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So, we are designing a spacecraft, Communication satellite, interplanetary probe, etc. Anyway, extra propellant for in-space maneuvers is always good.

Initially we have:

  • wet mass cap by rocket launcher
  • hypergolic delta V for nominal spacecraft mission (extended mission or repurposing of the spacecraft are desirable, of course)
  • dry mass cap (wet mass cap minus mass of propellant for the nominal mission delta V)

We are lucky enough and designed the spacecraft well below dry mass cap. (During development spacecraft have big enough margins for unforeseen components' mass growth - up to 30-40% for totally new component).

Propellant tanks of our spacecraft were sized for dry mass cap, so now we have more delta V for smaller mass. And also we have wet mass below launcher's capacity.

What can we do now? I think the answer depends on topic's question - should the spacecraft's hypergolic propellant tanks be always 100% full at launch?

-Was any real spacecraft launched with partially filled hypergolic propellant tanks?

-Are there technical restrictions for that? (vibrations at launch, etc.)

-Is it difficult to change propellant tanks' size at late phase of development? Is it possible to do this without a total redesign?

I suppose if the answer to the main question is "yes", designers are more flexible. They can make tanks with some extra size, so additional propellant can be added for ever longer life of the spacecraft. If the answer is "no", tanks size and amount of the propellant should be chozen at early phases of development.

P.S. In modern time extra mass is often used for cubesats. But I'd like to know how to optimize the main craft.

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  • $\begingroup$ As for "What can we do now?" is tack on a bunch of cubesats on the launch. This is what usually is done with the surplus of payload capacity, reducing the cost of launch of the payload. $\endgroup$
    – SF.
    May 26, 2018 at 19:26

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Spacecraft tend to be designed for one particular mission plan, so their propellant tankage is sized for that mission. If the designers are conservative, they may budget a little extra tankage against mass overruns on other components, or in the hope of extending the mission parameters (as in New Horizons' plan to visit a Kuiper Belt object beyond Pluto).

That said, some spacecraft designs do fly distinctly different mission profiles requiring different fuel loadouts; the example I'm familiar with is our friend the Apollo CSM. As described in this Q/A, the CSM propellant load for the low Earth orbit missions Apollo 7 and Apollo 9 was much smaller than on the translunar missions. In the case of Apollo 7, the underloading was partially forced by the use of the less powerful Saturn IB launcher, but Apollo 9 was an LEO, CSM/LM mission launched on a Saturn V, and so could have reached LEO easily with the full propellant load, yet the CSM propellant tanks were only 2/3 full. So this is a strong indicator that once the needs of the mission plus a safety margin are met, there are reasons to stop loading. Propellant cost is an obvious one; safety is another possibility (it would be slightly better to abort away from 12 tons of deflagrating hypergolics than from 18 tons), but both of these would be small influences at the margin. All other things being equal, a lighter upper stage prop load would also provide some protection in the event that a lower stage engine underperformed or failed.

Sloshing in spacecraft propellant tanks during launch is a concern whether underloaded or not. The tanks are not completely full of liquid even when "fully" loaded; there's ullage space containing pressurant gas (typically helium), and slosh baffles are required.

Increasing propellant tankage of a spacecraft late in design can be difficult. As tankage is often a large portion of the volume budget, it can have a strong impact on the overall design. The extreme case of this is a launcher stage, which is almost all tankage; we do see stages get "stretched" in later versions (Falcon 9 has done this a couple of times) which doesn't require too much redesign if the structural strength is there. On the other hand, if the spacecraft designer has been conservative from the start, they may have deliberately left open space with the idea of adding additional propellant tankage as a future upgrade.

Downsizing tankage is done less often, because underloading is simple to do, but it has been done. After the Apollo moon program wound down, the service modules used for Skylab and ASTP had a significant portion of their main propellant tankage removed to save mass, since they were to launch on Saturn IB.

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