Difference between revisions of "Tutorial:Low tech Duna sample return"
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==Back at Kerbin==
==Back at Kerbin==
You may have noticed that there's no second parachute to land at
You may have noticed that there's no second parachute to land at Kerbin. Blame minimalism :). A rescue rocket could have come to pick Jeb up. But a [[FAQ/Gameplay#Why_do_some_Kerbals_laugh_during_the_entire_mission.2C_while_others_panic_easily.3F|BadS]] doesn't need rescue. Just go on EVA and a jetpack .
Kerbin. Blame minimalism :)
rescue rocket could have come to pick Jeb up. But a [[FAQ/Gameplay#Why_do_some_Kerbals_laugh_during_the_entire_mission.2C_while_others_panic_easily.3F|BadS]] doesn't need rescue. Just go on EVA
Revision as of 15:13, 24 September 2021
This is an advanced mission to Duna with minimal tech. You don't need much tech for this mission, but this also makes some parts more difficult and you'll learn a lot. Read the tutorial before flying away and feel free to change the design as desired, like adding solar panels, strut connectors, landing legs or another parachute. Sorry for making this tutorial so long, but since it's a tutorial I made it comprehensive so less experienced players can do it, too.
- Orbit Kerbin
- Plan maneuvers to encouner Duna
- Execute maneuvers
- Get surface sample
- Orbit Duna
- Plan maneuvers to encouner Kerbin
- Execute maneuvers
- Basic Rocketry
- Engineering 101
- General Rocketry
- Advanced Rocketry
- Building level 2
- Launch pad due to rocket mass
- Tracking station for visible conics
- Mission control for maneuver planning
- Austronaut complex for EVA
- R&D for surface sample
Pilot with one or more stars (orbiting Kerbin will do this)
The delta-v map at https://i.imgur.com/gBoLsSt.png says we need
3400 m/s for low Kerbin orbit (LKO) (that's very generous)
950 m/s to get to the edge of Kerbin's sphere of influence (SOI)
130 m/s to encounter Duna
10 m/s or less for a plane change
0 m/s aerobraking
20 m/s landing burn
1450 m/s to low Duna orbit (LDO)
360 m/s to Duna's SOI
250 m/s to encounter Kerbin
6580 m/s total
These are all vacuum delta v's. From LKO we need 3180 m/s. That's a more accurate number.
- Mk16 Parachute
- Mk1 Command Pod
- TD-12 Decoupler
- FL-T400 Fuel Tank
- LV-909 "Terrier" Liquid Fuel Engine
- TD-12 Decoupler
- FL-T400 Fuel Tank
- FL-T400 Fuel Tank
- FL-T400 Fuel Tank
- FL-T400 Fuel Tank
- 4x Basic Fin
- LV-T30 "Reliant" Liquid Fuel Engine
- TT-38K Radial Decoupler
- Aerodynamic Nose Cone
- BACC "Thumper" Solid Fuel Booster
Delta-v: 6162 m/s
Mount the boosters at the lower end of the decouplers, so their top turns away at decoupling and aerodynamics drives them away from the rocket. I put the bottom of the boosters slightly below the Reliant so the rocket would stand more stably on the launch pad. Launch stability enhancers do a better job at that, but due to minimalistic tech I don't use them.
Remove the monopropellant from the command pod.
- Check list
- VAB Engineer report
The delta-v display disagrees with Kerbal Engineer Redux (KER) mod. If I put the Reliant in stage 4 they agree.
Click on the Crew button and select your 1+ star pilot.
Click the launch button.
Low Kerbin orbit
You are on the launch pad now.
For most rockets I work out an ascend profile to orbit by trial and error. For this one the "manual" is:
- Activate SAS
- Switch the lower left infobox to 'Maneuver Mode'. This show's you apoapsis (Ap) and periapsis (Pe).
- stage (press space)
- at 10 m/s: turn/yaw 20 degrees right/east (from 90 to 70 degrees)
- while turning, click prograde hold
- Just before the boosters burn out fully turn on the Reliant.
- When the boosters burn out: stage
- At 70.5 km Ap turn off the Reliant
- Switch to the map (you could save now with F5)
- Click Ap with the right mouse button (RMB).
- 16 s before Ap: full thrust; when the time to Ap rises, cut thrust
- 8 s before Ap: full thrust; when the time to Ap rises, cut thrust
- 4 s before Ap: full thrust; when the time to Ap rises, cut thrust
- 2 s before Ap: full thrust; when time to Ap is about 20 s: cut thrust
- Now you have ~10 s to RMB click Pe.
- Use low thrust to keep time to Ap between 1-10 s.
- When Pe reaches 70.5 km you have a great orbit. :)
I have 3251 m/s left now. I used 2911 m/s to get to orbit!
