Aerobraking:

The New Inexpensive Way to Travel

           Included- How Aerobraking can be used to "Aerocapture" a Space Vehicle

 

 

 

Chelsey Bryant

ASEN 5050 Spaceflight Dynamics

December 2002

 

ABSTRACT

A process referred to as aerobraking, has recently become an inexpensive technique to transform a high elliptical orbit to a lower circular orbit without using the spacecraft’s engine. The traditional method of lowering a highly elliptical orbit involved using the satellite’s engine to slow the spacecraft at the orbit’s point of perigee. This would result in the next pass at apogee to be at a lower altitude. Instead, a satellite can be directed into the upper fringes of a planetary atmosphere to lower its orbit. When the satellite encounters an atmosphere, some of its orbital energy is converted to thermal energy due to the aerodynamic drag on the vehicle induced by the particles the satellite encounters in the atmosphere, a decrease in velocity will then occur. Therefore, with every subsequent pass through the orbit’s point of perigee, the satellite’s velocity will decrease slightly and consequently the spacecraft will not climb as high on its next pass. The change in acceleration of the spacecraft due to aerobraking can be calculated. This is largely dependent upon the density of the atmosphere through which the spacecraft passes. A type of aerobraking called aerocapture, although risky, can even further the benefits gained during aerobraking.

 

 

Introduction

A process referred to as aerobraking, has recently become an inexpensive technique to transform a high elliptical orbit to a lower circular orbit without using the spacecraft’s engine. This innovative technique was developed by mission design engineers at the Jet Propulsion Laboratory and has since been used on several missions, including the Odyssey spacecraft in 2001 and the Magellan spacecraft in 1994.

 

                              

                       Figure 1: Odyssey and Magellan Spacecrafts both utilized aerobraking during their missions

 

The aerobraking process occurs through several orbital passes around a planet. With each pass the orbital altitude of the spacecraft lowers due to the drag the spacecraft encounters in the planet’s upper atmosphere. As atmospheric molecules strike the spacecraft, there is a transfer of energy and momentum. The momentum transfer creates the drag which is necessary for the aerobraking to occur. The use of atmospheric drag to reduce orbital energy, and therefore lower the orbit, conserves fuel, mass, volume, and cost.

In order for the aerobraking process to begin, the spacecraft is commanded to lower its altitude at periapsis to a point where the orbit encounters the upper fringes of the planet’s atmosphere. The spacecraft will then encounter air resistance, which will slow the vehicle slightly. The spacecraft will therefore not climb as high on its next pass at apoapsis. Eventually, the spacecraft will reach its desired altitude around the planet.

Unfortunately, aerobraking has several drawbacks. The aerobraking process is largely based upon the density of the planet’s upper atmosphere. This density is difficult to measure precisely and therefore makes aerobraking a risky technique. Also, aerobraking is a lengthy process, which can often last over several months.

 

The Aerobraking Process

The aerobraking process begins with the spacecraft configuring itself in an orientation such that would increase drag. Both Magellan and Mars Global Surveyor used their flat solar panels and high gain antenna dishes to provide a large profile area.MGS also released extra flaps located at the end of the solar panels to further increase the drag.

                                                    

                                                       Figure 2: MGS in its aerobraking configuration

 

Once the spacecraft is in its desired orientation, the spacecraft then enters into the first of three phases that makes up the aerobraking process. Once the spacecraft is in its drag position, it then performs several small propulsive maneuvers to lower the orbit periapsis to the altitude at which aerobraking will occur. The spacecraft will then remain at the aerobraking altitude until the apoapsis altitude is lowered to very near the desired orbital altitude. The length of this aerobraking phase is dependent upon the amount of drag the spacecraft encounters in the planet’s atmosphere. The final phase of the aerobraking process requires the spacecraft to reduce the apoapsis altitude, while slowly increasing the periapsis altitude. At this point aerobraking is terminated with a maneuver which raises periapsis out of the region of significant drag.

Figure 3 shows a diagram of the different phases that occur during the aerobraking maneuver.

 

Mission Phase

Action

Figure 3: Diagram showing the stages of the aerobraking process

1) Satellite enters initial orbit
  • Systems Checkout
  • Initial Aerobrake Orbit entered

2) Perigee lowering burn
  • With the use of thrusters, lower perigee to target altitude

3)Aerobrake drag near Perigee
  • With each pass through the atmosphere, the aerobraking drag reduces the orbit energy and lowers the orbit apogee

4)Apogee burns to control perigee
  • Apogee burns will be made as necessary to adjust perigee altitude to counter secular orbit disturbances and maintain perigee altitude within the target window

5)Perigee raising burns
  • As the apogee altitude nears the desired level, several perigee burns will raise the perigee out of the atmosphere, therefore stopping the aerobraking

6)Final Circular Orbit
  • Thruster burns will now set the satellite at the desired parameters

 

 

Heritage of Aerobraking

Aerobraking was first demonstrated during the Magellan spacecraft mission in 1994. While at Venus, Magellan used aerobraking to circularize its orbit and then later to study the Venusian atmosphere and improve the estimates of the aerodynamic properties of the Magellan spacecraft.

