Tutorial:Hopping around celestial bodies
So you've arrived on the Mun. Time to explore all those different biomes for science! Or maybe you're in career mode and have contracts to investigate various locations. Either way, you need to get around the place.
On planets with an atmosphere, spaceplanes will be the most efficient way of going from place to place, but without an atmosphere they are not an option.
Therefore, this tutorial explains various ways of getting there in a vaccum.
- Length: 15-30 minutes
- Difficulty: Moderate
- For version: Every version
Choose your vehicle
Your kerbonaut can walk at about 1 m/s. Around the Mun that would take 1.2 million seconds (about 2 weeks of game time, and you can only do 4x time warp while moving over the surface), so that's clearly only an option for very nearby sites.
Of course you can speed it up with the EVA jet pack, but you'll run out fuel pretty quickly.
Rovers are a little faster than kerbonauts, but even the fastest wheels only have a top speed of 23 m/s, so it will still take a long while to get around at 4x time warp. Plus there's the danger of tipping or accidental jumps ending in crashes.
Rockets are clearly the fastest way to get from A to B, without an atmosphere anyway. So if you're serious about exploring biomes, this is what you need. Of course rockets can crash too, but you're used to that by now, aren't you? At least you were going fast.
Design your hopper
You're free to design your hopper craft as you like. However you should remember the following points:
- Make it as light as possible, every kilogram counts if you want to be able to make multiple hops!
- Start with a small rocket and fuel tank.
- Add a small solar panel, maybe some batteries, and an antenna.
- Don't forget the science instruments.
Manned or unmanned?
- The unmanned flight computers are very light, so you save fuel. Make sure to take one with SAS. The Retrograde Hold SAS option will be helpful for landing.
- No support for crew reports or surface samples as required by many contracts.
- All possible sources of science (including crew reports and surface samples) are supported.
- Crew pods are heavy, so you'll need to bring a lot of fuel. An EAS-1 based hopper (see picture below) including almost half a ton of fuel is lighter than even just the Mk1 Lander Can!
- Extremely light
- Cannot be used to store data or surface samples for example, so you're going to have to return to a command pod after each site visit or lose science points due to transmission inefficiency.
- No support for crew reports (as required by some contracts), since the kerbonaut is in EVA mode without a crew pod, and can only make EVA reports. For crew reports in flight you should just do them in orbit. I don't think I've seen contracts for crew reports from the surface.
- Remember to bring a reaction wheel and batteries.
- Make sure the seat is positioned so that the kerbonaut faces away from the rocket, or the prograde/retrograde indicators will not work correctly! This is usually not how the vehicle designer will place it by default. If the rocket faces down, the foot pedals have to face up. Make sure to test it before launching.
Budgeting your Delta-v
So how much fuel do you need? I'll assume that you know the Delta-v required for orbit around the moon or planet you're on. For sites that are more than 90° away (in latitude and/or longitude), you'll need to budget twice the orbital Delta-V. For sites closer than that, the required Delta-V decreases the closer they are.
What good is an exploration vehicle if you can't refuel it? Carrying the fuel for numerous planned hops to every site is pretty wasteful after all.
However refueling under gravity is more complicated than in space. While you can use a Clamp-O-Tron Jr. docking port, getting the two ports to actually dock is extremely challenging.
For horizontal docking, you'll need to align both vehicles perfectly, while the dockings port are moved up or down by the landing strut suspension. The magnetic pull that works well in space just isn't strong enough against gravity to make the docking work.
Vertical docking can be completed more easily, but you'll probably burn through a lot of fuel trying to get the docking right. So make sure to have plenty of spare fuel.
Easier ways of refuelling are provided by various addons such as the Kerbal Attachment System that lets you link two vehicles for fuel transfer by EVA.
If your target is on the other side of the moon or planet (i.e. more than 90° away), it is generally better (fuel economy wise) to just go to a low orbit and deorbit to land where you want, as long as you perform a decent gravity turn. Because there's no atmosphere here, you should start that turn immediately after take-off with as low a pitch angle as supported by your TWR. Just don't hit any mountains. And note that the planet will rotate while you are orbiting, so adjust your orbit parameters accordingly or you will not be able to land near the intended site without a lot of corrections that cost fuel again.
Otherwise, here's what you do:
- If you have a contract, open map view and click on the site to enable navigation. Otherwise, figure out the heading manually.
- Enable SAS.
- Point at your target. If your vehicle is light enough, your pilot should be able to balance the hopper on one of its legs to facilitate targetting.
- Set your pitch to 45°. Maybe you remember that physics class where the teacher said throwing a ball at 45° will make it go farthest? So that's how we'll throw our Kerball for a maximum of distance with a minimum of fuel.
- Burn at full throttle until your trajectory ends where you want.
- Do not worry if your apoapsis is getting too high! This is normal, you're not going on a high orbit, since your horizontal velocity will be minimal.
- Do not make a gravity turn! We don't want to optimize our horizontal velocity for orbit, we want to land again, so a gravity turn actually wastes fuel here building up horizontal velocity for no good reason. Unless you're impatient, then it will get you there faster at the cost of a lot of wasted fuel.
- For sites far away, the moon or planet will rotate under your expected landing site while you are in flight, so make sure to adjust as necessary depending on the flight time and latitude of the landing site.
- If there are any mountains in the way you may need to burn higher than 45° to clear them, but afterwards you should switch to 45°.
Well, I don't think much needs to be said here, you've probably landed successfully many times already. Remember to quicksave (F5)!
However, here's some hints how to save fuel to have enough for more hops before refueling:
- If you were in orbit, add a maneuver node first to bring your trajectory near the surface at your landing site first.
- Don't just kill your horizontal velocity and then descend slowly, that wastes a lot of fuel fighting gravity for a long time. And often you'll miss the landing site anyway.
- Instead, add a maneuver node and pull the retrograde vector until your maneuver trajectory drops like a rock and the retrograde vector goes crazy.
- Compare the burn duration with that of your take-off burn, it should be about the same time or a bit less because your craft is lighter now.
- Make sure your navball is set to "surface" and not "orbit". Because of the orbital velocity of the surface itself (except at the poles), if you burn according to the maneuver indicator, you'll still have that velocity at the end of the burn, so instead just burn retrograde in "surface" mode.
- Timing the burn is a lot more important than in space. If you burn too soon, you'll end up at high altitude and need to waste fuel fighting gravity on the way down. If you burn too late, R.I.P.
- Depending on how close the maneuver node is to the ground, you will probably need to start the burn earlier than the usual T minus half the burn time. If the node is actually at the ground, you'll need to start burning exactly at T minus the burn time to make it! Note that the ground altitude in map view may not be completely accurate so don't rely on this too much.
- If you're at a high altitude with very little speed remaining, don't panic and burn fuel while slowly descending. Ideally you would wait as long as possible and then burn briefly at full throttle. This maneuver is called a "suicide burn" for a reason, though.
- Getting the timing of the landing burn right will take some practice for each new celestial body you explore. Addons such as MechJeb or Kerbal Engineer can help by giving a time and/or distance countdown to the optimal burn, but until you get the feel of the craft and the body, you should probably start a little earlier, just in case. You may also become good friends with the F5 and F9 keys.
Tada! You've learned how to hop around moons or planets without an atmosphere! Enjoy the many biomes you can visit this way.