A while back, a reader asked for fuel consumption figures for our favourite space freighter via the blog’s feedback form:
“I would like to have some detailed info on the consumed fuel in kg during the rendezvous phase of ATV towards the International Space Station, split for attitude and orbit manoeuvres. In detail I am looking for data on fuel consumption evolution to maintain the ATV with the wished attitude possibly for each phase (Yaw Steering, Target Pointing).”
A quick email to ESA's Laurent Arzel working on the Flight Dynamics team at ATV Control Centre in Toulouse gives all the answers we could need.
Laurent writes: “Between launch and docking, ATV burns fuel for the phasing period as the spacecraft raises its altitude to the first checkpoint (S-1/2), which is 5 km below and 39 km behind the International Space Station. In addition, it burns fuel to keep pointing in the right direction.”
“After the S-1/2 point, until docking, ATV continues to burn fuel for its flight and specific attitude control to the Space Station.”
The rough-average figures are 800 kg of propellant used for the phasing period, assuming the Space Station is flying at around 415 km altitude.
If the phasing lasts five days, an ATV requires 60 kg of propellant for yaw steering to constantly keep the solar panels basking in the light of the Sun. The amount of propellant used in this period for yaw-steering attitude control is a linear function, so adding a day for phasing would require 1/5 more propellant (so 12 kg more each extra day).
Another consideration is the slew manoeuvres that set ATV on the correct track before and after each orbital manoeuvre. This is a linear function depending on the number of manoeuvres. During an average 5-day rendezvous and docking period, the propellant consumed for slew is 10 kg.
Once ATV pass the S-1/2 point, a normal rendezvous requires around 300 kg of propellant.
How much does your fuel weigh?
Whereas on Earth we often speak in litres of fuel (or gallons of course), in space flight fuel is not always liquid. To make matters more complicated, most fuel uses oxygen to burn – the oxidiser. For vehicles that travel on the surface of our planet, oxygen is quite easy to find, just stick an air intake valve on an engine and you are done. Spacecraft (and submarines) however have to pack their own oxidisers.
Mix the fuel and the oxidiser together and you have what is called propellant. In your car, the fuel is petrol or diesel while the oxidiser is the same air you breathe. ATVs use MMH (MonoMethyHydrazine) as fuel and mix it with the oxidiser MON (Mixed Oxides of Nitrogen) in a ratio of 1.65 oxidizer-to-fuel.
Fuel consumption figures (extra-urban)
So once at its cruising speed of 28 800 km/h (thank you, Ariane 5), an ATV needs 1170 kg of propellant, of which 441 is MMH and 729 is MON.
Considering MMH density is 0.875 kg/l, we would need 504 litres of the stuff. Using a MON density of 1.114 kg/l we would need 654 litres of MON to dock an ATV.
Combining these gives a total of 654+504=1160 litres of propellant to dock an ATV, once in orbit.
Docking generally takes five days, so during that time period an ATV will travel: 24 hours in a day X 5 days X 28 800 km/h = 3.456 million km travelled on 1160 litres of propellant which translates to a 'fuel consumption' of just 0.03 litres per 100 km!
Disclaimer: These calculations show fuel consumption for educative purposes and may not reflect real driving results, your mileage may vary. We did have fun working this out, keep your questions coming…