After the success of this mission, aerobraking was then incorporated into the Mars Global Surveyor Mission (MGS) which reached Mars in September 1997. The MGS mission’s purpose was to analyze the physical and atmospheric properties of Mars in order to gain a better understanding of the planet and how it was formed. Through repeated aerobraking passes the MGS spacecraft slowly changed its high elliptical orbit to a low circular orbit. Due to the use of aerobraking rather than propulsive maneuvers to change the orbit, the cost of the mission was reduced dramatically. The aerobraking was scheduled to take six months to reduce the apoapsis and align the MGS in the proper polar orbit. Unfortunately, due to several unexpected events during the mission the aerobraking process did not proceed as planned and the timeline for the mission had to be extended for over a year. An unexpected structurally impaired solar panel on the spacecraft, as well as major fluctuations in the density of the Martian atmosphere were two of the major problems encountered during the mission. Eventually the aerobraking process for the MGS mission was successful as well as the fulfillment of all of the science requirements for the mission.

 

                                                              

                                                              Figure 4: MGS over Mars during Aerobraking

 

 

The Odyssey spacecraft, which was launched in 2001, also utilized aerobraking during its mission. The aerobraking phase was the most complex phase of the entire mission, yet is was completed successfully. Odyssey skimmed through the upper reaches of the martian atmosphere 332 times. By using the atmosphere of Mars to slow down the spacecraft in its orbit rather than firing its engine or thrusters, Odyssey was able to save more than 200 kilograms (440 pounds) of propellant. This reduction in spacecraft weight enabled the mission to be launched on a Delta II 7925 launch vehicle, rather than a larger, more expensive launcher.

             

Figure 5: Odyssey Spacecraft during Aerobraking

 

Calculating the Affects of Aerobraking

The affects of aerobraking are based largely upon the density of the atmosphere through which the spacecraft is passing. The density in the upper fringes of the planet’s atmosphere is difficult to determine due to the density changing often. The density of the upper atmosphere changes because of a complex interaction between three basic parameters: the nature of the atmosphere’s molecular structure, the incident solar flux, and geomagnetic interactions.

During the MGS mission, the common presence of a dust storms on Mars was an important consideration to take into affect. In such storms it is not the dust itself which is a concern, since the dust does not reach the altitudes at which aerobraking occurs, but it is the increase in density at the aerobraking altitude associated with the expansion of the atmosphere due to the atmospheric heating induced by the solar heating of the dust. It has been determined that the density at aerobraking altitudes could increase by as much as a factor of ten in just a few days following the start of a major dust storm.

Equation 1 represents the basic equation to calculate the acceleration due to aerodynamic drag.

                                          (1)

The coefficient of drag, cD, is a dimensionless quantity that represents how susceptible to drag the spacecraft is. This value is difficult to approximate, but can be calculated using the following formula.

, where cpmax varies with Mach number and atmospheric properties and a corresponds to the angle the satellite is oriented to for attack. For satellites in the upper atmosphere, the drag coefficient is often approximately 2.2(using a flat plate model).

The atmospheric density, r , indicates how dense the atmosphere is at a specified altitude. To calculate the density of the atmosphere at a specified altitude above the planet’s surface, equation (2) is used.

                                                      (2)

As can be seen above, the ideal-gas law relates the absolute pressure, r 0 , the mean molecular mass of all atmospheric constituents, M, the acceleration due to gravity, g0 , the universal gas constant, R, and the absolute temperature, T (Kelvin). Much of the difficulty in determining an exact model for the density is dependent upon the temperature.

Since atmospheric density is such an important parameter for determining the expected drag, the density most be continually approximated to make any needed modifications to the mission. If a large increase in atmospheric density occurs, the periapsis altitude must possibly be raised to prevent damage to the spacecraft.

In equation (1), A represents the exposed cross-sectional area of the spacecraft. This is another difficult parameter to measure. The exposed cross sectional area is defined as the area which is normal to the satellite’s velocity vector.

The mass, m, of the satellite, and the satellite’s relative-velocity vector are also needed to calculate the acceleration due to drag. The relative-velocity vector is the velocity vector relative to the atmosphere. This vector can be determined using equation (3).

                                          (3)

The total propulsive change for an aerobraking maneuver can also be calculated.

The following equations are applicable to an aerobraking maneuver that begins at a point in a large circular orbit of radius . The spacecraft then travels through a transfer ellipse with apogee at and perigee at R. Along the circular arc at Radius R the speed decreases due to the atmospheric drag the spacecraft encounters. The following pass will now have a radius at apogee of and perigee at R. Assuming the aerobraking maneuver is completed, the velocity is propulsively increased to circularize the orbit at . This explanation similarly follows the diagram shown in figure 4.

Beginning with an initial circular orbit, the velocity is

                                                        (4)

 

The semi-major axis of the transfer ellipse can be calculated using equation (5).

                                                   (5)

Therefore, from energy calculations:

                  (6)

The required impulse is and this reduces to

                                         (7)

For the second burn, the final circular orbit has a radius , therefore

                                            (8)

And the second transfer ellipse has a semi-major axis that can be found with equation 9.

