Delta V and Asparagus Staging

[GoSlash27]

3 hours ago, The Space Dino said:

Bad news: the 7000 m/s delta v design still didn't work, should I redesign it or go for other stuff in career mode?

@DrLicor any ideas ?

Space Dino,

 We would need to know why it didn't work. A Mun lander is feasible in 1.2 career, even under "caveman" conditions, so this is a design/ execution problem rather than an unrealistic goal.

Can you share a link to your craft file?

Best,
-Slashy

[The Space Dino]

@GoSlash27 I think the problem is I slowed down to high in my descent. I don't have the craft file, but I do have a screenshot.

[GoSlash27]

@The Space Dino,

 It's kinda hard to tell what you have going on from that pic. Are all of the engines in your asparagus arrangement LV-T30s? What about your vacuum stage? Is that an LV-909?

 When you say "it didn't work", what exactly do you mean? Did it run out of fuel? If so, when?

Can you describe the points in the mission at which you executed each staging event?

Thanks,
-Slashy

 

*edit* I'm attempting to recreate your ship from the pic. It looks like your vacuum stage has 2,400 m/sec DV (depending on what's in your cargo bay). That's enough to do the entire mission from munar circularization to Kerbin intercept. If you attempt to do the Mun intercept on this stage, you'll run out of gas later.

 *edit 2* I built a replica of your ship. I figured out that you were using LV-T45s. It got to orbit comfortably with 1,233 m/sec DV remaining. This is more than plenty for the Mun intercept and even circularization if you choose to do that.

Are you seeing similar numbers?

*edit 3* It made Mun intercept with no problem on the core LV-T45 stage. With 345 m/sec DV, it's also good for circ and beginning the descent (which will help dispose of unwanted space junk). Incidentally, I didn't notice any solar panels on your ship. If you don't have them, you need to fix that.

Best,
-Slashy

 

 

 

Edited October 30, 2016 by GoSlash27

[Plusck]

1 hour ago, GoSlash27 said:

@The Space Dino,

 It's kinda hard to tell what you have going on from that pic. Are all of the engines in your asparagus arrangement LV-T30s? What about your vacuum stage? Is that an LV-909?

 When you say "it didn't work", what exactly do you mean? Did it run out of fuel? If so, when?

Can you describe the points in the mission at which you executed each staging event?

Thanks,
-Slashy

 

*edit* I'm attempting to recreate your ship from the pic. It looks like your vacuum stage has 2,400 m/sec DV (depending on what's in your cargo bay). That's enough to do the entire mission from munar circularization to Kerbin intercept. If you attempt to do the Mun intercept on this stage, you'll run out of gas later.

 *edit 2* I built a replica of your ship. I figured out that you were using LV-T45s. It got to orbit comfortably with 1,233 m/sec DV remaining. This is more than plenty for the Mun intercept and even circularization if you choose to do that.

Are you seeing similar numbers?

Best,
-Slashy

Sounds reasonable to me.

@The Space Dino

Replacing the Swivels (LV-T45s) with Reliants (LV-T30s) - at the very least on the first booster stage to get dropped - should increase dv significantly. There's no need for gimbal in the lower atmosphere since there's plenty of aero control surfaces.

Personally, in early career, I hardly ever use the Swivel. Margins are too tight to afford the luxury of upper atmosphere/vacuum control, and I'm invariably ditching them about when I get to orbit, so their better vacuum stats don't ever really make up for their added weight and lower power.

[The Space Dino]

@GoSlash27 there are solar panels behind the capsule, 2 of them. Just to confirm with you, I used Swivels and LV-909s. By "didn't work " I mean that I did not have enough delta v to escape the Mun's SOI. If your're wondering the service bay has 2 Goo containers and 2 thermometers. There is a heat shield under the service bay.  I also see about 1000m/s when I enter Mun orbit but when I try to return to Kerbin it's not enough.

@Plusck thanks for the tips. I'll replace a pair of Swivels with Reliants.

 

Edited October 30, 2016 by The Space Dino

[GoSlash27]

Update 4:

It's in a 25x25 LMO with 53.6 m/sec remaining. Plenty to put the stage on a collision course with the surface.

4 minutes ago, The Space Dino said:

 I also see about 1000m/s when I enter Mun orbit but when I try to return to Kerbin it's not enough.

No, you should have over 2,400m/sec when entering Munar orbit with this design. 1,000 isn't going to cut it.

You're losing nearly 1500 m/sec before you reach Munar orbit. We need to back-track and figure out where...

