PH 711: Spacecraft mission design and selection: real examples

PH 711: Spacecraft mission design and selection: real

examplesDr. Mark Price: Spring 2011

We will go through the procedure of a real example of the mission proposal and selection process.

In 1 or 2 lectures I can barely scratch the surface! Even the smallest science mission costs >£100M and there is quite literally THOUSANDS of pages of documentation for even a modest mission.

However, hopefully, it will give you a flavour for the process.

Mission selection

Call for proposals:

◦ ESA regularly makes a ‘call for proposals’ – approximately once every 5 years. (NASA has a ‘decadal’ study).

◦ This is a chance for the scientific community to get together a proposal for a scientific space mission

◦ Each proposing team is normally made up of 20 – 200 scientists from the relevant field

◦ This call lasts approximately 3-4 months

◦ Warning: ESA and NASA love acronyms! The documentation is littered with them!

ESA mission selection (NASA is very similar, but US-centric).

Activity Date: ‘Cosmic Visions’ call 2007

5 March 2007: ◦ Release of 1st Call for mission proposals of Cosmic Vision 2015-2025

30 March 2007: ◦ Deadline for Letters of Intent

11 April 2007: ◦ Briefing to proposers at ESTEC

29 June 2007: ◦ Mission Proposals due

July - September 2007: ◦ Proposal evaluation process by ESA and Peer Review Teams

October 2007: ◦ Working group/SSAC evaluation and selection of: ◦ 3 Class M mission proposals (cost ceiling €470M)◦ 3 Class L concept proposals (cost ceiling €900M)

ESA selection timeline

ESA mission selectionESA headquarters science council meeting

ESA science programme has three distinct domains◦ Astrophysics (Astrophysics Working Group: AWG)◦ Solar System (Solar System Working Group: SSWG)◦ Fundamental physics (Fundamental Physics Advisory

Group: FPAG) Each group (depending on the mission proposal)

passes its recommendation up to the Space Science Advisory Committee (SSAC).

The SSAC then passes its recommendation up to the Science Programme Committee (SPC) which has delegates from each of ESA’s member nations. They have final say.

ESA mission selection

ESA mission selection ‘flow’ chartESA mission proposals

52 proposals received from the community Of those 6 for each ‘M’ and ‘L’ class

proposal are selected Principal Investigators (‘PI’s) from each

proposal are invited to ESA headquarters in Paris to give a talk to the selection board based on their ‘yellow book’ proposal.

These 6 are then whittled down to a just 3 to go through to the next evaluation phase.

2007 call: ESA mission proposals

Timeline prior to any spacecraft construction

ESA mission proposals

ESA mission proposalsVarious phases of a mission prior to launch

ESA received 52 proposals but whittled it down to 6 candidates

Class M candidates (2 will be successful)◦ EUCLID◦ PLATO (we will discuss in detail)◦ Solar Orbiter

Class L candidates (1 will be successful)◦ EJSM-Laplace (we will discuss in some

detail, if there is time).◦ LISA◦ IXO


Each proposer submits a ‘yellow book’ which is a document describing in detail the science and the feasibility of the mission.

These documents are hundreds of pages long!

The ‘yellow book’ must explain to the selection panel◦ Why your science is top priority◦ How can you do your science (does the technology

exist?)◦ What are the risks?


Plato – main science goals:

1) The detection of extra-solar planetary systems of all kinds, including small, terrestrial planets in the habitable zone of solar-type stars.

2) A precise characterization of the basic physical parameters of the detected exoplanets and the host stars: radius, mass, age.

3) The identification of suitable targets for future, more detailed characterization.

Yellow book example pages: Plato

Yellow book example pages: Plato(Effectively Plato is a super-sensitive photometer!)

Plato is a super-sensitive photometer (measures the amount of light from a star very accurately)

The proposal is to build on Kepler/GAIA results and detect thousands of Earth-sized exoplanets in a (near) all-sky survey.

