SPACECRAFT POWER BUDGET DONE RIGHT: POWER BUDGET SECRETS

As unbelievable as it sounds, not all space missions do power budgets, yes, unbelievable, but true.

I specialize in spacecraft power system and lasers, I have written 76 papers and participated in more than 50 space missions to the date of writing this article, and I say ‘more than fifty ’ because I stopped counting at 50, and in all that experience I’ve come across some of those cases and I have ended up helping prepare one or outright doing it myself.

In less than the 50% of the cases PWBs (Power budgets) are prepared by the engineering team of the customer and done right, we will see soon that ‘right’ looks like, but the rest of the cases, they (the customers) either did not know they need it, or it is done ‘wrong’ or is a ballpark figure that at the moment of launch is completely off by far or it is done prematurely. So, the first step in doing a good PWB is to know WHEN to do it.

BUT WHAT REALLY IS A SPACECRAFT POWER BUDGET?

A PWB is basically a balance sheet of all the energy inputs/gains and all the power outputs/losses in the spacecraft and it has to be positive by a good margin or else what you are launching into space is not a spacecraft but a brick.

DOING IT RIGHT: WHEN TO DO IT?

Most people would think that it should be done right at the start and this is correct to some extent: After all, how are you going to know the size of your solar arrays and EPS/PCDU and batteries if not? But there are caveats to this.

The above sentence is true, but is not the end of the story, why? Because in your pre-PDR phase you have a lot of assumptions and unknowns and invariably a PWB (Power Budget) based on assumptions will be wrong, along the way to CDR you will find that many of those assumptions were off for so many reasons or the components you are using were not well understood in all their power needs, like for example, a component that nominally needs 10W but if you read fine print in the last sections it also happens that needs 25W for 600 milliseconds as inrush current and voila!

You just got yourself a blackout and possibly an infinite reboot problem if you have not installed a capacitor bank at the input. So first of all you need to understand that you Pre-PDR PWB is just an estimate. But you still need to do your financials and need to know what to quote to the providers!

What to do? As a standard, DOUBLE THE SIZE OF THE BATTERIES, if your preliminary PWB needs 100W, budget 200W on batteries, or get batteries with something like PowerFlex, these are batteries than can serve 2x, 3x and even 4x their nominal capacity for long times without breaking a sweat and not causing a blackout. You cannot go wrong with this, especially if you select batteries with PowerFlex that will allow you to go as low as 50% above your preliminary PWB

DOING IT RIGHT: WHAT IS THE MINIMUM LEVEL ABOVE THE PWB THAT WILL SUPPORT MISSION SUCCESS?

The minimum safe number is 50% above your FULL PWB needs; this is calculated assuming that all your payloads, subsystems and functions are ON at the same time, your minimum safe level is 50% above that. Let’s say your full PWB says you need 100W on batteries, then you need to guarantee at least 150W on NOMINAL POWER or 100W on PowerFlex 2x.

But why so much power if I’m not going to need it? The answer is simple yet final: TEMPERATURE.

DOING IT RIGHT: KEEPING IT COOL.

As said above, temperature is key and unfortunately, final. In space, especially in LEO (but it is worse in GEO), the biggest risk is overheating, not freezing, why?, because in LEO the spacecraft spends 66% more or less of its life in illumination with temperatures that go as high as 110C or even 120C so if your PWB says you need 100W and you have battery banks rated at 100W/h it means that you will be depleting the batteries in a hour and making them run hot, very hot, THAT IS A BAD IDEA.

Why? Because it can lead to a thermal runaway, deplete the batteries, shorten their life cycle and all that heat will radiate to your surrounding subsystems, making them run hotter and consume more power, which in turn makes the batteries even hotter, add that up to the heat you are receiving from the sun and voila!, your mission is over.

This is why you oversize your batteries and solar arrays: so they run cool, without breaking a sweat, without leaking heat and start a thermal runaway than can kill your mission as soon as your third orbit.

DOING IT RIGHT: SIZE YOUR SOLAR ARRAYS CORRECTLY AGAINST YOUR BATTERIES

Now, if you oversize your batteries to run cool and support mission success that means you have to have the right size of solar arrays to charge the batteries in one orbit: this is no easy task.

The problem is that if you try to simply match your SAs (solar arrays) to your battery capacity let’s say: 100W batteries to 100W solar arrays you will end up with very big SAs, a 100W battery is a small thing, even in a CubeSat but a 100W SA is no small thing and your ADCS engineering team will be very unhappy with that. So, what is the right number?

