Duna and general interplanetary launches mechanics

[Actually_New_KSP_Player]

1.4.2 KSP only Kerbal Engineering installed.

I did my first launch to Duna without any guides or tutorials and it was decent until i realized that i need to slow down from crazy 5000 m\s Kerbol orbit to almost zero to get on Kerbin's orbit, which i don't think is possible with my upper stage terrier and spark engines and that is the first question.

Second one is how am i supposed to make a straight Duna launch (some guides make it even without orbiting the Kerbin) if it seems too precise and more like perfect planetary alignment? I did mine via orbiting the Kerbol and only then making some moves to collide with Duna and i'm not sure that this is the proper way of doing that since its cost.

Ship

Spoiler

Orbits

KerbalX

Edited September 15, 2018 by Actually_New_KSP_Player

[HebaruSan]

2 hours ago, Actually_New_KSP_Player said:

I did my first launch to Duna without any guides or tutorials and it was decent

Congratulations! That's not very easy to do!

2 hours ago, Actually_New_KSP_Player said:

until i realized that i need to slow down from crazy 5000 m\s Kerbol orbit to almost zero to get on Kerbin's orbit

Don't feel bad, a flyby is the first logical step of exploration; even in real life the first seven Mariner missions were all flybys.

2 hours ago, Actually_New_KSP_Player said:

which i don't think is possible with my upper stage terrier and spark engines and that is the first question.

Well first you don't need to get it down to zero, just to the local orbital velocity of the planet you're visiting at the altitude that your craft has. This is easier the closer you get to the planet; Duna's atmosphere is 50 km high, so if you can get your periapsis close in, you'll have a better chance of getting into orbit. Press tab a few times on the map screen to look at Duna before you get there.

Second, you can do things to lower your speed relative to the planet. If your orbit goes way above or way below your target's orbit, that will show up as extra velocity when you enter the target's sphere of influence, which will cost you more to cancel out. So it helps to use an orbit that just touches your starting and ending planets without going past them, which is called a Hohmann transfer and leads into your next question...

2 hours ago, Actually_New_KSP_Player said:

Second one is how am i supposed to make a straight Duna launch (some guides make it even without orbiting the Kerbin) if it seems too precise and more like perfect planetary alignment?

As you mention there are already a number of guides out there explaining transfer windows. Do you have a more specific question about them? Writing a whole guide in a response on this thread would be kind of redundant.

2 hours ago, Actually_New_KSP_Player said:

I did mine via orbiting the Kerbol and only then making some moves to collide with Duna and i'm not sure that this is the proper way of doing that since its cost.

That's an excellent way to learn. The next thing would be to figure out the most efficient way to do it once you're out orbiting the sun, and then once you've mastered that, experiment with what happens to your solar orbit when you exit Kerbin's sphere of influence in various directions. There are ways to choose an exit direction that will have the same effect on your solar orbit as if you had done a burn out there instead.

[Mahnarch]

Are you plumbing your fuel inward?

I don't see any ductwork. You could probably make your boat a lot smaller if you did.

[Rocket In My Pocket]

You should get Transfer Window Planner mod too, and check a DV map. 16,000Dv?! That's supreme overkill for Duna.

For comparison here's an early Career Duna flyby and return mission. Only has like 6k Dv. (Whoops, looking at it in the editor closer just now, and realized I was sending the top bit up unfueled, so I must have fueled it in orbit. So...roughly 9k Dv then. Still, a lot cheaper! Also a good way to save on launches as a side note/tip, send the mission critical bit up unfueled then rendezvous and fuel it up before it leaves.)

Spoiler

Seriously though, congrats on getting a fly by in the first place! Very impressive without any external help, besides KER. 

Transfer Window Planner:

Delta-V map:

 

Edited September 15, 2018 by Rocket In My Pocket

[Actually_New_KSP_Player]

9 hours ago, HebaruSan said:

Well first you don't need to get it down to zero, just to the local orbital velocity of the planet you're visiting at the altitude that your craft has.

When i got in Kerbin's SOI i made a maneuver node and pushed it full retrograde to make an orbit and it said that i need to slowdown from those 5000 m\s to around 100 m\s or so. But as you mentioned i was way below it, so maybe that's the case.