Three different spacecraft optical configurations (telescope design and CCDs) proposed from three different contractors:◦ Astrium (‘Concept A’)◦ Thales (‘Concept B’) ◦ PPLC (‘Concept C’) [PPLC = Plato Payload Consortium]


Plato: concept A (Astrium)•Uses 12 reflecting telescopes to achievea field-of-view of 1800 deg2

•Optical structure is silicon carbide to reducethermal expansion/contraction•Mirrors are aspheric and require special manufacture

•Has a sunshield deployed in space•Requires 176 CCDs with a resolutionof 2080x2574

Plato: concept B (Thales)

•54 refracting telescopes mounted on optical baseplate•Telescope primary lenses made of BaF2 to cover wavelength range 450 – 1000nm•Total field-of-view ~ 625 deg2

•Each telescope to have 3000x6000 pixel CCD

Telescope based on known design

Plato: concept C (PPLC)•42 refractive telescopes•Each with its own sunshade•Total field-of-view = 1800 deg2

•Each telescope has 6 BaF2 lenses

•Requires 168 CCDs witha resolution of 3584x3584





GANTT chart for Plato operations to launch

Spacecraft to be placed at ‘L2’ point to minimise radiation from the Sun (thermal and ionising). Ideal location for astronomy missions


How does Plato’s ‘Yellow Book’ address the environments we have discussed in this module?

Thermal control:◦ Telescope is passively cooled. The spacecraft structure is

kept at ~150K using a sunshield and passive radiative cooling

◦ The CCDs and optics are coupled to the spacecraft structure through a silicon carbide baseplate to cool them ~170K

◦ The electronics will operate at 320K and are thermally isolated from the focal plane array

◦ Spacecraft is at L2 point, and thus radiation from the Sun is minimised


Radiation environment“The radiation levels for PLATO are calculated to be

10 times higher than for GAIA. A proper radiation analysis is needed in a possible future phase in order to better characterise and estimate the radiation effects.

A possible mitigation action against radiation damage would be to add a protective silica plate on top of each focal plane. Assuming 10mm thickness for this plate, the mass impact is an additional 30kg.”

(Basically: we don’t really know yet).


Mass budget: optical array – concept A

Telescope mass budget for each telescope and Telescope Assembly (TA).

Telescope TA (3 telescopes) PLM (4 TA)M1 9.8kg 29.4kg 117.6kgM2 6.4kg 19.2kg 76.8kg

Protective silica plate 2.5kg 7.5kg 30.0kg

Total 124.4 kg


Mass budget: optical support – concept A

Item Mass [kg] incl. marginM1 supporting plate 171.2M2 supporting plate 62.7Struts 39.2Interfaces fixation devices 10.5Secondary structures and baffles 2.1Fixation H/W (screws, etc.) 6.3Interface base plate 104.7Isostatic I/F with SVM 20.0

Total 416.7



Engineering drawings of payload (concept ‘A’) and how it mounts inside the proposed Soyuz launch vehicle.


PlatoMass budget

Plato power budget for all three concepts


‘Technology Readiness Level ’ (TRL) – i.e. can we build what we say we can build.◦ “For design concept A, the Deployable Sunshield

is identified as a critical area in which the TRL needs to be raised before a possible implementation phase. In particular, the HDRM that will need to provide structural stability during launch needs to be developed...” (HDRM=Hold Down and Release Mechanism)

◦ This basically says that the technology needed to unfold the sunshield for concept A isn’t mature enough.


◦ “Common for all concepts is the need for an early CCD design activity so that there is sufficient time to produce all the CCDs needed. The specifications of the CCDs are not more complex compared to other programs such as GAIA, but the designs require many CCDs due to the large FoV. So the critical issue is rather in having enough time to produce and test all CCDs instead of new, complex CCD designs. This is why a development activity in phase A/B1 is needed to ensure that production of the FM CCDs can start as soon as possible.”

Plato requires a LOT of space qualified CCDs so need to start producing them ASAP.


Data download◦ Only (up-to) 48 hours of data will be stored on-

board.

◦ Data will be downloaded once-per-day for a period of 3.5 hours in a 24 hour cycle

◦ Data transmit rate will be 8.7 Mbps

◦ Data will be transmitted in the X-band (10 MHz) using standard GMSK modulation


The mission operations can be broken down into the following tasks:

Mission Planning: long term, medium and short term planning (24 hours to one week time frame, it is assumed that only very limited short term planning activities are required for PLATO.)

Spacecraft control, following the Flight Operations Plan and the short-term plan

Spacecraft status monitoring and off line performance analysis. Instrument control, implementation of the observation schedules Collection of the instrument HKTM telemetry and instrument status

monitoring. Orbit determination and control using tracking data and implementing orbit

manoeuvres. Off line attitude determination and control based on the processed attitude

sensors data in the spacecraft telemetry and by commanded updates of control parameters in the on-board attitude control system.

