Difference between revisions of "Tutorial:Basic Rocket Design"
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Currently, the game simulates drag only in a very simplified manner: Instead of accounting for the actual shape, each part simply has a flat drag value assigned. All drag values of your craft are then accumulated and used as a simple global drag value. This means that it is entirely irrelevant how you shape your rocket and it's the reason why currently you actually could send a pancake rocket up to space just as easily as a proper slim one. | Currently, the game simulates drag only in a very simplified manner: Instead of accounting for the actual shape, each part simply has a flat drag value assigned. All drag values of your craft are then accumulated and used as a simple global drag value. This means that it is entirely irrelevant how you shape your rocket and it's the reason why currently you actually could send a pancake rocket up to space just as easily as a proper slim one. | ||
| − | A more sophisticated drag system is planned to be included in | + | A more sophisticated drag system is planned to be included in version 1.0.<ref>[http://forum.kerbalspaceprogram.com/content/326-Beyond-Beta]</ref> Some players choose to use the [http://forum.kerbalspaceprogram.com/showthread.php/20451-0-21-Ferram-Aerospace-Research-v0-9-5-5-Aerodynamics-Fixes-For-Planes-Rockets Ferram Aerospace Research mod] which implements a more realistic drag and lift models. |
So the rules for designing rockets are simple: Make them light, make them smooth, and give them plenty of power. Fairly straightforward. | So the rules for designing rockets are simple: Make them light, make them smooth, and give them plenty of power. Fairly straightforward. | ||
Revision as of 08:17, 6 February 2015
[outdated]This tutorial describes all important basics on how to use the VAB and how to get your first rocket into orbit without a catastrophic failure. It will describe basic VAB functions, explain available part groups and their uses, and introduce you to basic staging while providing a multistage example rocket as well as describe how to complete your first launch and return successfully. It will also attempt to introduce various common science and engineering terminology that is frequently used in game without being too nerdy. Inter-Wiki-Links are used to allow you to gather more knowledge about specific aspects you may be curious about.
- Length: 45 minutes
- Difficulty: Beginner
- For version: Every version
Feedback is much appreciated on the Discussion page! If things are too complicated or confusing, the fault lies with the tutorial for not being elaborate and/or clear enough, so complain!
Intro to rocket science
The main rules for getting an object into space from Kerbin and keeping it there (which is called achieving a stable orbit) are fairly easy to explain: Your main adversaries to overcome are gravity and drag. Gravity decreases with the square of the distance as you move away from the center of the planet. Drag decreases almost exponentially until you reach the top of atmosphere (which is 69,078m for Kerbin). The effect of gravity increases based on the total weight of your vehicle. The effect of drag increases based on the shape of your object. The slimmer and "smoother" it is, the less drag you encounter. If you build a brick, don't be surprised if it flies like a brick. To overcome gravity and drag and achieve an orbit, you need to generate vast quantities of thrust (about 4000 m/s Delta-V for Kerbin). This is called orbital speed. Overall, the mass of your vehicle is far more important than drag, but a good design still makes valuable contribution to your rocket's efficiency.
Currently, the game simulates drag only in a very simplified manner: Instead of accounting for the actual shape, each part simply has a flat drag value assigned. All drag values of your craft are then accumulated and used as a simple global drag value. This means that it is entirely irrelevant how you shape your rocket and it's the reason why currently you actually could send a pancake rocket up to space just as easily as a proper slim one. A more sophisticated drag system is planned to be included in version 1.0.[1] Some players choose to use the Ferram Aerospace Research mod which implements a more realistic drag and lift models.
So the rules for designing rockets are simple: Make them light, make them smooth, and give them plenty of power. Fairly straightforward.
It is good practice to always first construct your payload and afterwards the rocket that is supposed to bring it into space (the latter is also often referred to as "launch vehicle"). Later, this will make it easier to better estimate what launch vehicle you'll need for your missions.
Placement helpers
This section discusses how to use placement assists. For how to use the VAB/SPH controls, see "controls" and "using the VAB/SPH controls.
Symmetry
On the bottom left portion of your screen, you can see a button with a yellow dot in the middle. If you click on it, you will enable the symmetry option. This means that now, whenever you place a part of your vehicle that is not in the middle vertical axis (top-down), it will be placed symmetrically on both sides. This option is immensely helpful when constructing launch vehicles, as this assures that you do not accidentally misplace parts which would shift your center of mass from the middle of the rocket. There will be cases where it becomes unavoidable to have a center of mass that is offset from the middle of the vehicle, but every offset has to be continuously corrected during the ascent, as it causes your rocket to "lean" into a specific direction. And energy used for correction is not available to push your rocket upwards. You can click this button various times to cycle through different symmetry modes (1,2,3,4,6,8).