9 hours ago, HebaruSan said:

As you mention there are already a number of guides out there explaining transfer windows. Do you have a more specific question about them?

I was just confused by how easy they do that so it looked like "Let's launch that thing, make it up to 50 km altitude and boom we are colliding with Duna".

 

7 hours ago, Mahnarch said:

Are you plumbing your fuel inward?

I don't see any ductwork. You could probably make your boat a lot smaller if you did.

No. KerbalX

[Rocket In My Pocket]

6 minutes ago, Actually_New_KSP_Player said:

I was just confused by how easy they do that so it looked like "Let's launch that thing, make it up to 50 km altitude and boom we are colliding with Duna".

Are you talking about direct ascent to intercept vs orbit then intercept?

It's really half of one, six dozen of the other. If you have the energy to escape Kerbin's gravity directly you had enough energy to orbit it, and then escape. Direct might be a smidge more efficient, but nothing to write home about.

Plus there is a lot to be said for having time to setup your maneuver node especially if you are new.

[Actually_New_KSP_Player]

3 minutes ago, Rocket In My Pocket said:

Are you talking about direct ascent to intercept vs orbit then intercept? 

First one.

[Rocket In My Pocket]

4 minutes ago, Actually_New_KSP_Player said:

First one.

Like I said, you can do it either way, it really doesn't make a big difference.

The Dv cost is pretty much the same. (I'll emphasize the importance of TWP here again, as it's insanely cheaper Dv wise to go to Duna at the right time.)

Also, just out of curiosity; why stay on 1.4.2 if KER is your only mod?

Edited September 15, 2018 by Rocket In My Pocket

[HebaruSan]

1 hour ago, Actually_New_KSP_Player said:

When i got in Kerbin's SOI i made a maneuver node and pushed it full retrograde to make an orbit and it said that i need to slowdown from those 5000 m\s to around 100 m\s or so. But as you mentioned i was way below it, so maybe that's the case.

I assume that you meant to say Duna's SOI here, right? If you were flying by Duna and then coming back to Kerbin, that's a different problem.

But it does remind me of one more option that you have at Duna: aerobraking. Duna does have some atmosphere, so if you can manipulate your orbit to pass below 50 km (but not too low!), you can collide with that atmosphere in order to slow down, so not all of that 5000 m/s has to come from your engines. Though note that this would typically require a heat shield if the speeds are high.

1 hour ago, Actually_New_KSP_Player said:

I was just confused by how easy they do that so it looked like "Let's launch that thing, make it up to 50 km altitude and boom we are colliding with Duna".

Sometimes people in videos will leave out the part where they time warped forward to a transfer window (as well as the part where they time warped to align the space center with the direction they wanted to be leaving Kerbin's SOI). You can generally assume that any trip to another planet has been carefully aligned before launch using one of the transfer window tools (one of which is linked in the left image in my forum signature, you'd be welcome to try it).

[Actually_New_KSP_Player]

2 minutes ago, HebaruSan said:

I assume that you meant to say Duna's SOI here, right? If you were flying by Duna and then coming back to Kerbin, that's a different problem.

No, Kerbin's. I made it to Duna's orbit very easy, almost like i did with Mun's, grabbed some science from it's high and low orbits and little piece from Ike's, then entered Kerbol's orbit and burned retrograde to collide with Kerbin. I'm kinda scared of aerobraking in Kerbin's atmosphere on speed higher than 2500 m\s because it's much thicker than, for examle, Duna's.

[HebaruSan]

1 hour ago, Actually_New_KSP_Player said:

No, Kerbin's. I made it to Duna's orbit very easy, almost like i did with Mun's, grabbed some science from it's high and low orbits and little piece from Ike's, then entered Kerbol's orbit and burned retrograde to collide with Kerbin.

Oh, well you're practically an expert already then!

1 hour ago, Actually_New_KSP_Player said:

I'm kinda scared of aerobraking in Kerbin's atmosphere on speed higher than 2500 m\s because it's much thicker than, for examle, Duna's.

