Mercury Return Mission

*Back to Mercury Activities*
A **Mercury return mission** is very difficult because of the high amount of fuel needed to spend all the delta-v required. In order to complete this mission, players have to build a massive rocket or conduct multiple gravity assists.

A mission to Mercury requires huge amounts of fuel. In addition, the planet lacks an atmosphere, so landing must be done with retrorockets. The best way to do this mission is to get the paid version which will allow you to build larger rockets, to merge rockets, and to exchange fuel. Still, the mission is possible with the free version. Some players suggest the use of helping rockets, get a few refueling stations orbiting Mercury or Venus, or use gravity assists in addition to in-orbit spacecraft assembly.

Helping rockets can push the main rocket out of Earth's atmosphere, push it out of Earth's sphere of influence, push a rocket out of Mercury's orbit or land on Mercury, return to Earth orbit and return safely and to help send the main rocket into orbit. In addition, if you use the paid version, helping rockets can refill the fuel tanks.

## Mercury transfer alternatives

There are 3 different ways to get to Mercury: directly from Earth, with the help of gravity assists, or with the help of Venus aerocapture.

### Direct transfer

This is the fastest and easiest way. However, fuel consumption will be huge. The rocket will fly by Mercury very fast because of the Sun's gravity pulling the rocket much faster (especially in its perihelion (Mercury's closest point in the sun)), so that it will need a lot of fuel to slow down to get to orbit. This technique can be performed well if you have a huge rocket, as allowed by the paid version. With the free version of the game, the rocket may not have enough fuel to return after a landing.

Mercury has a very small sphere of influence. To gain the correct trajectory, you need to do some very precise engine burns and some orbital maneuvers.

### Gravity assists

*see also: Gravity Assists*

The most commonly used method to reach Mercury is to use at least one Venus gravity assist meaning a mission to Mercury starts like a mission to Venus. This will lower the perihelion below the orbit of Venus and the aphelion below the orbit of Earth. It may be necessary to conduct two Venus gravity assists.

A more complex way to reach Mercury is the use of many gravity assists. A rocket can escape Earth's sphere of influence with the help of one or two Moon gravity assists. Then, with the use of up to five Earth gravity assists, it can lower perihelion to Venus orbit. Finally, two Venus gravity assists are enough to reach Mercury's orbit.

After all this, the rocket will need to be slowed down to enter mercury orbit. In order to do this, Entering Mercury orbit would require around 6 Mercury gravity assists to reach a orbit around the sun almost identical to Mercury's one.

The use of multiple gravity assists takes time, but save a lot of fuel, so it seems only interesting on free version of the application, or to learn how to master this technique.

### Venus aerocapture

This method is efficient, but risky. The rocket must first fly towards Venus, then slightly pass through the upper layers of the atmosphere. This will slow the rocket down enough to gain Venus orbit. This method is risky because you can burn up into the atmosphere failing the mission.

From the orbit of Venus, a rocket can use a transfer window to reach Mercury. With the help of one or two Mercury gravity assists, landing should be possible.

When a rocket enters Mercury's sphere of influence, it usually has a great speed because the sun's gravity, especially at its perihelion. Slowing down requires significant fuel. This is why it is important to decrease speed by using at least one Mercury gravity assist. If the rocket has a velocity below 1100 m/s when entering Mercury's sphere of influence, it can slow down without problems.

It is good to make the rocket first gain a low Mercury orbit before landing, because the the speed of the spacecraft at Mercury approach needs to be reduced in both cases: with or without orbit insertion before the descent.

## Mercury return mission without parts extension

It is possible to land on Mercury and get back to earth with the free version of this application, but this requires multiple launches to build the spacecraft in earth orbit, in addition to the use of gravity assists.

### Rockets

The rockets to use for Mercury return mission without parts extension are almost the same as for the Venus return mission without parts extension, the only differences are:

1. Three additional launches will be required to get 6 tugs and the descent stage refilled up to 100%.
2. The ascent stage is much smaller, due to the gravity 2.4 times smaller of Mercury compared to Venus

Due to fuel mass cascade effect, one less tug/launch may be sufficient, but unlike the mission described here, this hasn't been tested with only 9 launches instead 10.

#### Ascent stage

The ascent stage is the only part of the spacecraft to be different for Mercury return mission regarding Venus return mission. This stage with the booster to put it into earth orbit is visible in the picture above.

From top to bottom, the ascent is made of: parachute, capsule, heat shield, separator, 15t tank with 2 docking ports and 2 RCS on it, Hawk engine, separator, docking port to provide the connection with descent stage.

The ascent stage booster is made of a center core with 20t tank and Hawk engine for second stage, a separator, two 20t tanks and a Hawk engine for first stage, which is connected to two side booster with side separators. Both two side boosters are made of, from top to bottom: an aerodynamic nose cone, two 20t tanks and a Hawk engine.

#### Spacecraft

The spacecraft from Venus return mission can be reused for Mercury return mission, except for the ascent stage which can be drastically reduced.