Save as "1".
This rocket turned out a bit stubborn. It tended to drift south because the radial decouplers were flexing and the twisted boosters would create an aerodynamic force. This probably could be mitigated with (auto)struts (not used due to minimal tech). Correct for the drift early with WASD.
The rocket flys very shallow. This is efficient, unless it really flies too low, creating too much air drag, heat and explodes or passes Ap. It's ok if the fins explode :). We don't need them at that altitude any more and it saves mass. If the Ap is over the next peninsula when you shut off the Reliant the first time, great. It even overshot for me. Too much overshoot is bad.
Let's find a Duna encounter.
Press escape and click 'Tracking Station'.
Zoom out (keypad '-' saves you from sore fingers from excessive mouse wheel scrolling) till you see the Sun. Double click the Sun. With RMB tilt till you see the solar system from above, the planets moving counterclockwise. Zoom so Duna's orbit fills the screen.
With RMB turn Duna to the top of the screen, the 12h position.
While keeping Duna at 12h, do 100,000x time warp, till Kerbin is at 2h.
From the list on the left select your rocket and click the green rocket button, or just double click the rocket.
Go to map mode.
Double click Kerbin.
Look at it from above (at it's north pole).
Turn the day side to 12h.
Click at your orbit at the 8h position.
Click 'Add maneuver'.
You can pull the prograde/retrograde/normal/antinormal/radial in/out handles away and towards the maneuver node to add/subtract delta v to/from these directions. For fine tuning, move the mouse at a handle and scroll the mouse wheel. You can RMB click a node to switch between delta v and next/previous orbit mode. With the circle you can move the node on your trajectory. You can close the node by clicking away from it and open it by clicking at it.
Add about 1000 m/s prograde till your planned trajectory changes from ellipse to just going away, without coming back.
Zoom out. Uhh, I got a Mun encounter. Select a few orbits later, till the encounter disappears.
Zoom out till you see the Sun. Double click it. Turn Kerbin to 3h. With RMB tilt the solar system away by about 45 degrees. Zoom till Duna's orbit fits the width of the screen. Click at Duna or its orbit and select 'Set as target'. Duna's orbit must be yellow, now.
Adjust the prograde delta v till the trajectory touches Duna's orbit.
LMB grab the grey circle of the node, move the mouse a bit away, about 1.5 times the handles length and move around the node in a circle, till Ap is highest (if nothing happens, you moved to far away from the node). You are moving the node on your orbital trajectory around Kerbin till your escape trajectory goes in a near prograde direction relative to the solar orbit. Adjusting till Pe is at Kerbin should work well, too. Adjust prograde delta v till Ap touches Duna's orbit.
You should now have two markers on your trajectory: 'Closest approach' (CA) and 'Target position at closest approach' (TP). When you execute the maneuver and fly to CA, Duna will be at TP.
TP should be ahead of CA in our case. Since Kerbin's orbit is smaller and thus faster than Duna's orbit, you can catch up and reduce your lag behind Duna (the phase angle). We'll do just that in successively smaller steps, making CA and TP move closer together till we get an encounter.
Save with F5.
Go to the tracking station again.
Look at the game time. Time warp for about 50 days (or less, if this is your second or further iteration).
Switch back to your rocket.
Delete the maneuver by clicking the x next to the delta v next to the navball. Or you could go to the map, open the maneuver with LMB click, then RMB click it and click the x there.
Do the maneuver planning again.
The markers should be closer together now. If CA is ahead of TP then the 50 days were too much. Load with F9 and warp less. But not just yet ...
Moving your maneuver around on your Kerbit orbit (while looking at Duna's orbit) changes the distance between the markers and may get you an encounter. If you went too far, you can use a bit more delta v, overshoot Duna's orbit and that delay may get you an encounter when you come back down to Duna's orbit. That's not ideal, but may save you more planning effort, if you have enough delta v.
If CA is close behind TP (you can get a close up if you rotate the view so the markers move to the front), then instead of warping in the tracking station, click '+' in the orbit +/- mode of the maneuver node, till the markers move together and you get an encounter. If that makes the markers to disappear, you have to adjust the delta v and maybe the node position on Kerbin's orbit, if you advanced many orbis later.
If a Mun encounter messes up your planning, go a few orbits forward or backward.
I just got an encounter 15 days in the future by clicking about 30 times orbit+ and adjusting delta v and moving the maneuver a bit.
Duna turned green and I got 'Duna encounter' and 'Duna escape' markers right next to it.
LMB click Duna or its orbit and select 'Focus view'. Zoom in. You'll see your Duna flyby trajectory. This must be moved into Duna's atmosphere.