Your fear is justified, but this is what a heat shield is for. If your craft is aerodynamically stable and not super heavy (ideally just one of the cone shaped pods), it's safe to enter Kerbin's atmosphere with a periapsis around 30 km at high velocities as long as you have a heat shield in front to protect the other parts. If you don't believe me, you can always press F5 or shift-F5 to create a quicksave, then try it and if it doesn't work, press F9 or shift-F9 to restore.

[Actually_New_KSP_Player]

2 minutes ago, HebaruSan said:

Oh, well you're practically an expert already then!

Your fear is justified, but this is what a heat shield is for. If your craft is aerodynamically stable and not super heavy (ideally just one of the cone shaped pods), it's safe to enter Kerbin's atmosphere with a periapsis around 30 km at high velocities as long as you have a heat shield in front to protect the other parts. If you don't believe me, you can always press F5 or shift-F5 to create a quicksave, then try it and if it doesn't work, press F9 or shift-F9 to restore.

On screenshots you can see that there's large service bay on top of whole structure with heat shield below it, that part completely leaves the craft upon landing so all science and probe stored inside. I'll try aerobraking on Kerbin and see if that works.

[Actually_New_KSP_Player]

Apparently even ascending\descending nodes at 0 degrees doesn't make it much simplier, i still need to burn retrograde like crazy.

[Mahnarch]

I wouldn't wait to that Pe to start burning. I'd start burning right away, further out - aiming for a ~35km Ap inside Kerbin atmo.

 

That's just me, though.

 

Maybe you can use Mun to slow yourself down, too, if you want? I'm not your real dad.

[Actually_New_KSP_Player]

9 minutes ago, Mahnarch said:

I wouldn't wait to that Pe to start burning. I'd start burning right away, further out - aiming for a ~35km Ap inside Kerbin atmo.

 

That's just me, though.

 

Maybe you can use Mun to slow yourself down, too, if you want? I'm not your real dad.

I mean i would do that, but it takes that much to even get into no-matter-what orbit, not even talking about LKO, i just can't afford it. I tested way earlier than this launch that if you burn retrograde for orbit earlier than you reach Pe you will need little bit more of burning.

[Mahnarch]

Hmmm... *strokes imaginary beard*

 

Have you tried getting out and pushing? I don't know the numbers that you can get out of an EVA jet pack but, that far out it might help a bit.

 

[Edit: You don't have Kerbals onboard, do you? If not, then, nevermind.]

Edited September 15, 2018 by Mahnarch

[HebaruSan]

1 hour ago, Actually_New_KSP_Player said:

Apparently even ascending\descending nodes at 0 degrees doesn't make it much simplier, i still need to burn retrograde like crazy.

You're burning outside of Minmus's orbit. Try adjusting your flight path to pass near Kerbin first (say, a periapsis of 150 km instead of 50000 km).

[Actually_New_KSP_Player]

2 hours ago, HebaruSan said:

You're burning outside of Minmus's orbit. Try adjusting your flight path to pass near Kerbin first (say, a periapsis of 150 km instead of 50000 km).

Here's my Pe is 80 km, but it's still requires around 15 minutes of actual time burning, which i did and for some reason it made my Pe inside the planet. Still 15 actual minutes seems too much. Some youtube videos somehow made it to 50 km Pe with 2500 m\s speed and somehow didn't lost their engines, fuel tanks etc, which i tried and lost whole ship including service bay even at 60 km Pe.

[Rocket In My Pocket]

Wait...20 minutes of burn time and a TWR of 25?

What kind of engine are you using?

....You also have 179 alerts...lol?

[HebaruSan]

1 minute ago, Actually_New_KSP_Player said:

Here's my Pe is 80 km, but it's still requires around 15 minutes of actual time burning

Still 15 actual minutes seems too much.

That's because your upper stages' TWR are very low (0.23 - 0.6). That's not a serious problem in real space travel, but it does make for rather tedious gameplay. I usually keep it above 1 for that reason, which would be accomplished by putting stronger engines on those stages.

2 minutes ago, Rocket In My Pocket said:

TWR of 25?

I think KER is doing something weird there, maybe considering "weight" based on the strength of gravity at the current altitude, or using another reference body. The original VAB list shows nothing over 0.7 after the first stage.

1 minute ago, Actually_New_KSP_Player said:

which i did and for some reason it made my Pe inside the planet.