On-board S/W maintenance (OBCP programming and maintenance, and application and payload software update management).

Data distribution and data archiving (science data are archived at ESAC)


ESA has an internal review to determine the feasibility (scientific and engineering) and risk associated with each mission.

Referred to as an ‘Internal Review Report’ (shortened to ‘IRR’).

This document outlines the risks to the Science Programme Committee (SPC) and makes recommendations.

These recommendations effectively ‘make’ or ‘break’ a mission proposal.

ESA response to Plato proposal

ESA IRR for PlatoIRR risk summary for Plato: 4 areas are identified

“Given the high number of CCDs to be produced in all studies, a key element which will impact the spacecraft development is their manufacturing time. In order to evaluate the proposed schedules for the development and production of the potential PLATO CCDs, the GAIA CCDs development, manufacturing and test rate was used as a benchmark. In Figure 2-1 the time line of the GAIA CCD program is shown.

Based on that, a reasonable production rate is about 3 CCDs per month.”

Plato: CCD requirement

“When the Gaia bench mark is applied to the two industrial studies and the PPLC the following schedule can be deduced for the Phase B2/C/D:”

Astrium (concept A): ◦ 214 CCDs (BB + EM + FM + FS) - 6 years

Thales (concept B): ◦ 128 CCDs (EM + FM) - 3.6 years

PPLC (concept C): ◦ 168 CCDs (FM) - 4.7 years

(This is just the length of time needed to produce enough space-qualified CCDs for the mission!)

Plato: CCD requirement

The IRR flags concerns about the optical configuration for all three concepts:◦ The mirrors for concept A are aspherical, non-standard and

made of SiC.◦ The estimated time to manufacture one M1 mirror is ~27

months. Therefore total manufacture time is >6 years (estimate).

◦ The refractive designs are simpler, and use known technologies but BaF2 lenses are expensive and the crystal has to be custom grown.

◦ Estimated time to grow a large BaF2 crystal ~2-3 months.◦ Lenses are hygroscopic and thus need environmental control◦ 250-300 lenses are needed for either concept B or C. Lens

manufacture time is unknown.

Plato: optical configuration

Plato: optical configurationIRR optical design assessment and recommendations

The IRR summarises the risk of the technology for Plato as follows

Plato: technology readiness

The following summarises the main conclusions of the review team:

The mission requirements are reasonably well defined to start the Phase B1. The spacecraft design does not show peculiar complexities beyond the state

of the art achieved with Gaia and/or Corot. The mission is reasonably straightforward.

The mass is underestimated. Compatibility with the Soyuz launcher requires downsize of the S/C by, at least, 5 %.

The only identified critical technology is the manufacturing of the primary mirror M1 in the reflective system due to the very peculiar shape of the surface.

The schedule is strongly underestimated. Main driver is the high number of CCDs to be manufactured (other elements are the high number of optical elements and the not fully thought verification at payload system level).

Assuming start of the phase B2/C/D middle of 2012, a “realistic” launch date will be 2021. This assumes that the phase B1 is used to freeze the CCD design and prototyping; otherwise a further year shall be added.

Plato: IRR summary

Main recommendation to SPC

“The current design is not compatible with a 2017/2018 launch. To achieve end 2018 a major reduction (order of > 30%) of the payload module (number of CCDs and optics) is required. This would result in a positive mass margin, which can be used for increasing the pupil diameter and recovering at least part of the science performance. Considering mission extension in orbit could also help recovering performance. Using the ESA-developed instrument performance model for PLATO, preliminary results obtained by the study team at the end of the review indicate that the proposed way forward is viable with moderate impact on science performance if any”

Effectively says that for a 2018 launch, the mission has to be ‘descoped’ by 30%! But, due to the nature of the mission (no cryogenics, exoplanets aren’t going anywhere etc) science could be recovered by extending the baseline mission from 6 years.

Plato: IRR summary

The procedure to select a space mission is complicated!

Lots of documentation has to be produced and meetings held before a single nut is purchased.

Missions can be ‘descoped’ (‘Exomars’ for example) to cut costs and to meet launch windows.

Past a certain point in the process, the external science community has very little say in the final mission profile.

The time from ‘call’ to ‘launch’ is normally 10+ years!

Conclusions

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PH 711: Spacecraft mission design and selection: real examples

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