You can edit your manoeuver with the node's controls. But now is the time to do it in the lower left box:
Click the purple Maneuver mode button. Select maneuver 1 with the white triangle arrows. Use the tab with the maneuver node symbol (looks like a star, but the rays are the node handles). (The table tab allows you to edit numerically).
There you can scroll, drag and click the handles and the time arrows (make your maneuver a little earlier or later, i.e. moving it around on the Kerbin orbit). The vertical lever to the right adjusts the sensitivity.
Move the trajectory right under or above Duna. You can reduce prograde delta v a bit if you optimize the departure orbit and time. Don't touch (anti)normal/radial in/out. If your trajectory disappears or changes color, you may have a Mun encounter. Try a few orbits earlier or later. Sometimes the trajectory disappears for reasons I don't understand.
You could save now with F5.
Now we need to get the trajectory up (or down) towards Duna.
When you make a fist with your right hand and stretch your thumb away and think of your fingers to be your orbit, direction is towards your finger tips, then your thumb is the 'normal' direction. The other way is 'antinormal'. That's the right hand rule.
When you accelerate (anti)normal, your direction of motion (your velocity vector, which is speed and direction) changes a bit towards 'up' ('down'). This also changes your orbital plane, like holding out your hand flat and turning your arm along its axis. On the left/right side your orbit (or hand) will move up and down, while along the turn axis it will not. So to move your flyby trajectory, you must make an (anti)normal maneuver 90 degrees before your encounter, or half way between Kerbin and Duna:
Zoom back out, double click the Sun, look at the solar system from 'above', (optionally turn till Kerbin and Duna are on the same horizontal line), and click the trajectory half way between Kerbin and Duna.
Focus Duna again and zoom in.
In the control box select maneuver 2. Adjust (anti)normal delta v to bring the trajectory up/down towards Duna. I want an equatorial orbit, so I want the trajectory to be horizontal.
Keep adjusting pro/retrograde at maneuver 1 and (anti)normal at maneuver 2 and zooming in till your trajectory is at Duna. Put your Duna Pe at about 25 km. Fly by at the side that will result in a prograde Duna orbit. Duna rotates counterclockwise when looking at it from above.
You can avoid Ike encounters by changing prograde delta v and compensating with departure time.
BTW, another way to get your trajectory up/down to Duna is to make an (anti)normal maneuver at the ascending/descending node (AN/DN). The way I do it uses less delta v, but has higher approach velocity at the encounter. I don't worry about that, cause aerobraking takes care of that.
Departure from low Kerbin orbit
Next to the navball you see three times, e.g.
- Node in t-13d, 0h, 35m
- Burn Time: 42s
- Start burn in: 13d, 0h
1 says when the maneuver node is. 2 says how long you have to thrust with your rocket engine(s). 3 says when to begin burning.
Now KSP maneuver nodes assume you accelerate your rocket instantly by e.g. 1052.7 m/s. But actually it takes 42 seconds. To come closer to the planned result, you do half your delta v before and the other half after the maneuver node. But your rocket becomes lighter as you burn and exhaust fuel, so the acceleration increases during the burn, and the distance covered during the first half of delta v is longer than during the second half. To compensate for that, you could use the buttons for how much delta v you burn before and after the maneuver node. It only allows 10% steps, which is probably too coarse. Or you could manually start burning a little sooner or later. Would you know which way to adjust? I don't think it's worth it, and go with 50%. It won't be exact anyways, and you'll have to make correction maneuvers, like it's done irl.
Time warp till 3-5 min before burn start. I'll time warp 13d in the tracking station. There I select my rocket which shows the maneuver node so I can RMB click it so I can see the time to the node. Double clicking Kerbin pacifies the node.
Turn the rocket to the blue maneuver marker, which is the direction you should burn. If it's not visible, a blue arrow will point to it.
Wait till burn start. Now the rocket should still point to the maneuver marker. But to simplify the game KSP switches to a rotating frame of reference fixed to the orbital rotation, I think, during time warp, which makes the rocket rotate slowly relative to the stars ... and the maneuver marker/direction. So watch out if it changed significantly and correct if necessary.
Burn. But wait, what's that line in the bar? And the numbers 4 and 2? Stage 4 doesn't have enough fuel for the burn, so mid burn you have to stage as seamless as possible.
Also, your burn may not be exactly accurate, so it's a good idea not to rely just on the instruments, but watch where your real trajectory is. Focus on Duna, zoom out, so you can at least see your whole planned trajectory (this way you should have the SOI of Duna in view, where your real trajectory will appear. Whoa, the planned trajectory is not there. In the 90 before the burn I quickly correct the maneuvers. KSP is unstable at times ...