Angular Snap
Just next to the Symmetry button is another button with a yellow circle and a dot within. This is a simple on-off toggle. Enabling this will cause all your placed parts to be angularly snapped to sets of 22.5° . This also is very useful in assuring that your vessel will have a mostly symmetrical center of mass, even if you place singular parts opposite of each other. This option should be enabled throughout this tutorial (actually, there only rarely is a case where disabling this ever is useful).
Physics indicators
Next to the yellow buttons are three more, green buttons, vertically aligned. From top to bottom these allow you to get a visual marker for: Center of mass (CoM), center of thrust (CoT) and center of lift (CoL). "Center of lift" is mainly useful for atmospheric vehicles (planes) and will be rather useless for rockets. The other two can be helpful to verify that both center of mass and center of thrust are well placed. Both should be centered horizontally. Center of mass should be in the lower third of your rocket (or rather: Close to your center of thrust). If it is far above, your rocket will be "top-heavy" and be more difficult to control during ascent. Center of mass will later change dynamically during flight as fuel is burnt up or stages are decoupled.
Rotating pieces
This is described in the Controls in detail. In short: Use the WASDQE buttons to rotate pieces in 90° steps before placing . When holding Shift while pressing WASDQE, rotation will be 5° per rotation (useful for angling panels, ladders, thrusters, etc.).
Payload Design
First, you need to decide on what the objective of your mission is. Then you figure out how to achieve said objective with the least cost and effort. The main objective of this tutorial mission is to get a satellite into a stable orbit around Kerbin. It is good practice to separate the design process into two parts: First you design the payload (the craft you want to have in orbit), then you design the launch vehicle (the rocket that will bring your payload into orbit). Upon creating a new vehicle, you are first required to choose a suitable command module. The available modules fall into two major categories: Manned and unmanned. If you want to send your Kerbals into space, make sure you pick a crew pod. If you want to keep your Kerbals safe while you dabble with your first excursions into the void, you can pick any of the unmanned modules. Remember the golden rule to keep overall mass as low as possible while maintaining your mission objective.
Reaction Wheels
- → See also: Reaction wheel
To facilitate rotation of a vessel, reaction wheels can be used to apply torque to the vessel at its part location, so bear in mind that reaction wheels far off the center of mass can induce undesired rotation effects. They are mostly used to rotate your craft precisely without the need to use up fuel. Instead, reaction wheels require electric charge. All command pods (manned and unmanned) contain basic reaction wheels. The torque provided scales roughly with the pod's mass (the bigger, the more torque). Reaction wheel torque is grossly overpowered ingame compared to reality, making it much easier to control crafts.
If you have a very heavy and bulky payload, you might want to add additional reaction wheels to speed up rotation time. There are several inline wheels available in the "Controls" panel.
SAS
- → See also: Stability Augmentation System
This toggleable assistance system included in all command modules (default: "T") supports rotation control (pitch, yaw, roll) of a vessel. Manual control input while SAS is active will be dampened and slow rotation rate to zero ("dampening mode"). This can be useful if you have very powerful rotation wheels that would cause too strong of a torque otherwise or to "brake" rotation.
If your vessel does not rotate, SAS switches to "Locking" mode. It will now try to hold the current orientation (which is not to be confused with heading).
SAS uses the same means of control available to you as user (reaction wheels, RCS, control surfaces, engines, etc.).
Electric Charge
- → See also: Electric charge
If your vessel runs out of electric charge, you will lose control over all devices that need it (losing reaction wheels being the most threatening) and all engines will be automatically cut. Unmanned probes require a continuous current for operation, and all reaction wheels require energy while activated. The easiest way to provide electricity is by attaching battery packs to your payload. Placement of these is irrelevant, all parts of your vehicle will be considered "wired" and can be supplied by the batteries, no matter what's in-between.
However, they have finite reserves, and if you intend to keep a satellite in space for a prolonged period of time it is advisable to provide a way to generate energy. There are three ways to do so in the current version (all except the engines are in the Utility tab):
- Radioisotope thermoelectric generators (Currently there is only the PB-NUK Radioisotope Thermoelectric Generator available)
- Some liquid engines produce a small amount of electric charge while running as a by product (Low power output, try not to rely on it.)
