# Flying fuel tanks! Which deep-space spacecraft had the largest fuel mass fraction?

The Los Angeles Times' Deborah Netburn just wrote a really nice retrospective; ‘OK. Let’s do it!’ An oral history of how NASA’s Cassini mission to Saturn came to be with plenty of photos and quotes from contributors to the spacecraft.

Some of the images really surprised me! Everyone views a spacecraft differently, but from a perspective of size, Cassini is two big things; a giant fuel tank to go far away from home, and a giant dish to stay in touch with home, and possibly to protect the spacecraft from direct impacts from ring particles during the more daring maneuvers.

This made me look up the wet and dry masses of Cassini. From the Wikipedia page for Cassini-Huygens I see a launch mass of 5,712 kg and a dry mass of 2,523 kg. I am not sure if these both include the Huygens lander or not.

If I just use those two numbers, I get about 56% of the mass as fuel, but I am not sure if this is the correct usage of the numbers because of how Huygens factors in, so I thought I would pose the question to the experts, and let you decide how to handle the math:

Question: Which deep-space spacecraft had the largest fuel mass fraction?

above: "Engineers and technicians at JPL work to complete the stacking assembly of the Cassini spacecraft in 1996." From here. Ken Lubas / Los Angeles Times

above: "Parts of the Cassini spacecraft are assembled at JPL in 1996." Cropped, from here. Iris Schneider / Los Angeles Times

above: "A diagram of the Cassini spacecraft and Huygens probe." From here. NASA Jet Propulsion Laboratory. Open in a new window for full size so you can see the detail and read the descriptions!

• Deep space meaning beyond the moon? So Pioneers, Voyagers, and any of the interplanetary missions? – Russell Borogove Sep 14 '17 at 16:47
• @RussellBorogove yep, that's exactly what I meant. – uhoh Sep 14 '17 at 16:53
• I think the dry mass figure does not include Huygens (319kg) but can't swear to it. For e.g. getting the total ∆v capability from the Tsiolkovsky equation you have to run the numbers twice, once with Huygens' mass and the pre-deployment fuel expenditure, once without Huygens & the remainder of the fuel. – Russell Borogove Sep 14 '17 at 18:36
• @RussellBorogove that sounds like quite a fun project. Would need a fairly complete and accurate tabulation of propulsive maneuvers that distinguished between main engine and thruster, luckily I'm working on that. – uhoh Sep 15 '17 at 1:42

I'm not up to a complete exhaustive survey of every possible contestant, so I've focused on relatively recent orbiters. I found a few that beat Cassini. The figures I've found so far are occasionally a bit contradictory or squirrelly -- masses are rounded differently here and there, some sources give the design tankage while others give the actual flown amount of propellant, for instance, or include pressurant or other consumables mass in propellant mass -- so any of these may be off by a couple of percent.

I had assumed that the winner was going to be a Jupiter or Saturn orbiter, since they have the greatest ∆v requirement for orbital insertion, but multiple Venusian and Martian orbiters seem to beat Cassini.

The Venus orbiter Magellan is the leader at a hair under 70% -- 2414 kg of propellant on a total launch mass of 3449 kg.

MAVEN tops the list of Mars orbiters at 66%. It will need to expend some fuel to accomplish several "deep dips" into the upper Martian atmosphere, and the intent is for it to move into a high circular orbit to serve as a communications relay once its science mission is complete.

Mangalyaan/MOM had to do its own trans-Mars injection burn (!), so it required a propellant fraction of 63%, with 852 kg propellant on 1337 kg launch mass.

ExoMars is 63% sans Schiaparelli or 54% with the lander included.

Juno appears to tie with Cassini at 56% (2032 kg propellants, 3625 kg launch mass).

Strangely, I can only find the combined propellant + pressurant mass for MESSENGER, putting it something less than 54%.

Mars Reconnaissance Orbiter is close behind at 52%.

Galileo is about 41%; like Cassini-Huygens, it's complicated by being an orbiter-plus-probe configuration.

Mars Express, 40%.

Aktsuki, 38%.

Mars Global Surveyor, 34%.

Even though Dawn's ion propulsion system achieves 6-10 times the specific impulse of storable bipropellant thrusters, it still has a respectable 35% mass fraction.

New Horizons only needed to make small course corrections, no orbit insertions, so only carried 16% of its mass in fuel.

• This is amazing, and absolutely contrary to what I initially expected, but in hind sight it make sense. I had thought that the farther the craft, the higher fuel fraction. But of course it's usually gravitational assists (and time) that are used for distance, and fuel is used for capture and subsequent orbital maneuvers. This puts MAVEN and MOM at the top and New Horizons at the bottom of this compilation, even though New Horizons is of order 30x farther from the sun. – uhoh Sep 15 '17 at 1:51
• I'm also surprised, I would have thought capture at Jupiter was the most expensive mission. – Loren Pechtel Sep 15 '17 at 2:12
• It's not so much the gravity assist as the launcher upper stage (often a Centaur or Fregat) that's getting you to your target. It's still very surprising to me that the Mars/Venus orbiters are so fuel-fat. – Russell Borogove Sep 15 '17 at 2:31
• What about Polyus, the Soviet military anti-anti-ICBM satellite that was launched on Energia in 1987? It had a launch mass of 80 tons and must've been the most massive spacecraft to ever have orbited Earth, in one launch. I guess most of that was fuel for maneuvering. – LocalFluff Sep 15 '17 at 7:22
• @LocalFluff OP asked for deep space craft; Polyus doesn't qualify. I also can't find detailed mass specifications for it, but it would surprise me if it had more than 25% propellant mass ratio. – Russell Borogove Sep 15 '17 at 12:16