As you come near the end of the burn, slow down or stop and restart slowly. When your trajectory appears in Duna's SOI, stop, cause it may move by quickly. You may proceed carefully, slowly, perhaps in short pulses. For the very end, you may limit your thrust: RMB click your engine and set the trust limiter to 2%.
Oops, I overshot a bit before limiting thrust. So now with limited thrust I carefully burn a bit retrograde.
That brings up another topic. I turned from prograde to retrograde with the SAS button. That works nicely, but it uses more electricity that necessary. And ... to minimize tech, I didn't include a battery or solar panels. The Reliant produced EC while burning. The Terrier doesn't, so since we decoupled the Reliant the rest of the mission must complete with the 50 EC (electric charge) stored in the command pod. So a low power way would have been to turn off SAS, give the rocket a little nudge in the right direction and wait till it reaches the desired direction, potentially with some additional corrective nudges. Then either stop manually, but that will never stop completely. Rather you could turn on SAS, which will stop rotation. Or press 'F', which will also stop rotation. Maybe not completely if you don't press long enough. But there's a neat trick. Time warp instantly stops rotation :). So 1 s time warp and the rocket is still without use of EC :). Alternatively you could RMB click the command pod and set (reaction) wheel authority to 1%. Then you can use SAS as usual which will work slowly and use much less EC. Manual still uses lower power, I think.
I'm now at 48.07/50 EC, SAS is at 1% and off.
Delete the maneuver.
Fiddling with maneuver planning shows me Duna Pe changed to 251 km. That's because our burn was not exactly as planned. That's normal. We're not a computer and even computers aren't perfect. I'll correct that later.
In map mode I open the plane change maneuver and click the little fast forward "Warp to Next Maneuver" button. This gets me 60 s before the next burn. We should have prepared for the maneuver before the time warp, but timing is not critical this time. I turn the rocket to the next maneuver marker. Low power :). Stopped with time warp. 48.06 EC left.
The next maneuver is 8.0 m/s. That burn length is 21 s at "full thrust". That's because the thrust limiter is at 2%. Good. Otherwise the burn might be so short we'd be unable to do it accurately. I often set the thrust limiter to get a ~10 s burn and do the rest of the burn with fine control.
Ahh, but first I add some prograde to the maneuver to get Duna Pe back to 25 m/s. Barely a change in the maneuver marker. This time I'll adjust the rocket attitude to follow the marker during the burn as it moves to tell me I'm off and need to burn somewhere else to correct. Don't bother to follow the maneuver marker for the last bit of the burn.
6 min after the node I start the burn, watching the trajectory move, stopping in between to follow the maneuver marker. Nice, 25.412 m Pe, flyby plane well aligned with Ike's orbit, so I'll get a perfect equatorial orbit.
Full time warp, till the ship automatically slows down to 50x time warp as it changes SOI. This slow down takes a little while, and at 100.000 time warp the ship can move through small SOIs before coming to a halt.
Arrival at Duna saved as "2" and with F5.
Landing at Duna
I'll let it go through the atmosphere as is and see what happens ... Ship turns prograde, low drag, doesn't slow down enough.
I tried a lot ... stopped saving EC ... different Pe resulted in different elliptic orbits after which I came down to different terrains ... canyons, a crater, lowlands. One interesting attempt was to come down retrograde and before crashing I'd turn prograde, pull up, lift would let me fly up again, till I turned around and was retrograde again and clicked retrograde hold. Then a short burn would slow me down enough to throw the chute, but the terrain was to sloped to land standing, like Starship. That used up a lot of EC, tough.
Successful attempt was when by chance came down retrograde from a medium high elliptic orbit, Ap about halfway to Ike's orbit and 21 km Pe, before midland sea, a low altitude terrain with relatively high air pressure/drag, generally flat with soft hills. My impulse let me fly relatively horizontal and as I came down a short burn slowed me down enough to throw the chute. I used retrograde hold. Shortly before landing I did a short landing burn to reduce touchdown speed. I landed at a level enough spot where SAS could stop me from falling over. Otherwise vary the time you throw the chute or change the altitude it fully opens (using it's menu) to land on different spots.
Jeb got out, planted a flag, got a surface sample and made an EVA report, got back in, and I flew back to orbit at 45-50 deg before switching to prograde. Delta v left in 60 km orbit is 938 m/s and I need 620 m/s to Kerbin encounter. 39.59 EC left, easy peasy.
You learned how to fly to Duna, you'll be able to fly back to Kerbin on your own :).
Back at Kerbin
With some aerobraking and delta v left, LKO was easily achieved. You may have noticed that there's no second parachute to land at Kerbin. Blame minimalism :). A rescue rocket could have come to pick Jeb up. But a BadS doesn't need rescue. Just go on EVA and do a Jetpack deorbit.