Do not reuse a Quicksave from Venus mission to start Mercury one with same spacecraft, because you won't be able to get both Venus and Mercury achievements at the same time. Thus, 10 launches and 9 earth orbit rendezvous will be needed, but with some practice, each of them would only takes 15 minutes.

### Mission

Once the entire spacecraft has been assembled on 50 km earth orbit, the trip to Mercury will go as described below.

#### Trip to Venus

Use "Navigate To" feature to put the spacecraft on a trajectory to Venus, with a delta-V of approximately 700m/s. Once outside earth influence sphere, perform a trajectory correction to make a flyby of Venus at 76km (or a little less providing it doesn't enter Venus atmosphere) and take care to make this trajectory crosses the line linking Venus and the Sun in order to make gravity assists slow down the spacecraft. Transfer fuel to spacecraft tanks used by the three engines performing the burns, and jettison empty tugs.

#### Trip from Venus to Mercury

Once outside Venus influence sphere after the flyby, the orbit perihelion should be close to Mercury orbit. Burn retrograde to make this orbit tangent Mercury one, but without being closer than it to the Sun (because "Navigate To" feature would give misleading values in the part of the spacecraft orbit below Mercury one).

Click on Mercury and select "Navigate To". See gravity assists article for further details, but the delta-V given by this feature will be different depending on the relative position of Mercury and the spacecraft, and this will be cyclic.Therefore, wait for this value to decrease and try to get the lower one or at least a delta-V lower than 100m/s.

Each time you get an encourter with less than 100m/s of delta-V, perform the necessary trajectory corrections to do a flyby of Mercury, crossing the line linking the Sun to Mercury (be careful that the position of Mercury towards Sun during the flyby is not the same than during the trajectory correction) In order to get as much gravity assists as possible, without taking the risk of impacting Mercury. Tune the flyby altitude between 500 and 1000m by using 0.1% engine thrust or RCS if the flyby altitude is still changing too fast with this thrust level.

Once outside of Mercury influence sphere after the flyby, check that apohelion has decreased and repeat the procedure of the previous paragraph untill the spacecraft orbit is visually very close to Mercury orbit (this time the seen delta-V may follow a sequence as -145m/s and +145m/s on the next proposed encounter, showing that gravity assists won't help further). Perform perihelion raise after each flyby if spacecraft orbit goes below Mercury one, in order to avoid misleading delta-V values. After performing 7 flybys of Mercury, the spacecraft orbit should be close enough to Mercury orbit to perform Mercury Orbit Insertion burn during the next flyby, as shown in the picture below:

#### Mercury orbit insertion

During the next flyby, tuned at 48km, perform a retrograde burn at the periherm (trajectory's closest point to Mercury) to enter a circular orbit at 48km around Mercury. If necessary, perform trajectory correction at apoherm and periherm to get a clean circular orbit at 48km, this will help rendezvous procedure when getting back from ground. At this time, the spacecraft should have one last tug with approximately 50% fuel.

#### Mercury powered descent and landing

Once on 48km circular Mercury orbit, undock the descent stage from the spacecraft tug, deploy its landing legs and ignite the descent stage retrograde to start power decent. Always keep the engine facing the moving direction shown by the white arrow, and try to on/off the engine at 100% thrust to keep the speed ten times smaller than the altitude. Once the spacecraft vertical, try to pitch it a little bit to keep horizontal speed at 0 (horizontal speed is not shown when at 0m/s). Keep doing on/off of the engine at 100% to still have a vertical speed of one tenth of the altitude, and pitch to keep horizontal speed to zero until touching the ground. You should have approximately 25% fuel in descent stage once landed.

#### Return from Mercury surface to orbit

To get back to orbit, ignite descent stage and the next ones each time one is empty until rendezvous with the tug leaved on 48km circular orbit. Use the rendezvous method described in method without Navigate To functionality in Rendezvous page.

#### Return trip to Earth surface

Once ascent stage docked back to the tug in mercury orbit, transfer all ascent stage fuel to the tug, which should bring it to 56%. Use "Navigate To" feature to get a trajectory back to earth. You should be able to find a +1200m/s delta-V. Use this trajectory to perform the burn and once outside Mercury influence sphere, perform trajectory correction to do a flyby of earth at 15km, this will ensure atmosphere reentry at reasonable incidence. Once close to earth, burn retrograde all the 25% of fuel remaining in the tug to slow down as much as possible before atmosphere reentry, which should bring spacecraft speed from 3000m/s to approximately 1700m/s. Undock the capsule from the spacecraft and pitch it to make heat shield face moving direction. Heat shield temperature should raise up to 2700°C during atmosphere reentry. Open the parachute at 600m and click on "Recover" once on the ground.

You should then have the 2 Mercury achievements.

## See also

* Mercury
* Getting to Mercury
* Mercury Activities
* Mercury Landing Strategies
* What to do in Spaceflight Simulator?