Sancho study: designing the minimum Earth escape spacecraft

Orbiter (Sancho) operations

Following the industrial assessment of Don Quijote mission concept, engineers from ESTEC and ESOC participated in a common and interdisciplinary effort to analyse the massive quantity of documentation relating to the possible solutions put forth by industries. As a result, a set of recommendations and key lessons learned were compiled. This precious knowledge inspired and suggested alternative ways to optimize the orbiter spacecraft design that have been further analyzed by ESA's General Studies Programme.

As a first step the efforts have concentrated on only the Sancho spacecraft (Small Autonomous Navigation CHaracterisation Orbiter), which has since become the subject of a dedicated study at the Concurrent Design Facility (CDF).

Instead of simply reproducing previous studies, the Sancho study tested the limits of interplanetary mission design. The aim was to produce a new set of key 'lessons learned' which can be used in the future in many other mission design studies and concepts. "Beyond the Earth-Moon system, the Near Earth Objects are our closest and most accessible neighbours. Therefore by assessing the design of a mission to an easy-to-reach NEO, we are automatically investigating the most modest but valuable form of interplanetary spacecraft" says Naomi Murdoch from ESA's Advanced Concepts Team.

Here several options have been analysed starting from a new target asteroid selection based on engineering parameters to the choice of appropriate spacecraft propulsion systems.

Mission Objectives

The main objectives of the Sancho study were to:

  • Analyse alternative options and design a low-cost asteroid mission which maintains a link with the general Don Quijote objectives. It is part of the "In-Orbit Demonstration" series of technology mission and should focus on:
    • Deep-space precise orbit determination
    • Autonomous asteroid rendezvous, orbit insertion, orbit maintenance, and close proximity operations.
  • Provide budgets, configuration, subsystem designs with required performance, definition of space and ground operations and facilities, definition of programmatics and performing a risk assessment and cost analysis for such mission.

The assessment considered two possible mission scenarios: the "Cheap Option" and the "Flexible Option". The results presented do not represent how the final mission would look, but can be used as guidelines or for reference purposes.

“Cheap Option” Design Scenario

The deployed configuration of the Cheap Option
In the “Cheap Option”, Vega, the future smallest European launcher, is used to place the spacecraft into a 300 km Low Earth Orbit (LEO) and then a Star 48 upper stage is then used to place Sancho onto a direct injection transfer to its target.

A simple chemical propulsion system is selected for the spacecraft in order to perform both the trajectory correction manoeuvres and to rendezvous with the asteroid upon arrival. Finally, a spacecraft design philosophy based on simplicity has also been adopted, leading to reduced complexity and cost.

The target selected for the “Cheap Option” mission scenario is the Amor Asteroid 2003 SM84 as it represents a good compromise in terms of size and distance from the Earth.

2003 SM84 characteristics

Parameter

Value

Absolute Magnitude (H)

22.74

Density

Unknown, 2.00 gr/cm^3 assumed

Diameter

70-160 m, 115.00 m assumed

Mass

1.59*10^9 kg (spherical shape assumed)

Semi-major Axis

1.12583 AU*

Perihelion

1.0332 AU

Aphelion

1.2185 AU

Orbital Period

436 days

Eccentricity

0.082285

Inclination

2.795 deg

Next observation

October 2009

* 1 AU = 149 598 000 km
The small mass of this object results in Sancho orbital velocities which are in the order of a few cm/s and only hundreds of meters in altitude. However, simulations prove that stable orbits do exist and it is guaranteed that close-proximity operations can be safely conducted whilst avoiding any risk of collision.

A direct interplanetary trajectory to this target was found and the characteristics are shown in table below. It will take approximately 18 months for Sancho to complete the transfer, followed by approximately 14 months of operations.

Cheap Option Trajectory Characteristics

Parameter

Value

Departure Date

2015/4/7

Vinf

2.18 km/s

Initial Mass

462 kg

DSM ΔV

404 m/s

Arrival Date

2016/10/5

Arrival Velocity

477 m/s

Arrival Mass

287 kg

The timing of the operations has been specifically planned such that the radio science experiment (RSE), necessary to calculate very precisely the trajectory of the asteroid, is done while the Sun-spacecraft-Earth angle changes the least. The time spent waiting for the optimum period of the RSE is not wasted, as it can be used to complete the mapping of the asteroid and investigate the surface properties.

The distance to the Sun during both the transfer and the operations remains steadily between 1 and 1.2 AU which is of great advantage while sizing the different spacecraft subsystems.

