Model Philosophies and Model Types for Space Projects

Launching a spacecraft is a one-shot opportunity, as most of the satellites cannot be repaired or replaced after launch. For these reasons, space systems must undergo an extensive qualification and acceptance test campaign before launch to ensure that every requirement is met within an acceptable level of risk.  To achieve this, physical and digital models must be used.

However, testing every aspect of a spacecraft on every model selected would be extremely expensive and time consuming. That is why a model philosophy must be selected at the beginning of the project, selecting the minimum number of models required to achieve confidence in the final product while maintaining a reasonable balance between cost, schedule and risk.

The Model Philosophy defines the optimum number and the characteristics of physical, virtual and hybrid models required to achieve the confidence in the product verification with the shortest planning and a suitable weighting of costs and risks.

It is selected by means of an iterative process that combines different factors simultaneously:

• Programmatic Constraints
• Verification Strategy
• Integration and Test Programme
• Development Status.

The starting point to define the correct model philosophy is to consider the development status and maturity of the hardware, as new technologies requires more extensive qualification and verification campaigns, while previously qualified or recurrent designs may benefit from a more relaxed qualification plan.

Also, note that it influences the project schedule, due to the selection of the overall verification and test strategy. A sequential use of a model on different levels and for different purposes can increase the schedule duration, whereas parallel work on various models can shorten it.

Every mission has unique requirements, but there are three types that are mainly used:

It is used in projects for which all affordable measures are taken to achieve minimum risks, also where new or complex technologies are involved or systems cannot be recovered after launch.

Its main advantages include:

• Low risks.
• Capability to perform parallel activities on different models.
• Completion of qualification activities prior to acceptance
• Capability to use QM or EQM as integration spares during higher level activities

Its main disadvantages are:

• High cost
• Increased programme complexity.

It reduces the number of hardware models by qualifying and accepting a model that will be flown. Mainly used in projects with no critical technology, also with extensively qualified hardware and where a moderate level of risk is accepted.

Its main advantages are:

• Lower development costs
• Reduced hardware manufacturing effort
• Simpler programme management

However, its main disadvantages are:

• Increased risk
• Serial activity performed on the same model
• Spare units unavailable.

It combines elements of previous approaches and is one of the most common solutions in new projects, as it always results in a protoflight model to be flown after a protoflight test campaign whose is scope is reduced with respect to that of the pure protoflight approach.

This philosophy allows to perform some parallel activities, use QM and EQM as integration spares during high level activities, and conform to the delivery dates of high reliability components as well as the accommodation of commercial components.

The choice of model philosophy is strongly influenced by the type of project:

• In the case of one-of-a-kind projects (scientific missions), the tendency is to choose a pure protoflight approach supported by virtual models.
• In the case of a series of satellites, a pure protoflight is normally used where a QM undergoes a full qualification test campaign and the recurring FMs sees only a limited acceptance campaign.

According to the verification and qualification requirements defined by the model philosophy selected, different types of models can be employed.

Space projects employ a wide variety of models, each serving a specific purpose during the project.

These can be classified in 5 groups:

• Design and development

Used for early design activities, where Mock-ups and Development Models are used to validate concepts and new technologies.

• Thermal and Mechanical Qualification

Mechanical and thermal verification is performed using structural, thermal and thermal-structural models.

• Design and Functional Qualification

Engineering and qualification activities make use of electrical and functional models (EFM), engineering models (EM), engineering qualification models (EQM), qualification models (QM) and life test models (LTM) to validate performance and reliability.

• Flight

It includes the protoflight model (PFM), flight model (FM) and flight spare (FS)

• Other models

Finally, dedicated models for specific purposes developed for training, ground segment validation or human related models.

Selecting the appropriate EEE parts is an important aspect of model philosophy. They are often available in both commercial and space grade, depending on packaging, validation, general quality and traceability of the manufacturing process.

Also, it is possible to find an EM version or even a dummie version of a qualified copy intended for FM, that offers the capability to use a representative alternative component at a lower cost for some models when certain aspects are not needed.

There are so many critical aspects regarding space missions and most of them cannot fail after launch, that is why it is so important to define the correct model philosophy, as well as the physical and digital models that will be used at the beginning of the project, in order to get a correct balance between the different constraints that are involved into the mission.