8
$\begingroup$

After seeing Elon Musk's proposal of using rockets to travel from city to city I was wondering how much fuel such a rocket would use, and how would that compare to traditional flights in terms of environmental impact (CO2 emissions).

Is it possible to provide an estimate based on current technology?

$\endgroup$
  • 3
    $\begingroup$ It burns methane, the most "friendly" (or, "least-hostile" to the environment) hydrocarbon fuel of all ( one molecule of CO2 and two molecules H2O for every molecule of methane - CH4 - burnt. Hope he gets it working. Also potential for several very good environmental things to happen as eg. atmospheric CO2 has much lower potential for global warming than CH4 which makes it into the atmosphere from indescriminate fossil fuel mining methods. $\endgroup$ – Stan H Oct 16 '17 at 7:29
  • 2
    $\begingroup$ Um, no. Musk showed dedicated suborbital flights. $\endgroup$ – Hobbes Oct 16 '17 at 15:53
  • 5
    $\begingroup$ Back of the envelope calculations show 17,500 gallons of fuel used to fly a 747 from New York to London. A 747 carries 400-600 passengers. I don't know how many passengers are shown in Musk's slides for this application of his proposed vehicle, but I doubt it is anywhere near that many. And given that even a Falcon 9 first stage holds 25,000 gallons of kerosene, the comparison does not look favorable for the rocket. $\endgroup$ – Organic Marble Oct 16 '17 at 17:54
  • 1
    $\begingroup$ @OrganicMarble very nice. Maybe if large wings were added to the F9, and the trajectory remained at about 12 km altitude, and only one engine were used, and the thrust were used to drive a fan, and ... oh, then it would be a "747". Perhaps the four passengers could remain seated in the roadster inside the custom, pressurized fairing. $\endgroup$ – uhoh Jan 4 '18 at 1:07
  • 2
    $\begingroup$ @OrganicMarble BFR and not Falcon9 So methane instead of kerosene. $\endgroup$ – jkavalik Jan 4 '18 at 6:49
4
$\begingroup$

Loose estimate based on preliminary data for BFR:

BFR first stage gross weight is 4400 tons (IAC presentation 2017). If I set the mass fraction to 90% that's 4000 tons of methane + LOX. The second stage has another 1100 tons. Assuming all of that is burned on a flight (worst case).

The reaction is CH4 + 2 O2 = CO2 + 2 H2O, ratio 42:36, is 2746 tons of CO2.

Payload to LEO is 150 tons, that's (my estimate) enough to carry 800 passengers.

For comparison, a Boeing 747-400 uses 60 tons of fuel for a 5500 km long flight carrying 400 passengers, producing 189 tons of CO2.

So for a transatlantic flight, a BFR produces 7x as much CO2 as an airliner. For the longest flights (London-Australia, 20,000 km), fuel consumption of the Boeing increases by 4x. I already assumed the BFR consumes its entire tank capacity above, so for this flight the BFR produces 1.4x as much CO2 as an airliner.

$\endgroup$
  • $\begingroup$ The CO2 in the exhaust is even more suborbital than the rocket, so it will all fall into the atmosphere. The molecules are too heavy for any noticeable amount to escape due to solar radiation. $\endgroup$ – Mark Adler Jan 4 '18 at 18:25
  • $\begingroup$ One thing of note is that jet engine combustion happens in a nitrogen rich environment leading to the production of NOx. $\endgroup$ – Lex Jan 4 '18 at 22:27
  • $\begingroup$ ​Thanks, nice answer. These figures could then be normalised on the number of passengers to get a CO2 per person moved indicator. $\endgroup$ – Andrea Casaccia Jan 5 '18 at 9:01
  • $\begingroup$ the relative figures (7x as much/1.4x as much) are also valid per passenger. $\endgroup$ – Hobbes Jan 5 '18 at 9:05
  • $\begingroup$ Sorry I didn't read carefully enough. The 747-400 has a payload of 125 tons and carries 400 passengers. How did you reason for the 150 tons → 800 passengers estimate for the LEO? $\endgroup$ – Andrea Casaccia Jan 5 '18 at 9:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.