- Solar panels
Long-term satellites often use solar panels, which have a high output at no cost and are very lightweight. However, they require space to generate meaningful amounts of energy, need unobstructed sight of the sun and cannot take a lot of physical stress (drag, impacts, bumps, etc.). The amount of energy generated also decreases the further the probe moves away from the sun. The radioisotope generator is heavier than solar panels and provides little energy, but it has an infinite lifetime, is independent of solar radiation and is much more compact, making it popular for planetary probes that are active during night time. However, they are also very sensitive to physical stress and have to placed securely.
When relying on solar power to provide energy for a satellite on equatorial orbit, it is advisable to have sufficient amounts of batteries on board which can provide energy while passing the dark side of the planet. Once the sun rises again, battery packs will get recharged. 2-3 Z-100 Rechargable Battery Packs or a single Z-1k Rechargable Battery Bank will comfortably suffice for our mission. Batteries in KSP do not deteriorate in capacity or recharge ability as real life batteries do.
Attaching two OX-4W solar panels or three OX-STAT panels in a symmetric fashion at a minimum will ensure sufficient energy for most uses. Symmetric placement ensures that at least one panel will (almost) always face the sun, disregarding the vessel's current orientation.
RCS
- → See also: Reaction Control System
The reaction control system is a special system that creates thrust by expelling compressed gas. This gas is stored in a compressed form called Monopropellant (often shortened to Monoprop or Mono) as it does not require a mix of two propellants like all other engines. The thrust generated by these thrusters is very low, but - unlike in reality - they do not apply force on nearby other objects (the exhaust of regular solid or liquid thrusters can propel another object away), making them the first choice for all kind of high-precision rotations and translations in vacuum, where no significant resistance (drag, gravity) have to be overcome. The low thrust also allows a very fine control (even more so in precise control mode, default "Capslock"), perfectly suited for docking maneuvers. Another possible field of application is for ultra light probe propulsion on low-gravity, no-atmosphere planetoids or moons. A lightweight probe with lots of RCS thrusters can be controlled there effectively. They have two major upsides: While they require special fuel to be carried along (meaning extra mass), the actual thrusters are very lightweight and omnidirectional. Also, it is not necessary to take care of fuel lines for the thrusters, as they can be placed anywhere, and do not have to be directly connected to the monopropellant tank. Another upside is that fuel consumption is extremely low. Even a small tank of 25 units of monoprop can last for minutes of thrust. When mounted, RCS is not active by default. Like SAS, it can be toggled anytime (default: "R"). If RCS and SAS are active, RCS will be used for course corrections. This can be important if you have to speed up rotations or translations, but take care to not leave it active for prolonged periods of time, as SAS likes to do "mini"-boosts of RCS for continuous corrections. If you do not require fast course alterations or precise orbital corrections in space (as for docking maneuvers), you can simply choose not to install RCS at all and save valuable tons of weight.
If you decide to put them on, there are some things to consider: As the RCS thrusters are very low-power, you need many of them. A regular satellite module with ~3 tons weight that is intended to be docked at a space station should have 6-8 thrusters. As explained earlier, RCS is (almost) omni-directional. Still, in order to complete orbital maneuvers like rotating in a timely fashion, you need to be able to thrust into various directions.
For thruster placement, you should enable the "center of mass" indicator. Using the Symmetry-3 or Symmetry-4 option, place one set of thrusters radially near the bottom of your payload. Place another one equidistant from the center of mass near the top of your payload. For bigger crafts, you might want to add an additional ring of thrusters around the CoM to facilitate translating movements.
Failure to space your RCS thrusters equally from the center of mass will result in diverted torque application. This means rotational maneuvers will introduce slight movement, and movement thrusts will also introduce rotation. This is particularly important during docking when a steady focus on the target is necessary.
Science
- → See also: Science modules
These parts provide scientific data for research as well as measures to transmit said data back to Kerbin. They also mean extra drag and weight, so I'll forgo them in this tutorial.
Parachutes
- → See also: Parachute
If you don't want to keep your satellite permanently in space or be able to deorbit your Kerbals safely, there's no better way than using parachutes. Placing one on top of your payload is the best bet for a safe descent. If your payload is rather long in shape, it might be better to use a single radial parachute mounted at the center of mass to prevent toppling over on touch-down. Parachutes are very strong and can decelerate your payload safely onto Kerbin, thanks to its thick atmosphere. Don't be alarmed if parachutes are "floppy" after being deployed, they only fully deploy for the last 500m. Using the Mk25 heavy drag parachute is only necessary for heavy payloads to slow down enough for the final parachute to be able to do the rest. If your payload is descending too fast when the parachute deploys (aim for ~100 m/s vertical speed), the parachute gets ripped off, leading to a rather unpleasant touchdown.
