Sylvie Willis
The amount of fuel required to return from Mun depends on several factors, including the type of mission, weight, and landing efficiency. For instance, a soft landing requires more fuel than a flyby. The choice of engine also affects fuel efficiency, with the poodle engine having a vacuum specific impulse (Isp) of 350, while the twin boar engine has an Isp of 300. Additionally, the Oberth effect describes how burning horizontally at a low altitude can be more efficient than burning straight up. Returning to Kerbin from Mun involves burning towards the retrograde vector of Mun and getting into orbit before burning prograde at the right time. Some players suggest skipping the Mun orbit stage on the return journey, while others recommend using external fuel tanks to transfer fuel into the main tanks before takeoff.
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What You'll Learn
Use gravity to your advantage
To return from Mun to Kerbin, the most fuel-efficient method is to burn retrograde in the Mun's orbit. This involves pointing your ship in the direction opposite to the Mun's motion and burning your fuel, which will allow you to escape the Mun's gravity and slow down relative to your orbit around Kerbin.
Another important consideration is the specific impulse of your engine, which is a measure of its efficiency. Engines with higher specific impulse values will provide more delta-v for the same ratio of wet to dry weight. Therefore, selecting an engine with a higher specific impulse can help you maximize your fuel efficiency.
Additionally, you can take advantage of the Oberth effect. Instead of burning straight up, burn horizontally to remain at a very low altitude. Burning horizontally allows you to take advantage of the extra 41.4% efficiency gained at low altitudes when accelerating from orbital velocity to escape velocity.
Prior planning is also crucial. Ensure that your trajectory is optimized to minimize fuel consumption. For example, when setting your initial interception orbit, aim for a periapsis no lower than 10km. This will put you in the lowest, slowest, and most stable orbit. As a result, you will have less velocity and time to manage during landing, making it easier to time correctly.
Furthermore, consider using external fuel tanks on your lander. Transfer the fuel into your main tanks upon landing and jettison the external tanks before taking off. This will provide you with additional fuel reserves for your return journey.
By combining these strategies and carefully managing your fuel usage, you can take advantage of gravity and increase your chances of a successful and fuel-efficient return from Mun to Kerbin.
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Use more fuel-efficient rockets
To ensure a fuel-efficient return from Mun to Kerbin, consider the following:
Firstly, the choice of rocket engine is crucial. The "twin boar" engine, for example, is designed to push objects out of the atmosphere and may not be the most efficient choice for manoeuvring in space. Engines with higher vacuum-specific impulse (Isp) values will be more fuel-efficient for a given thrust. The "poodle" engine has an Isp of 350, while the "nerv" nuclear engine boasts an impressive 800. For even greater fuel efficiency, the xenon ion engine "dawn" offers an exceptionally high Isp of 4200. Selecting the right engine for the specific requirements of your mission is essential for optimising fuel efficiency.
Secondly, the number of engines utilised can significantly impact fuel efficiency. Employing multiple engines can decrease delta v, which is a crucial factor in determining fuel efficiency. Whenever possible, opt for a single engine or a smaller engine that can accomplish the task. Reducing weight is akin to adding more fuel, so consider ways to minimise unnecessary weight.
Additionally, the specific impulse of an engine is a critical factor in fuel efficiency. Engines with higher specific impulse values will yield more delta v for the same ratio of wet to dry weight. This means that you can achieve greater velocity changes with the same amount of fuel. Therefore, selecting engines with higher specific impulse can significantly enhance the fuel efficiency of your mission.
Furthermore, the trajectory and direction of your burns can also impact fuel efficiency. Burning prograde very slowly and monitoring your projected periapsis around Kerbin can help lower your periapsis efficiently. Aim for an apoapsis of about 10km and stabilise your orbit. Rather than focusing on achieving a stable Kerbin orbit, a periapsis of 50-60km should suffice. Additionally, consider the Oberth effect, which suggests that burning horizontally at a low altitude can be more efficient than burning straight up, especially for bodies with higher gravity.
By carefully considering the choice of engine, the number of engines, the specific impulse, and the trajectory of your burns, you can significantly enhance the fuel efficiency of your return journey from Mun to Kerbin.
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Burn prograde slowly
When returning from Mun, the most fuel-efficient method is to burn prograde very slowly. This is because prograde motion is the normal motion in the same direction as the primary body (in this case, Mun) rotates. By burning prograde, you can lower your periapsis efficiently and escape Mun's orbit. Here are some detailed instructions and considerations for burning prograde slowly:
First, it is important to understand the concept of the Oberth effect. This effect states that a spacecraft can gain additional speed and kinetic energy by falling into a gravitational well and then using its engines to accelerate further during the fall. This is a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well. By taking advantage of the Oberth effect, you can burn your fuel more efficiently at the lowest possible orbital periapsis when the kinetic energy is greatest.
When attempting to return from Mun, aim for a periapsis no lower than 10km. This will put you in the lowest, slowest stable orbit. Then, when you come to land, you will have lower velocity and more time to time your landing correctly. This will help you avoid crashing or running out of fuel before reaching the surface, as mentioned in some players' experiences.
Next, try to make your escape burn by burning prograde very slowly. This will lower your periapsis quite fast. By burning prograde slowly, you can gradually decrease your orbit and escape Mun's gravity. This method is safer than the "Suicide Burn" technique, which involves a single long burn that ends only when you touch down. The Suicide Burn method is extremely dangerous and nearly impossible without computer assistance.