The "Cheap Option" Platform Design & Configuration

Proteus Solar Array

The “Cheap Option” design scenario consists of:

  • An aluminium structure with a central cylinder holding the propellant tank and the shear panels.
  • Two deployable solar arrays, totalling 7 m^2, based on the Proteus Solar Array (see image)
  • One battery for launch mode and safe mode.
  • The monopropellant propulsion system has a single tank, used for spacecraft attitude control as well.
  • The communication subsystem consists of a 1m dish high-gain antenna, with no pointing mechanism.
  • There are also low-gain antennas for back-up and Low Earth Orbit (LEO) operations.
  • A passive thermal design has been selected.
  • In fact, the only mechanisms on-board are the solar array and HGA hold-down and deployment mechanisms.

While no specific purely scientific payload was selected to be on-board, the spacecraft is equipped with a narrow- and wide-angle camera for the autonomous-rendezvous phase with the asteroid. A range finder is also selected for close proximity operations. However, other payloads could be considered and eventually included thanks to a robust margin philosophy allowing possible extra mass to be available. The Attitude and Orbit Control system (AOCS) also consists of star trackers, Sun sensors, inertial measurement units, and reaction wheels. The on-board computer has also been simplified by placing the electronics and memory required for navigation and mapping in the camera itself. To imagine the simplicity of the overall system, the maximum power demand during operations around the asteroid is a mere 400 W, which is less than the demand of a common hair drier!

The mass budget for the Cheap Design Option SANCHO spacecraft is:

Total Dry mass (with margin) 289.03 kg
Propellant mass 175.87 kg
Total Wet mass 464.90 kg

“Flexible Option” Design Scenario

The launch configuration inside the Vega fairing with Star-48 PM and adaptors

In order to provide the most flexible design, capable of reaching multiple targets, an electric propulsion architecture was selected. For the same launcher/upper stage configuration as in the “Cheap Option”, over 10 targets were identified complying with a set of engineering constraints such as size, trajectory type and the feasibility of the radio science experiment (RSE). Among these Apophis was selected as it represents the perfect candidate for the Sancho mission: obtaining a very precise orbit determination of this object might be absolute necessary one day in order to rule out a possible Earth impact in 2036.

Apophis characteristics

Parameter

Value

Absolute Magnitude (H)

19.6

Density

Unknown, 2.00 gr/cm^3 assumed

Diameter

270±30 m

Mass

2.06*1011 kg (spherical)

Semi-major Axis

0.922261 AU

Perihelion

0.7461 AU

Aphelion

1.0985 AU

Orbital Period

323.5 days

Eccentricity

0.191059

Inclination

3.331 deg

Next observation

October 2009

A spiral trajectory with long periods of thrusting was found which takes over two years for the spacecraft to rendezvous with Apophis. The main characteristics of the trajectory are shown in the table below. Shorter interplanetary transfers are possible, however, there is an engineering constraint which limits the power available to the propulsion system. In return this meant the solar array size was constrained, which also brought with it benefits in the form of a smaller mass, smaller disturbances from solar radiation when orbiting around the asteroid and a simpler attitude control design.

Flexible Option Trajectory Characteristics

Parameter

Value

Departure Date

2013/4/1

Vinf

3.16 km/s

Initial Mass

390 kg

Total ΔV

390 m/s

Arrival Date

2015/5/11

Arrival Mass

368 kg

“Flexible Option” Platform Design & Configuration

The “Flexible Option” design consists of:

  • An aluminium structure with a central cylinder holding the propellant tanks with shear panels, just as the “Cheap Option”.
  • Two deployable solar arrays (4.1 m^2 in total), with a 1 degree-of-freedom pointing mechanism for Sun-tracking.
  • One battery for launch and safe mode.
  • A 1m high-gain antenna for which a pointing mechanism is also required.
  • Two low-gain antennae for Low Earth Orbit operations and contingencies.
  • The passive thermal design consists of heaters, radiators and MLI.
  • The attitude is guaranteed by resistojets which share their Xenon and tank with that on the electric propulsions system.
  • The electric propulsion system is made up of a gridded ion engine with a system sized for 700 W power available at 0.75 AU.

The mass budget for the Flexible Design Option Sancho spacecraft is:

Total Dry mass (with margin) 318.85kg
Propellant mass 44.00 kg
Total Wet mass 362.85 kg
Total mass at launch 390 kg

Last update: 23 May 2012

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