Launch vehicle
Propulsion
There are two types of main propellants used to generate the large amounts of thrust needed to push you into space: Solid and liquid fuel. The main difference is that liquids are burnt in a controlled fashion, allowing you to alter and adjust thrust levels during flight. Solid fuel on the other hand burns with full power as soon as it is ignited, and will continue to burn so until its entire fuel reserve is depleted. The upside of solids is that they have a somewhat better thrust-to-weight ratio (abbreviated TWR) than liquid systems and are cheaper to produce and recycle. As by far the most energy of every space flight is required for the effort to escape Kerbin's atmosphere and reach a stable orbit, solid engines are often attached to the main rocket and will be decoupled after they burn out (often called "booster rockets").
Even a basic rocket design such as this should use several stages for an efficient ascent. Later you can refine your stages in advanced tutorials or experiments.
At the bottom of your payload you should place a single TR-18A Stack Decoupler. This ring allows you to trigger a small explosion that separates the top from the bottom. This will be used to discard the chubby main launch vehicle once you have reached space.
Below that follows the launcher. This should be composed of a very strong first stage that can push you through the thick first kilometers of atmosphere where gravity and drag are the strongest. Once you pass the first kilometers, resistance (drag, gravity) is considerably less and subsequent thrust can be lower and still be effective. The other upside of burning much and hard at the start is that you immediately begin reducing your weight by burning fuel and soon after by discarding empty fuel containers, making the following ascent phase more and more easy.
Using a two stage launch vehicle with three powerful boosters for takeoff will give us a lot of (surplus) energy to ensure we can establish a stable orbit.
The main launch stage should be a FL-T800 Fuel Tank to ensure escaping the atmosphere. An additional FL-T200 Fuel Tank and a light low-thrust LV-909 Liquid Fuel Engine will serve to fully establish the orbit and allow us to deorbit again. Three radially attached boosters will give us the kick we need (and then some) for the thick lower atmosphere. Use TT-38K Radial Decoupler with a three-symmetry option and place them radially on the lower end of the main fuel tank. After that, attach three Rockomax BACC Solid Fuel Booster on these decouplers. You will notice that the symmetry function works even on these "child objects", placing them perfectly on the other decouplers as well.
That should be enough total thrust.
Staging
Once you have finished adding parts to your vehicle (and really only then) you have to set up the stages of your launch. These are groups of actions that can be triggered by toggling the next stage (default: Spacebar). Stages always count backwards from the highest to the lowest stage. For the example above the staging would look like this:
- Stage 0: Deploy Parachute
- Stage 1: Decouple orbiter engine
- Stage 2: Decouple launch vehicle and enable orbiter engine
- Stage 3: Decouple booster rockets and enable main engine
- Stage 4: Enable booster rockets
The only thing one could argue about is whether to start the main engine right from launch or not. As we have a relatively light vehicle overall (~33 tons), the boosters provide plenty of thrust already. Enabling the main engine later will conserve its fuel which is needed for the upper atmosphere, but more importantly has to be used in a controlled fashion when circularizing the orbit (prograde boost at Apoapsis). The downside is that solid boosters have no thrust vectoring, so you have almost no means to correct your ascent trajectory while the main engine is shutdown.
Checklist
For the sake of this tutorial, I'll contribute a simple "This works"-list of a basic vessel you might want to check against the design you have come up with. If your vehicle is similar to this, don't bother altering it, I only provide a working example. The point here is to see if you missed anything necessary (see Catastrophic failure). From the top to the bottom:
1: Mk16 Parachute
2: Command Pod Mk1
- 3: Z-100 Rechargable Battery Pack 2x, symmetrically attached to the command pod
4: TR-18A Stack Decoupler
5: FL-R25 RCS Fuel Tank
- 6: RV-105 RCS Thruster Block 4x, symmetrically attached to the R25 fuel tank
- 7: OX-STAT Photovoltaic Panels 4x, symmetrically attached to the R25 fuel tank
- 9: RV-105 RCS Thruster Block 4x, symmetrically attached to the T-200 fuel tank
10: LV-909 Liquid Fuel Engine
11: TR-18A Stack Decoupler
12: FL-T800 Fuel Tank
13: LV-T30 Liquid Fuel Engine
- 14: TT-38K Radial Decoupler 3x, symmetrically attached to the FL-T800 Fuel Tank
- 15: Rockomax BACC Solid Fuel Booster 3x, symmetrically attached to the TT-38K Decouplers
- 16: Aerodynamic Nose Cone 3x, placed on top of the boosters . These are purely cosmetic: The current atmosphere model means they actually increase drag of your vessel, so the bare flat-top boosters would be more efficient than capping them.