Experience dictates that the minimum number is 50% and the maximum should be 75% of the nominal battery capacity, so if you have a 150Wh installed battery capacity, you can get away with it using 75W SAs, but how does that work?

If you read carefully and did it right, it means that in the above case, your PWB demands 75W from a 150Wh battery so during an illumination cycle in LEO of a little more than an hour, you would have consumed half the battery capacity, but at the same time you will have collected 75 J/s (yes, per second) charging your batteries as they are being used (if your batteries have that capacity, many do not) so the net result will be that you will enter eclipse cycle with your batteries charged (SoC) at about 80% or even 95%, meaning that you will have a nice run while in darkness and

emerge from it at about 60% to 75% charged to start the new cycle again and you will be almost fully charged in illumination before entering eclipse again. It also means that your batteries will enter eclipse warm after charging and will emerge cooler after giving that heat away to your electronics, in turn keeping them warm during the cold eclipse and reducing the thermal cycle inside the spacecraft.

Bottom line: Your spacecraft will never run out of power, will never overheat and will never freeze, because you can use your electronics in eclipse and the waste heat of it helps to keep it warm.

DOING IT RIGHT: KNOW YOUR PAYLOADS, REALLY!

When designing a spacecraft many tend to go for a Bus solution, which means that they acquire a whole spacecraft from a manufacturer ready to be stuffed with payloads, other go for designing their dream spacecraft using the best components in the market. In both cases a problem always arises and that is the payload selection.

So many times, it happens that the engineers only look briefly at the power requirements, like how much does it consumes at peak operation or the maximum/minimum power requirements, and that is OK, BUT IT IS NOT ENOUGH.

When selecting a payload, from the power systems engineering point of view, here are the bare minimum parameters to look for in the datasheet:

1. Typical power consumption: This is the typical power usage of the device, but is only a ballpark figure, it does not take into account the temperature and this is something you need to watch

2. Maximum/Minimum ratings: These are the limits of the device, the ranges that your PWB needs to serve.

3. Inrush needs: This one is very tricky, it may be that a device may only need a low of power during a very small time, like 60W over 10 milliseconds, but if your EPS hardware cannot manage this (i.e capacitor banks) or your PWB does not takes that into account, you are facing a blackout

4. Quiescent current usage: Another tricky gremlin, this is the parameter responsible for so many mysterious problems in space and even in ground, this is the power the device consumes while idle and it can be a problem if your spacecraft is going to be parked inside the SHM (Space Head Module, the rocket head) for many weeks or even months, why?, because it behaves like an insidious leak and can even deplete your batteries if they are not sized right: Imagine that your PWB requires your batteries to be 90% charged at BOS (Beginning Of Service), but your spacecraft was 4 weeks parked inside the rocket without charging, and while many LSP (Launch Service Providers) allows you one final charge a few weeks of even days before the launch, you did not do that or could not and did not take into account the Quiescent current usage of your payloads and when you go into orbit, your batteries are almost depleted or outright DOA (Dead on Arrival).

What to do?: Plan for those parameters right, especially those last 2 insidious ones.

DOING IT RIGHT: CALCULATE IN JOULES, NOT IN WATTS

This one can be confusing, but is critical, so bear with me:

We all know that energy is not the same as power, but few really understand why: Energy is the capacity to do some work, such as making your devices function, while power is defined as the rate at which the energy is used, while the work is completed. The unit used to measure energy is joules, power is measured in watts, in fact, watts are joules per second.

So energy is what you need for your devices to work and power is how much energy they need to complete a task: Let’s say that a camera payload consumes 5 watts as per its datasheet, that means that it needs 5 Joules per second and you are going to operate the camera for 10 minutes, so you will need a total of:

5 x 10 x 60 = 300 J

That is very different than 5 watts!, it means that the camera uses 300 J to operate for 10 minutes at a consumption rate of 5 J/s (Watts)

Then you can see that your PWB has to be calculated in Joules, not in Watts, your battery capacity must be expressed in Joules, so a battery that has a rate of 25W/h, in Joules means that the battery has a capacity to serve 25 J/s during an hour before being depleted, which means:

25 x 3600 = 90,000 Joules

And your camera is going to use 300 J of those 90,000, so you should be fine, but what about the solar panels?

Solar panels or solar arrays produce energy instantaneously, the idea here being that you channel that energy to charge your batteries and that’s OK, if were not for the fact that some PCDUs or EPS use that energy to charge the batteries WHILE THEY ARE BEING DISCHARGED and while it is a wonderful technology it is very tricky to calculate it properly.