How much DV do you show remaining when you reach LKO?

Best,
-Slashy

 

 

[The Space Dino]

6 minutes ago, GoSlash27 said:

Update 4:

It's in a 25x25 LMO with 53.6 m/sec remaining. Plenty to put the stage on a collision course with the surface.

No, you should have over 2,400m/sec when entering Munar orbit with this design. 1,000 isn't going to cut it.

You're losing nearly 1500 m/sec before you reach Munar orbit. We need to back-track and figure out where...

How much DV do you show remaining when you reach LKO?

Best,
-Slashy

 

 

Hmm... I think I saw about 3 to 2 thousand m/s after I reached LKO. I still had the core stage after entering Mun orbit but it  was pretty much almost out of fuel. I'm pretty certain I messed up somewhere, because my piloting skills are kinda bad.

[GoSlash27]

17 minutes ago, The Space Dino said:

Hmm... I think I saw about 3 to 2 thousand m/s after I reached LKO. I still had the core stage after entering Mun orbit but it  was pretty much almost out of fuel. I'm pretty certain I messed up somewhere, because my piloting skills are kinda bad.

That's okay. We'll get through it

 If you had your core stage almost out of fuel in Munar orbit, then you're fine up to that point. It's probably your descent to the surface that's killing you.

This is what I ended up with at touchdown:

This is 1,566 m/sec DV on the surface. How much do you have at landing?

 

[GoSlash27]

Back in LMO with 947 m/sec remaining.

The burn back to Kerbin shouldn't take any more than 270 m/sec.

Reentry with plenty of leftover fuel.

Recovered Jeb and the science without incident, so the ship works as designed. I didn't use a max- efficiency technique for the Mun landing, but rather a more expensive "pinpoint landing" technique.

So here's what you should be seeing for DV remaining:

At LKO, about 3,600 m/sec.

At LMO, about 2,450 m/sec

On the Munar surface, 1,500 m/sec

Back at LMO, about 950 m/sec

Stage jettison for reentry, about 680 m/sec remaining.

 

Wherever your numbers diverge from this is where you're having a problem. You will either have to add DV to that stage to compensate, or I can give you some pointers on how to improve your technique.

Best,
-Slashy

Edited October 30, 2016 by GoSlash27

[DD_bwest]

if a mun landing is giving you alot of trouble for now,  maybe plan a science mission to scoop up a lot of easy science and unlock propulsion systems.   that node makes mun landings significantly easier because of the spark and the tiny tanks.  There is also a boat load of easy science where you wouldnt even have to leave kerbin orbit, and even more if you do a mun orbit + return.

[The Space Dino]

@GoSlash27 yes you're right, I certainly did mess up my landing looking at your delta v. When I landed I had less delta v then you had after entering Mun orbit after landing. Can you tell me how to land properly?

@DD_bwest I would like to give myself a challenge

[DrLicor]

@The Space Dino, sorry I was off for the weekend so had no time to respond. But it seems you're beeing helped quite well by the rest  

About the landing, the learning progress is quite simple, however, it's easy to learn, hard to master. For landing you want to slow down your craft until your trajectory is hitting your desired landing spot. Most of the time I do it in 2 burns, a breaking burn and a land burn. For the last burn it can be a little tricky, but I've read a great tip for 'calculating' it on the forum.

The ideal landing would be to burn for example 30 seconds, after those seconds your craft should have a speed of 0m/s and is standing on the ground. But that's what you will do later on   What you want is to burn retrograde. To plan this burn, you can easily place a manouver node just above the ground, you retrograde the manouver until the flight-path 'flips' over. Now you have your delta-v and the time you need to burn for what we call a 'suicide burn' When it says you need to burn for 30 sec's. Make it 45, just to be sure, you can always fine-tune it when you've got the drill of it. During the descent, keep burning retrograde, but be carefull, if you've sett the SAS to retrograde, and you brake to hard, your retrograde can flip, and so does your lander. I think many fellow kerbonauts can confirm.

However, when you reach the speed of 5 m/s you want to find the amount of thrust that keeps you at that 5m/s, so you slowly decent. That way you should land safely 

So to make a long story a bit less long:

1. longer your trajectory to a desired landing spot.
2. make a manouver node to calculate the amount of delta-v and time needed. (optional)
3. Do almost a suicide burn, but keep it at a stable 5m/s
4. land
5. ...
6. profit

 

Some background stuff  : For a powered descent to the surface of a massive body like the Moon, it turns out to be most fuel efficient to do all your deceleration at the very end of the trajectory, right before impact.

