The Lucy trajectory described in several excellent answers to this question involves a number of fairly large (in delta-V terms) deep space maneuvers, totalling (same source) about 1.5 km/s over almost ten years.

What form of propulsion will be used for these? I can't see any information about the spacecraft on any of the project web pages, just the destination and instrumentation. What fuels are efficient enough for such large delta-V but stable enough for such long term storage?

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    $\begingroup$ There's some sketchy info on page 70 of this GAO report: gao.gov/assets/700/691589.pdf It contains these hinty comments: main engine was the same model used on a series of environmental satellites that have had engine performance problems. The engine has failed in flight more than once and was a single point failure for the Lucy project. As a result, the project completed an engine trade study in July 2017 and decided to select a different engine. $\endgroup$ Jan 31 '19 at 19:43
  • $\begingroup$ The mission is still in the design phase, so engine selection may not be final yet. The design will be finalized by the end of 2019. $\endgroup$
    – Hobbes
    Jan 31 '19 at 21:17
  • $\begingroup$ @Hobbes fair point, although given the unusually large delta-V needed, I would imagine they must at least have checked that some feasible options exist. I would accept an answer that gave credible details of engines that could meet these requirememts. $\endgroup$ Jan 31 '19 at 21:24
  • $\begingroup$ Perhaps the question Flying fuel tanks! Which deep-space spacecraft had the largest fuel mass fraction? needs a companion question "...largest delta-v?" $\endgroup$
    – uhoh
    Jan 31 '19 at 23:23
  • $\begingroup$ @SteveLinton I've just asked Where can I read about Lucy's complete propulsion system? $\endgroup$
    – uhoh
    Feb 5 at 4:00

This article on the Psyche mission says Lucy's propulsion is to be chemical (in contrast to Psyche's electrical propulsion).

Since the fuel needs to be stored long-term, it's almost certainly a non-cryogenic hypergolic combination like MMH/NTO.

There are a number of proven thrusters using that combination which are used for primary propulsion in deep space missions; another question here is dedicated to figuring out exactly what engine Lucy intends to use, but let us say for sake of argument that it's the Aerojet R-4D. With a moderately large nozzle that thruster achieves 311 seconds of specific impulse. We can apply the rocket equation to work out the necessary mass ratio. I saw another source give 1.68km/s required for Lucy's maneuvers, so I'll use that figure.

$$\Delta v = v_\text{e} \ln \frac {m_0} {m_f}$$

$v_\text{e}$ is 311s x 9.801 m/s2 = 3048 m/s

Thus the log of the mass ratio is 0.5512, and so the initial-mass to final-mass ratio is 1.735.

This requires ~43% of the mass of the spacecraft be propellant at the beginning of these maneuvers. The QA that @uhoh referred to in comments shows that this is towards the high end of tankage ratios for deep space missions, but not unusually high.

R-4D might be overkill for a small spacecraft like Lucy, but most smaller thrusters will have poorer specific impulse, so they might cost more mass in propellant than they save in engine hardware.


A few Lucy team members were asked this at a Q&A event. Wilfredo Santiago, a thermal engineer at Lockheed Martin, answered:

...there's a few tanks onboard. One of them is carrying the fuel, which is hydrazine — highly energetic, toxic, so we have to be really safe around [it] once the spacecraft is fueled and working through it — and then there's an oxidizer.

The event was aimed at middle and high schoolers, so the team didn't go into much technical detail, but this is one of the only references to propulsion I could find from public Lucy Mission material.


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