Here you have to do a balance act: you have to calculate the energy from the SAs in Joules properly, like a SA that produces 15 watts is actually that it produces 15 Joules per second, so if you want to estimate you PWB using the example of the camera and the battery above and add the total energy consumption of the spacecraft as, let’s say 6000 joules in an hour.

(15 x 3600) – Tcl + (25 x 3600) – 300 - 6000 = 137,700 Joules or 38.25 Wh

*Tcl here represents the transmission/conversion losses of the system, more about it later

While in systems that only charge the batteries while they are idle, the calculation will be like:

– Tcl + (25 x 3600) – 300 - 6000 = 83,700 Joules or 23.25 Wh

Because the solar arrays are not an input to the system, only to the batteries. As you can see in the first case you will have an energy pool of 38.25 Wh and in the second case you will have an energy pool of 23.25 Wh total for the spacecraft operation in an hour. Here is where the PWB starts, with the real size of your energy pool.

DOING IT RIGHT: NEVER UNDERESTIMATE YOUR LOSSES

Losses are tricky and insidious, they hide behind the fact that they are normally small and many tend to discard them and not take them into account, but the fact is that so many times they can knock off your PWB out of balance and lead your mission astray.

Why? Because in space, losses tend to confabulate with their favorite accomplice: Heat , which makes the losses greater and greater until they are not trivial anymore, that is why so many engineers try to get rid of it. Take the case of a simple magnetorquer, which is basically a coil of wire, that turns on an off a magnetic field to produce a torque for changing the attitude of the spacecraft. But being a coil also means that it is basically a resistor that will warm up during operation.

The fact is that the magnetorquer has a nominal resistance, but you need to note at which temperature is that resistance valid, because at higher temperatures that resistance will grow and if your ADCS CONOPS mandates that you have to operate the magnetorquer for 10 minutes to produce a

constant dipole moment of let’s say 3 Am2, and the magnetorquer says it will use 10W (10V@1A) to produce that dipole moment, you need to see how much the nominal resistance will grow over those 10 minutes to accurately calculate how much energy you are REALLY going to use from your pool.

And that is only one case; all electronic devices have losses, specially the solar arrays that are normally very hot. Be mindful of that and do not ignore them.

What to do? Never underestimate small loses, never , and test all the devices for the times they are s upposed to work to really see how much they will consume.

DOING IT RIGHT: NOT JUST A BALANCE OF WATTS

Many do think that doing a PWB is just summing up the watts and subtracting them against the battery capacity: This is not correct.

The right way is to sum up the total energy consumption, this is not done in watts but in joules as said before, but the point here is that you need to sum up not only the energy but also how much time you are going to need that energy

So, a PWB done right has a power rate for the device and also for how much time it is going to be used and that gives of the energy balance against your energy pool

As seen in the table above, in the Power (J/s) column, the sum is only 23.32 J/s or watts and many will be happy with just that, but when you see the column Daily Energy in Joules, the amount is much more, it amounts to:

460,185,84 J / 24 h = 19,174.41 J/h / 3600 s = 5.32 J/s or 5.32 W constantly, now:

5.32 W x 24 h = 127.82 W means that if you have a 25 W/h (or 90,000 J) you will be OK as the system only needs a rate of 5.32W/h and your energy rate or power, is 5 times that, but….

And here comes the but…

If you are in LEO, in 5 hours you will deplete your battery; you will be stopping your mission every 5 hours if your EPS cannot charge your batteries at the same time they discharge. But if your EPS is capable of simultaneous charge/discharge, you charge the battery for 1 hour per orbit, meaning 15 times per day, your PWB is highly positive and your mission soars to victory.

Also, the Me column details the Median Error (losses) per each device; note the hours per day that a device is being used and the daily energy that device consumes is taken into account and that gives the energy consumed per orbit and the duty cycle of that device and that allows you to fine tune your mission and expand or restrict the use and duty cycle of your devices in order to secure a successful mission.

There are still many things to take into account, but these are the basics, I sincerely hope that this information leads to more successful missions and better informed planners.

Ronnie Nader

Rnader@exa.ec - ORCID: 0000-0002-1399-6973 - Scopus ID: 36125329900

Chief Designer – EXA

Academician – Engineering Sciences – International A cademy of A stronautics - IAA

Senior Member - Nuclear Propulsion Technical Committee – American I nstitute of A eronautics and A stronautics - AIAA