude. [SIZE=1](Yes, I know you didn't do that, I just postulate it because it makes the math simpler.)[/SIZE] ;)

[Red Shirt]

Just got home from stuffing my face. Immediately fired up KSP, and Hyper Edited the Moho Lander to a 30k Moho orbit. Took about 1200 dv to land. Believe my landing is also suffering from 'significant gravity loss'. Changed the ascent to a far more horizontal path. Circularized at 30k and still have 600 dv surplus (plenty for rendezvous and docking). I am still 100 dv over the chart but I'm OK with the result. Thanks.

[Snark]

Woot! Glad it worked out.

For your landing, how's your suicide-burn technique? Might be able to shave off some dV there, too. Basically, you want your landing procedure to be the same as taking off, in reverse. Come in low and shallow, slam on the engine at the last possible second so that you brake to a halt right when you get to the surface. Judging that "last possible moment" is tricky, which is why it's called a suicide burn. :) There are some tricks you can do with a surface-placed maneuver node to assist with that, not sure if you're familiar with the technique.

[Red Shirt]

With the spiders thinking I can slow down quickly is suicidal. :D That said my approach was terrible. Pretty much bled off the horizontal speed and dropped almost vertical from way up, engines burning most of the way. The intent today was to test if the tiny (from my design perspective) lander was capable of completing the job. Now, since it has proved worthy, learning some tricks with maneuver nodes would be great.

[Snark]

Red Shirt said:

With the spiders thinking I can slow down quickly is suicidal.

That said my approach was terrible. Pretty much bled off the horizontal speed and dropped almost vertical from way up, engines burning most of the way. The intent today was to test if the tiny (from my design perspective) lander was capable of completing the job. Now, since it has proved worthy, learning some tricks with maneuver nodes would be great.

Here's the simple way to get a fairly efficient vacuum landing:

1. From orbit, do a burn that lowers your trajectory into the surface. Ideally, you want to be going at a fairly shallow angle when you get to the surface-- i.e. coming in mostly horizontally, not straight down. Doesn't have to be really horizontal, a descent of 20 degrees below the horizontal is fine. Basically, the closer to horizontal it is, the more efficient, but the trickier the piloting will be. Ships with higher TWR can get away with shallower angles.
2. Place a maneuver node right at the spot where your trajectory intersects the surface.
3. Drag the retrograde handle, and hold it until your projected orbit shrinks to a point and the maneuver-node marker starts flipping back and forth.
4. Note the estimated burn time. This should be available to you (i.e. not "N/A") since you just did a max-thrust burn in step 1. Or if you want it to be a more accurate indicator, install my BetterBurnTime mod.
5. Put your navball into "surface" mode. If you have SAS level 1 or higher, put your ship into "hold retrograde", otherwise you'll need to do it manually.
6. Hit F5, because the next step is the one that puts the "suicide" in "suicide burn".
7. Watch the time until the maneuver node. Wait until the time remaining is about 80% of the estimated burn time. (i.e. if your estimated burn time is 20 seconds, wait until you're around 16 seconds before maneuver node time).
8. At that point, hit your engines to max and burn your way in. (Hold surface-retrograde, don't try to hold the maneuver-node marker.)
9. Watch your relative velocity. You'll likely have a little safety margin, meaning that if you keep burning, you'll slow to a halt when you're still a fair distance above the surface. So when you slow to a speed that according to your eyeballs is safe, you can back off the throttle.
10. Manually land.

 

[Red Shirt]

Awesome. Copied this into Word so I'll have it tomorrow while the forum is down. I'll let you know how it goes. I'll have a look at your mod as well when I download 1.05.

[Snark]

[quote name='Red Shirt']...so I'll have it tomorrow while the forum is down...[/QUOTE]

Yeah, gonna be mighty quiet on Black(out) Friday...

[Warzouz]

If you have a very low TWR you can't really go fully horizontal or you'll crash into the ground. You have to burn partially radially to keep vertical speed positive and increasing horizontal speed.

But that's true that vertical speed does only have to be slightly positive.

Anyway, beware of mountains at low altitude, especially on Bop (I nearly crashed into one even on a 45° ascent angle...). :D


EDIT : BTW, I always overfuel lander by a lot to allow multiple hops and science gathering. My typical lander as 3000m/s. When targeting nice landing zone, you can explore a lot of biomes. Edited November 27, 2015 by Warzouz