Quick guide to your first orbit
Launch
Engage SAS before launch, then start the first stage to initiate takeoff (default: "Space"). While your boosters burn out, increase throttle to 100% (default: "Shift" and "Ctrl"). This has no effect on solid boosters, but will be used on the main engine in the next stage. When the boosters are empty, launch the next stage which decouples the boosters and ignites your main engine. Now switch to the Map View (default: "M"), enable the navball there and monitor your apoapsis point on the trajectory you see.
Gravity turn
- → See also: Gravity turn
An efficient ascent does not go "straight up", but uses gravity to prepare an orbit trajectory by angling your rocket in-atmosphere. For this tutorial,a simplified gravity turn is suggested: At 12–15 km (should be shortly after you decoupled the solid boosters), angle your vessel towards 45° pitch, keeping an eastern heading (90°). Maintain this angle until your map's apoapsis point is at ~75 km (edge of atmosphere is at ~69 km, but a buffer never hurts), then cut your engines (default: "X") and float into space! Remember to monitor your fuel reserves in the main tank; Once it is empty, trigger the next stage to use the last engine.
Establishing a full orbit
While floating to your apoapsis, click on the apoapsis point of your orbit. This will open a small maneuver control gizmo. Use the green lever without the central cross (prograde) and drag it out until you see your planned orbit's periapsis appear on the other side of Kerbin (some fiddling with the map camera is required, yes). Now slowly continue to drag it out until you see the orange apoapsis and periapsis markers "flip" positions. Now both the apoapsis and periapsis should be around 75 km, meaning a stable, round orbit. Your navball will now show a blue marker (look for it near the prograde marker), so you can fly the maneuver either in map view or viewing your ship directly. Align your vessel so it faces this blue indicator perfectly (ensure SAS is enabled for better precision and stability). At the right of the navball there's a bar showing how much thrust will be required to complete your planned maneuver, as well as when your maneuver point will be reached. As your burn will take some time, it's best to start before reaching the maneuver point. Divide the estimated burn time by 2 and you know how soon you should start your maneuver (e.g. if estimated burn duration is 30sec, start when the timer hits T-15sec). Use full burn (unless you overheat) to complete this burn as quickly as possible. Always monitor the navball to ensure you still face the blue marker. If you notice you are far off the marker and want to correct, always cut your engine first (X key). Only then correct your course and restart your engine. If you have completed the burn and check the map screen again, you'll see you now have an orbit that's very close to the maneuver orbit. You can delete the maneuver node now by clicking on it again and clicking the small red X symbol.
Congratulations, you safely put the first satellite into orbit which could circle Kerbin for centuries! If it's a manned mission, you now can test out the EVA suit and take a space walk (Don't get lost! There's no safety line to keep you near your ship!), perform RCS maneuvers or just enjoy the planet zipping past below you.
Reentry
When your Kerbal notices he forgot to bring food, you can initialize reentry. To aim for a specific landmark along the way, use the maneuver tool again, this time you'll need the other green lever (retrograde) as you'll have to retroburn to lower your orbit. When targeting for a ground point, bear in mind that once you enter atmosphere, atmospheric drag and gravity will provide additional deceleration energy, so your actual touchdown point will be considerably short of the one the map trajectory shows (maneuver and orbit prediction do not take atmospheric braking into account).
Decreasing periapsis too much will mean your flight through the atmosphere will be shorter. This means that the atmosphere will also have lesser impact on slowing you down, and there will be a much harsher heating and deceleration when you hit the thick lower atmosphere. A safe and slow reentry would lower the periapsis to ~32km. This way the craft will fly almost horizontal in the upper atmosphere, bleeding speed and only slowly heating up.
Once your deorbit burn is complete and you are confident to land near your target, decouple the orbiter's fuel tank. Reduced weight means a more efficient aerobraking and less stress on your vessel. You can toggle the last stage (which arms the parachute) whenever you want, though it will only semi-deploy below ~23 km. Even semi-deployed it will increase your drag, so if you fear you'll overshoot your target (probably not: The strongest braking effect only occurs in low atmosphere), this can help shorten your trajectory significantly. ~500m above ground the parachute will fully deploy, abruptly slowing your vessel down. Luckily Kerbals are too cool to care about G-shock. After that you just enjoy the view while floating for a safe touchdown, which hopefully will not be on a steep mountain slope.
Currently reentry heat effects are merely visual, but this is planned to be expanded with real heat and G-stress simulation on vehicle parts in a future update. Realism fans rely on the Deadly Reentry Mod to simulate this already.