Additionally, consider the specific impulse of your engine, also known as vacuum specific impulse (Isp). Engines with a higher specific impulse will provide you with more delta-v for the same ratio of wet to dry weight. Using more fuel-efficient rockets with higher specific impulse values will help you escape Mun's orbit with less fuel consumption. For example, the "poodle" engine has a higher vacuum specific impulse than the "twin boar" engine, making it a better choice for Mun landings.
Finally, remember that everything is in motion, so aim for where Mun is going to be, rather than directly at it. Mun's gravity can speed you up or slow you down, depending on how you use it. By understanding and utilizing the gravity of Mun and Kerbin, you can optimize your trajectory and fuel efficiency.
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Burn retrograde in LKO
To return to Kerbin from the Mun, it is important to consider the most fuel-efficient methods to ensure a safe landing. One key consideration is to use the Mun's gravity to your advantage. By burning retrograde in the Mun's orbit, you can escape the Mun's gravity and slow down relative to your orbit around Kerbin. This manoeuvre can be done by waiting until your ship is pointing retrograde to the Mun's motion and burning straight up. However, it is worth noting that burning straight up is less efficient than burning sideways.
Another approach is to perform a gravity turn by shooting for an apoapsis of about 10km and then stabilising the orbit. This is followed by a burn home, where you need not bother achieving a stable Kerbin orbit. Instead, a periapsis of about 50-60km should suffice.
When landing, it is generally better to burn retrograde at periapsis to lower your orbit and eliminate your horizontal velocity. This is because, at periapsis, your velocity is highest in your orbit, and burning retrograde maximises the Oberth effect, which states that the faster you travel, the more effective your rocket is at converting chemical energy into useful changes in velocity. However, it is important to note that burning retrograde at apoapsis is more efficient for lowering periapsis, as you are moving slower at apoapsis, requiring less energy to lower periapsis.
To ensure a soft landing, it is crucial to aim for a periapsis that is slightly above the surface. Once you reach this periapsis, burn retrograde to zero out your surface velocity, leaving only a small vertical drop to the surface.
Additionally, consider using more fuel-efficient rockets with higher specific impulse (Isp) to maximise delta-v and minimise fuel consumption.
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Use external fuel tanks
When it comes to returning from Mun, fuel management is critical. One effective strategy is to use external fuel tanks, which can provide the necessary fuel reserves for a successful return trip. Here are some detailed instructions and considerations for employing external fuel tanks:
Firstly, it is essential to understand the concept of delta-v, which represents the change in velocity and is a crucial factor in space travel. The delta-v required for various maneuvers depends on the specific impulse of the engine, which measures its efficiency. Engines with higher specific impulse values will yield more delta-v for the same amount of fuel. Therefore, selecting engines with higher specific impulse values can help optimize fuel usage.
When utilizing external fuel tanks, consider using multiple smaller tanks instead of a single large tank. This approach offers greater flexibility in fuel distribution and allows for a lower thrust-to-weight ratio. For instance, pairing each liquid engine with two FLT 800 fuel tanks instead of one can significantly extend burn time while reducing the thrust-weight ratio. This configuration is particularly advantageous when additional burn time is needed without exerting excessive force.
The choice of engines plays a vital role in fuel efficiency. Engines like the "twin boar" are powerful but less fuel-efficient, making them more suitable for lifting heavy payloads out of the atmosphere rather than maneuvering in space. Opt for engines with higher vacuum-specific impulse (Isp) values, such as the "poodle" with 350 Isp, the nuclear "nerv" engine with 800 Isp, or the xenon ion engine "dawn" with an impressive 4200 Isp. These engines will enable you to achieve the same results with less fuel consumption.
In addition to engine selection, careful flight planning can further enhance fuel efficiency. Prioritize lightweight designs and minimize unnecessary weight to emulate the effect of adding more fuel. Ensure that your gravity turn is initiated promptly after liftoff and completed before reaching an altitude of 10 kilometers. This early initiation helps conserve fuel, especially if you have limited control over your first stage. Maintain a steady trajectory by keeping your heading marker within the prograde marker's circle to avoid spinning out and wasting fuel.
Finally, consider employing creative techniques to further optimize fuel usage. For instance, take advantage of the Mun's gravity by using it to slow down or speed up, depending on your trajectory. Additionally, when returning to Kerbin, imagine the Mun as your ship and burn retrograde in its orbit to lower your periapsis efficiently. This retrograde burn will help you escape the Mun's gravity and slow down relative to your orbit around Kerbin, potentially bringing you home safely.
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Frequently asked questions
You need around 800 delta-V for the return trip from Mun to Kerbin. However, this number can vary depending on how efficiently you can perform your gravity turn, the weight of your craft, and the efficiency of your landing. It is recommended to aim for around 10,000 delta-V to account for potential mess-ups.
The most fuel-efficient way to return from Mun is to burn towards the retrograde vector of Mun. This is typically done by getting into orbit and then burning prograde at the right time. Burning straight up from the surface can also save a lot of fuel, but it is less efficient than getting into orbit first.
To ensure you have enough fuel, you can use external fuel tanks on your lander. Transfer the fuel into your main tanks before taking off and jettison the external tanks. Additionally, using more fuel-efficient rockets with higher specific impulse can help reduce fuel consumption.