                                       (9)

so that from energy calculations

                                (10)

The required impulse is and this reduces to

                            (11)

Therefore, the total required propulsive change for the aerobraking maneuver is

Benefits of Aerobraking

An easy way to view the benefits of the aerobraking process is to compare the MGS and Mars Observer Missions. A comparison between these two missions makes sense because the mapping orbits for each mission are the same. The payloads of the two spacecraft’s differed by 81 kg, yet due to the extra needed fuel on the Mars Observer Mission for its propulsive manuevers, the total weight difference was 1512 kg. By using aerobraking, MGS was able to use a much smaller and less expensive launch vehicle. The money saved during the MGS mission due to aerobraking resulted in being $200 million dollars. Savings, such as this, have led to the incorporation of aerobraking in present and future mission to increase.

Drawbacks of Aerobraking

Aerobraking is a slow process that can often take several months to complete. Also, there are many fluctuations in a planet’s atmosphere, that require constant interpretation during a mission for the aerobraking maneuver to proceed as planned. The risk in the process can be higher than a normal orbital entry due to the heat a spacecraft can experience during the drag. Therefore, a minimum altitude must be carefully calculated based on the planet’s atmospheric density, to predict how low is too low for the spacecraft to fly.

Extension- How Aerobraking can be used to Aerocapture a Space Vehicle

Aerocapture

It has been shown that the use of aerobraking can save millions of dollars to a spaceflight, yet the time needed to perform aerobraking can be too time consuming for a mission. There is a type of aerobraking called "Aerocapture" that has yet to be used, but would increase the benefits that a mission would gain from using aerobraking.

Aerocapture is an aerobraking flight maneuver performed by a spacecraft to transfer to its desired orbit using only one atmospheric pass. The difference from the above explained aerobraking is that aerocapture transfers from a hyperbola to an ellipse. Therefore, avoiding the several elliptical passes necessary during aerobraking. This technique is very attractive for planetary orbiters since it permits spacecraft to be launched from Earth at high speeds, to give a short trip time, and then reduce the speed by aerodynamic drag at the target planet. As during aerobraking, aerocapture requires the spacecraft to raise out of the atmosphere at apoapsis through a propulsive maneuver (once the desired orbit is obtained). Figure 6 shows a diagram representing the course a spacecraft would follow during the aerocapture maneuver.

 

Figure 6: Path of spacecraft during aerocapture

To perform the aerocapture maneuver, the spacecraft will descend into relatively dense layers of the planet’s atmosphere, much denser than a spacecraft would experience during aerobraking. Also, the aerocapture entry velocity into the planet’s atmosphere will be much higher than that experienced during aerobraking, therefore causing the heatload the spacecraft will experience to be high. Due to the high temperature rise that could occur, a heatshield is necessary to protect the spacecraft. Once the spacecraft is ready to raise its orbit periapsis, the heatshield must be jettisoned to minimize heat soak.

Aerocapture is an extremely risky process and therefore a flight test is necessary before any major planetary science missions. Yet, there is a problem; Aerocapture flight speeds can not be duplicated in a lab. Therefore, flight test data such as flow field structure, aerothermochemistry, and aerodynamic forces is needed.

Therefore, the benefits from aerocapture include reduced mass of the spacecraft and shortened mission time. To perform the same reduction of velocity without aerocapture, a large propulsion system would be needed on the spacecraft.

Conclusion

As can already be seen through past missions, such as MGS and the Odyssey Mission, aerobraking is a successful maneuver to lower a spacecraft’s orbit. Also, it is a useful maneuver that can reduce the cost of spaceflight missions dramatically. As more information is gathered about each planet, the difficulty of performing aerobraking decreases. And as can already be seen through past missions, such as MGS and the Odyssey Mission, aerobraking is a successful maneuver to lower a spacecraft’s orbit.

The process known as aerocapture hastens the aerobraking process, requiring only one pass through the atmosphere and rapid deployment of a braking device to reach an intended orbit. The technique promises to reduce braking time dramatically ; from months to hours. The shortened flight time alone makes aerocapture suitable for human missions. The challenges include the need for heat-shield and, just as during aerobraking, detailed atmospheric information for each destination.

References:

Bridges, Andrew “Mars Odyssey Navigates Atmosphere” Associated Press  13 December 2001

 

Cancro, George J., Robert H. Tolson, Gerald M. Keating. “Operational Data Reduction Procedure for Determining Density and Vertical Structure of the Martian Upper Atmosphere From Mars Global Surveyor Accelerometer Measurements.” October 1998.

 

Kaelberer Monte, Kopman, Brain, Perin, Valentine. MGS Aerobraking 13 February 2001

 

Lee, Gentry. “The Tricky Science of Aerobraking.” Tech Wednesday (03 October 2001): 1-3.

 

Vallado, David A.  Fundamentals of Astrodynamics and Applications. El Segundo, CA: Microcosm Press, 2001.

 

“Inward Spirals”  Mars Global Surveyor  7 October 2002.

“An Explanation of How Aerobraking Works” Mars Global Surveyor 7 October 2002