(This is because if you decelerate sooner than that, you will be in flight longer; the longer you're up, the more fuel you need to spend counteracting gravity accelerating you toward the surface.)

So the fuel-optimum landing involves falling faster and faster toward the surface before doing your deceleration burn, which looks suicidal (and will be, if you don't leave sufficient margin for error).

 

 

[The Space Dino]

@DrLicor thanks for that another guide! The point at which I really mess up is the landing part, because my speed is too low too high up. I'll give your tips a try when I play KSP.

[DrLicor]

7 minutes ago, The Space Dino said:

@DrLicor thanks for that another guide! The point at which I really mess up is the landing part, because my speed is too low too high up. I'll give your tips a try when I play KSP.

That doesn't matter, just burn a bit later next time. And with what I've said, you now what to do  And if you're 500 meters above the ground and going 5m/s seconds. I would say you did a pretty well job for a first timer  .

Just drop your trust for a moment so you fall back a bit, then burn again to lower your speed for a 'soft' landing

Edited October 31, 2016 by DrLicor

[The Space Dino]

Well, at that time I didn't bring a Kerbal Engineer part so I could only know the sea level altitude of the stock game (which is hopelessly useless). I'll try to aim to slow down later.

[DrLicor]

Ah indeed, and you can just quick save just after your 'breaking burn'. So you can quick load any moment when you did something wrong. Just have a quick look at your altitude before doing things, so you know when and where you were doing something wrong.

[GoSlash27]

Space Dino,

This is the technique that I used:

It's not as efficient as the "zero descent rate" technique, but it's easy to do and very precise.

I can't seem to find a tutorial for the zero descent rate technique, but I'll try to find you one.

Best,
-Slashy

[DrLicor]

@GoSlash27 nice one! Indeed the big pro is that it's so precise. It's a shame I didn't notice this technique a year or 2 earlyer haha

10 minutes ago, GoSlash27 said:

It's not as efficient as the "zero descent rate" technique, but it's easy to do and very precise.

I can't seem to find a tutorial for the zero descent rate technique, but I'll try to find you one.

The technique I use for the zero descent rate or how space-x lands is how I describes above. Just place a manouver node as far as possible to the ground and keep increasing retrograde until it flips. After that it basicly gives you the rough amount of time and delta-v needed for the suicide burn. Not a 100% precise indeed. (has to do with the time your burn takes, most efficient with a high TWR)

[DrLicor]

I did some further research and couldn't find a good tutorial for the zero descent rate too, however I found something on the steam forum that could help you. It's not the best technique, but it's good to start with:

Quote

First, do a quicksave in stable Mun orbit. That way when you screw it up you can try again repeatedly without having to go all the way back to launch. SAS on. Landing gear deployed.

What you're going to do is pretty much come to a dead stop over your target zone and then drop straight down. Lower the periapsis of your orbit to pass fairly low over the surface - say about 5000 meters - over your intended landing zone (so, from the opposite side of the Mun of where you intend to land, you burn retrograde). You're doing this so that you don't have so far to drop which means using less fuel spent keeping the speed of your drop managable for less time.

Now, when you get close to your intended landing site burn retrograde and zero out your orbital velocity (that is, get as close to 0 as you can). You'll start to drop straight down and eventually the nav-ball will change from saying "orbital" speed to "surface" speed. You'll be picking up speed.

I generally let it get up to 150m/s or so and then burn until it drops back down below 50. We don't have a radar altimeter - it shows the distance to "sea level" not to the actual surface underneath us. Some people do as Vaprak suggested and set a node to get a distance reading but it's probably good practice (and in some ways a lot more fun) to be able to start to judge it by eye.

It helps if you orient the camera so that the sun is at your back because that allows you to see the approaching shadow of your craft on the ground which is a great visual cue as to how fast you're approaching it for the last bit. You will almost certainly have SOME horizontal velocity but usually it won't be enough to be overly concerned about. Don't get overly preoccupied chasing the retrograde marker on the navball as you get closer to the surface. You should be able to handle the little bit of horizontal movement on touchdown.

Keep your attention moving between the altimeter (even if it doesn't show the distance to terrain it's another indicator of how fast it's approaching), the navball pointer, the navball velocity reading, and the overall view of the approaching ground. Get too focused on one of them for too long and it's easier to get into trouble. You need overall awareness of what your craft is actually doing.

