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The Good, The Bad, and The Ugly… “Class D” Payload Explained


The TROPICS mission has been in the news recently as a consequence of the mission's first two being lost when the vehicle failed.

TROPICS is classified as a “Class D” mission.

This article focuses on Class D: what is the classification; how is it determined; what does it mean to a mission; and how did it result in the TROPICS launch failure.

What is a Class D classification?

The “Class” classification of a payload represents the “risk tolerance” of the payload. There are four classifications Class A, B, C, and D. Class A has the lowest risk tolerance and Class D has the highest risk tolerance. Class D “high risk tolerance” means that many possible mission failure mechanisms may exist.

How is the classification determined?

NASA establishes the risk tolerance for all NASA payloads and NASA sponsored payloads, according to NPR 8705.4A. There are four distinct risk tolerance classes. The risk tolerance classes are characterized in the following table.

Source: NASA

Class A: The lowest risk tolerance that is driven more by technical objectives. This would normally represent a very high priority mission with very high complexity, e.g. , Europa Clipper, James Webb Space Telescope.

Class B: Low risk tolerance that is driven more by technical objectives. This would normally represent a high priority mission with high complexity, e.g. TERRA, AQUA, TESS.

Class C: Moderate risk tolerance that is driven more by technical objectives. This would normally represent a medium priority mission with medium complexity, e.g. GOLD, SWOT, ICESAT–2.

Class D: High risk tolerance that is driven more by programmatic constraints, e.g. cost/schedule are equal or greater considerations compared to mission success. This would normally represent a low priority mission with a medium to low complexity, e.g. demonstrators, TROPICS, NICER, ECOSTRESS.

What does it mean to a mission?

A Class D risk tolerance classification affects the mission in the following areas: program and project management, access to space options, and safety and mission assurance. The classification brings with it both advantages and disadvantages.

Streamlined processes

In January 1, 2018, NASA instituted a new policy that streamlined processes for Class D missions with budgets less than $150 million excluding .

The made changes to program and project management (NPR 7120.5F), the changes included: reducing the number of program/project reviews; streamlining the number and levels of approvals; shrinking the documentation needed; and reducing the performance management requirements.

With the policy changes, Class D missions benefit from reduced administrative overhead, as well as shrinking associated schedule and labor burden.

Access to space options

NASA manages and provides to mission payloads. Class D payloads will typically have two options for access to space: Commercial –licensed launch services selected from vendors of the Venture–class Acquisition of Dedicated and Rideshare (VADR) program; or access to space.

Commercial FAA–licensed launch services

New projects such as VADR, provide Class D missions greater schedule and operational flexibility for delivery to space. VADR vendors include a range of established and emerging and launch service aggregators and brokers:

VADR domestic are certified by NASA as Category 1 consistent with NPD 8610.7D Launch Services Risk Mitigation Policy for NASA Owned and/or NASA Sponsored Payloads/Missions. A Category 1 launch vehicle is not required to have a flight history and is classified as having a high risk of failure.

Rideshare access to space

The NASA Rideshare Office maintains a list of available rideshare opportunities on scheduled NASA launch vehicles as well as external sources for rideshare.

Rideshare access to space is provided using an Evolved Expendable Launch Vehicle (EELV) via an EELV Secondary Payload Adapter (ESPA). An ESPA is an adapter for launching secondary payloads on orbital launch vehicles. Rideshare details are provided in the Rideshare Users Guide.

NASA launch vehicles such as V, Antares, and support ESPA. offers commercial rideshare launches approximately every four months.

The expendable launch vehicles that NASA uses are illustrated below.

Source: NASA

Safety and mission assurance

The mission's risk tolerance class determines the NASA–level Safety and Mission Assurance (SMA) expectations for design management controls, system engineering processes, mission assurance requirements, and risk management processes. Specifically, the mission must comply with SMA objectives laid out in NPR 8705.4A, APPENDIX D: Program and Project Safety and Mission Assurance Objectives for Class A to Class D.

The NASA Launch Services Program provides modified technical oversight for the launch mission consistent with NPD 8610.7D and NPD 8610.23C.

How it resulted in the TROPICS launch failure

As a Class D mission, TROPICS was able to take advantage of the streamlined processes. However, by using VCLS dedicated launches the mission took on the risk that the launch vehicle selected may never have been flown before, or even worse, is known have a high failure rate.

The VADR program was not finalized at the time of TROPICS. Consequently, a one-off VCLS contract was issued using an RFP process. Based on the RFP, NASA selected Astra as the launch services provider for TROPICS. When Astra was awarded the contract for TROPICS on February 23, 2021 – Astra was known to have a 100% overall launch failure rate. On the day before the first TROPICS launch, Astra had a 78% overall launch failure rate, and a 50% failure rate on the current rocket version. For more insight into how Astra was selected check out this article How did the TROPICS smallsats end up using a launch service provider with a 100% failure rate?

Unfortunately, it's not surprising that the Astra launch vehicle failed to deliver the TROPICS' cubesats to orbit.

If you found this article interesting, please like and share!

Additional information for the curious

NASA policies and directives related to Class D

NASA Online Directives Information System

NASA Rideshare

NASA small satellites

Class D streamlining initiative

NASA Policy Provides New Approach to Space Science and Technology Inventions

Class D policy and related links

TROPICS mission documentation



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