That's because you started burning significantly before the maneuver node. If you watch the map screen during the burn you can watch the effect that it has; some radial-out component would be needed to rotate your orbit and keep your periapsis from dropping lower.

1 minute ago, Actually_New_KSP_Player said:

Some youtube videos somehow made it to 50 km Pe with 2500 m\s speed and somehow didn't lost their engines, fuel tanks etc, which i tried and lost whole ship including service bay even at 60 km Pe.

Did your ship remain aerodynamically stable during re-entry, with the heat shield facing into the oncoming air/plasma? If it flips around, then your vulnerable components will be superheated and probably destroyed.

[Actually_New_KSP_Player]

6 minutes ago, Rocket In My Pocket said:

Wait...20 minutes of burn time and a TWR of 25? 

What kind of engine are you using?

....You also have 179 alerts...lol?

I think KER shows current TWR and depends on how close you are to Pe, which results in increased speed and harder maneuvers, screenshot below shows that i have 4700 TWR which is obviously not "real".

I'm using here terrier and spark, alerts is just completed contracts notifications so i don't check them.

5 minutes ago, HebaruSan said:

That's because you started burning significantly before the maneuver node.

I don't know where to start my burn because i have to decouple my lower stage with terrier and go on with a spark.

 

7 minutes ago, HebaruSan said:

Did your ship remain aerodynamically stable during re-entry, with the heat shield facing into the oncoming air/plasma?

Yeah, completely stable, it just reaches critical temerature to 100% and explodes.

[Rocket In My Pocket]

1 minute ago, Actually_New_KSP_Player said:

I think KER shows current TWR and depends on how close you are to Pe, which results in increased speed and harder maneuvers, screenshot below shows that i have 4700 TWR which is obviously not "real".

I've used KER for literally years and have never seen any behavior even remotely close to that... I think you may want to update to 1.4.5 and the latest version of KER? Sounds like a bug.

[HebaruSan]

4 minutes ago, Actually_New_KSP_Player said:

I think KER shows current TWR and depends on how close you are to Pe, which results in increased speed and harder maneuvers, screenshot below shows that i have 4700 TWR which is obviously not "real".

Note that "TWR (Surface)" is 0.37. I think that's the real number we're looking for, and it's consistent with the values shown for mass and thrust. You're 67,000,000 meters away from Kerbin, still out beyond Minmus, which backs up my earlier speculation about the TWR field using the force of gravity at your current altitude to determine the "weight" part of "thrust-to-weight ratio".

4 minutes ago, Actually_New_KSP_Player said:

I don't know where to start my burn because i have to decouple my lower stage with terrier and go on with a spark.

Well you've got 22 minutes of fuel, and falling to Kerbin will take 223 minutes at your current rate of travel, so maybe wait a bit longer?

4 minutes ago, Actually_New_KSP_Player said:

Yeah, completely stable, it just reaches critical temerature to 100% and explodes.

Does the heat shield explode first? Does it use up all the ablator?

[Actually_New_KSP_Player]

5 minutes ago, Rocket In My Pocket said:

I've used KER for literally years and have never seen any behavior even remotely close to that... I think you may want to update to 1.4.5 and the latest version of KER? Sounds like a bug.

Not sure if this is a bug though, it becomes "0 / 4700 TWR" if i turn off the engines and so does the thrust above it, it also decreases if i turn off the engines and let it fly closer to Pe.

 

6 minutes ago, HebaruSan said:

Does the heat shield explode first? Does it use up all the ablator?

Heat shield will survive if you do it right, but the service bay explodes within first like 10 seconds of reentry. I got 240 ablator out of 800 on 2.5 m heat shield and i think i should've grabbed little more, but service bay explodes before it runs out.

[Actually_New_KSP_Player]

I successfully landed it on ~2500 m\s speed, but i had to make another circle around the sun (it showed up that i have a collision on next circle if i stop burning retrograde so i decided why not), then burn almost all my fuel to get into orbit around Kerbin.

But i still don't know how to do return mission because many guides (and people here mentioned too) that i don't need such big rocket to do duna's launch because it's overkill, at the same time i wasted almost all my fuel to make an orbit around Kerbin...