Without a more convenient altitude-to-terrain reading I soon start keeping my speed below 50 m/s and deciding when to do that is just a matter of judging visually how close I'm getting to the surface. When I can see my approaching shadow I start trying to keep it below 20. This requires a fairly light touch on the throttle - tap on/wait/tap off. Speed on touchdown should be under 10m/s. If you're not too far above the surface you can just hit x and let it drop - the landing gear will absorb some impact as it's supposed to. SAS should help prevent you from rolling over and will actually keep you perched at angles on less than all landing legs. If you are just starting to roll over then maybe give the keys a bump in the opposite direction.

Land somewhere FLAT! Try to target the bottom of any crater or wide, flat spaces in between craters.

PRACTICE. And be patient. Things will start to happen faster as you get closer so don't try to rush to get to that point. Leave time accelleration alone.

That's how I got it done anyway.

note this is from 2013, some things could've changed

[Plusck]

3 minutes ago, DrLicor said:

I did some further research and couldn't find a good tutorial for the zero descent rate too, however I found something on the steam forum that could help you. It's not the best technique, but it's good to start with:

note this is from 2013, some things could've changed

That is not a "zero descent rate" landing - that's a "stop and drop" which is the least efficient method.

Constant descent rate (not zero) is a safe and efficient method for low-TWR craft.
It involves bringing your Pe to just above the surface, burning retrograde shortly before that point, and then gradually adding radial-out as your trajectory becomes suborbital, so that you are descending at a constant rate towards the surface.

So if Pe is originally at 1km, and your burn to cancel out horizontal velocity is 200 seconds (this would mean you had a TWR of about 1.5 on the Mun) you aim to keep your vertical descent rate at about 4-5 m/s the whole time. It'll take longer than 200 seconds (since you're burning radially out for some of that time) but, crucially, gravity will only be dragging you down at full whack towards the end of the burn: to start with, the curvature of the surface and your own suborbital trajectory limits the about of gravity you're having to fight.

[DrLicor]

1 minute ago, Plusck said:

That is not a "zero descent rate" landing - that's a "stop and drop" which is the least efficient method.

Yep, that's what I said or meant. haha

[Plusck]

7 minutes ago, DrLicor said:

Yep, that's what I said or meant. haha

Ah right, yes I see.

 

I was going to add that constant descent rate can be used for higher-TWR craft too, on a trajectory that impacts the surface.

Basically you start as you say, with a flipped node to get your time to impact.

Then, rather than doing a direct retrograde suicide burn at 2/3 of your time to impact (which brings the point of impact closer to you), you add radial out so that you stop accelerating towards the surface. The end result is that you should end up landing closer to where you were originally aimed, it'll take a little longer to do so (therefore less efficient, but less terrifying) and you're in a better position to add more radial-out to avoid terrain if necessary.

[GoSlash27]

1 hour ago, Plusck said:

That is not a "zero descent rate" landing - that's a "stop and drop" which is the least efficient method.

Constant descent rate (not zero) is a safe and efficient method for low-TWR craft.
It involves bringing your Pe to just above the surface, burning retrograde shortly before that point, and then gradually adding radial-out as your trajectory becomes suborbital, so that you are descending at a constant rate towards the surface.

So if Pe is originally at 1km, and your burn to cancel out horizontal velocity is 200 seconds (this would mean you had a TWR of about 1.5 on the Mun) you aim to keep your vertical descent rate at about 4-5 m/s the whole time. It'll take longer than 200 seconds (since you're burning radially out for some of that time) but, crucially, gravity will only be dragging you down at full whack towards the end of the burn: to start with, the curvature of the surface and your own suborbital trajectory limits the about of gravity you're having to fight.

^ This is what I'm referring to as "zero descent rate". It is the most efficient procedure in terms of DV expenditure. I'm playing hell finding a tutorial for it, though. I saw a really good video on it, but now I can't find it.

Still lookin'...

-Slashy

[The Space Dino]

@GoSlash27 thanks for that landing guide! It is quite helpful!

This is kinda becoming a hunt for the best way to land

[Plusck]

19 minutes ago, GoSlash27 said:

^ This is what I'm referring to as "zero descent rate". It is the most efficient procedure in terms of DV expenditure. I'm playing hell finding a tutorial for it, though. I saw a really good video on it, but now I can't find it.

Still lookin'...

-Slashy

Ahh - the video of a lander which started with TWR ≈1. He actually used about 640 m/s dv to land (with a high TWR suicide burn, it would have been about 570).

And yes, it really was zero descent rate for most of the time.

 

So it's less "most efficient for dv" and more "most efficient cost, engine mass, payload etc"