Systems Engineering

Budgets

Once the requirements have been defined, the next step is the allocation of resources to each subsystem. In other words, we must decide exactly how much of the spacecraft resources will need each subsystem to do its job. This allocation will depend on the subsystem requirements and will be an iterative process. The initial estimates are provided by the system engineer and are based on experience on previous missions. Subsystem designers will use these estimates as a starting point and will provide more accurate estimations as the design progresses.

The fundamental resource of a spacecraft is mass. Because of the high cost of launch vehicles and the step function in cost when you outgrow a vehicle, the system design must stay within established mass limits. Power becomes another limited resource once you choose the solar array size (a choice that must generally occur early in the development). There systems engineers can track multiple resources budgets although it may not be necessary to track them all.

Margins

In order to ensure that the system stays within the mass capability of the launch vehicle, we set aside margin, which is extra mass that is not assigned to any particular subsystem. We also carry margin for all the other spacecraft resources, like power, processor utilization, and memory. One of the biggest challenges for a spacecraft systems engineer is properly managing margin. If you hold too much margin, the subsystem designs become overly constrained and the cost of the system rises, or you lose out on capability you could have had in your system. If you hold too little margin, you will find yourself with an assembled spacecraft that doesn’t work (it is too heavy for the launch vehicle, its solar arrays don’t generate enough power to support its function, there is insufficient fuel, or some other such crisis). The consequences of exceeding capability are severe. Margin covers the uncertainty of the design, so you can reduce it as the design matures.

On top of the allocation, system engineers carry addition system margin to handle unforeseen situations.

Mass

To create a first mass budget:

  1. Start with the target launch mass
  2. Subtract the propellant mass
  3. Determine a reasonable allocation for the payload. This allocation must include a margin for contingencies that will depend on how new is the design of the payload
  4. Set aside system margin
  5. Allocate mass for each subsystem based on previous missions
  6. Iterate. Subsystem designers provide new estimations of the subsystem mass based on their calculations. They must aim at the target mass established in the budget for their subsystem
  7. Adjust the budget as appropriate

In Table 15, the average mass by subsystem for 4 types of spacecrafts is listed. It has been extracted from [WL99], consult the appendix A of the reference for more information.

Table 15 Typical percentages per subsystem in a mass budget.
Subsystem (% of Dry Mass) No Prop LEO with Prop High Earth Planetary
Payload 41% 31% 32% 15%
Structure and Mechanism 20% 27% 24% 25%
Thermal Control 2% 2% 4% 6%
Power (incl. harness) 19% 21% 17% 21%
TT&C 2% 2% 4% 7%
On-Board Processing 5% 5% 3% 4%
Attitude Determination and Control 8% 6% 6% 6%
Propulsion 0% 3% 7% 13%
Other (balance + launch) 3% 3% 3% 3%
Total 100% 100% 100% 100%
Propellant 0% 27% 72% 110%

Propellant budget

The propellant budget follows directly from the \(\Delta V\) budget. Using the rocket equation

\[\Delta V = I_{sp} ln \frac{M_{0}}{M_{f}}\]

we can calculate the propellant mass necessary for a maneuver given the \(\Delta V\) and the \(I_{sp}\) of such maneuver.

\[M_p = M_f(\exp^{\Delta V/(I_{sp}g_0)}-1)\]

where \(M_f\) is the spacecraft mass after the maneuver. Using this equation for each maneuver, the propellant budget is built.

A typical propellant budget contains four elements:

  1. Velocity-control propellant
  2. Attitude-control propellant
  3. Margin (a percentage of the total)
  4. Residual (unavailable propellant)

Power budget

The initial estimate for the solar array size should be based on an estimate of the power consumed by the spacecraft. Just like with mass, you should itemize the subsystems and eventually the individual components in a subsystem and add up the total power consumption based on type of spacecraft and on historical data. Some rule of thumb on power consumption:

  • The faster a data system operates, the more power it will consumed
  • Communications systems trade power for antenna size
  • Generally, the spacecraft will need more heater power if the environment has large variations

The estimation of the spacecraft power requirements is carried out in three steps:

  1. Preparation of an initial operatin power budget by estimating the power required by the payload an the spacecraft subsystems. In case the spacecraft has several operating modes that differ in powe requirements, we must budget separately for each mode (survival mode, nominal mode, science mode, etc)
  2. Selecting the battery capacity appropriate to the spacecraft power requirements and battery cycle life.
  3. Accounting of the power-subsystem degradation over the mission life by computing radiation damage to the solar array.

In Table 16, it is listed the average power by subsystem for 4 types of spacecrafts. It has been extracted from [WL99], consult the appendix A of the reference for more information.

Table 16 Typical percentages per subsystem in a power budget.
Subsystem (% of Total power) No Prop LEO with Prop High Earth Planetary
Payload 43% 48% 35% 22%
Structure and Mechanism 0% 1% 0% 1%
Thermal Control 5% 10% 14% 15%
Power (incl. harness) 10% 9% 7% 10%
TT&C 13% 12% 16% 18%
On-Board Processing 13% 12% 10% 11%
Attitude Determination and Control 18% 10% 16% 12%
Propulsion 0% 0% 2% 11%

References and Other Resources

[DRDF12]Anton H De Ruiter, Christopher Damaren, and James R Forbes. Spacecraft dynamics and control: an introduction. John Wiley & Sons, 2012.
[FSS11]Peter Fortescue, Graham Swinerd, and John Stark. Spacecraft systems engineering. John Wiley & Sons, 2011.
[MB14]Malcolm Macdonald and Viorel Badescu. The international handbook of space technology. Springer, 2014.
[SB16]George P Sutton and Oscar Biblarz. Rocket propulsion elements. John Wiley & Sons, 2016.
[Uni07]California Polytechnic State University. Poly Picosatellite Orbital Deployer Mk. III Rev. E User Guide. The CubeSat Program, Cal Poly SLO, 2007.
[WEP11]James R Wertz, David F Everett, and Jeffery J Puschell. Space mission engineering: the new SMAD. Microcosm Press, 2011.
[WL99](1, 2, 3) James R Wertz and Wiley J Larson. Space Mission Analysis and Design, Space Technology Library. Microcosm Press and Kluwer Academic Publishers, El Segundo, CA, USA, 1999.
[Wer78]James R. Wertz, editor. Spacecraft Attitude Determination and Control. Springer Netherlands, 1978. URL: https://doi.org/10.1007/978-94-009-9907-7, doi:10.1007/978-94-009-9907-7.