About That Tesla Roadster Flying Through Space: What Kind of Gas Mileage Is It Getting?
- February 19, 2018
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Elon Musk and his SpaceX team made huge news last week when they successfully completed the maiden launch of the Falcon Heavy on the afternoon of February 6, 2018. This launch was such a monumental accomplishment because the private company venture (the heaviest commercial rocket ever launched) could one day be used to take astronauts to the Moon and Mars, and it demonstrated the ability to do so with the ability to guide the rocket boosters back to Earth for reuse.
While all of this news was one of the most amazing accomplishments by a private sector company in terms of scale and implications for humanity, one of the most gripping aspects of the project ended up being the fact that the test payload Musk chose to attach to the rocket was his personal Tesla Roadster, painted cherry red to represent the launch’s step towards getting to Mars. The reason behind launching this $100,000 car into space (never to return) was purely to capture people’s attention and imagination, a goal that was undeniably achieved as Musk was able to give the world this image that mindbogglingly is real and not using any sort of Photoshop and was compelling enough to get everyone to take notice of this amazing accomplishment.
Given that the mission statement of Tesla is “to accelerate the advent of sustainable transport by bringing compelling mass market electric cars to market as soon as possible,” I found it cheekily ironic that fossil fuel– rocket fuel, no less– had to be used to get this Tesla mobile. This not entirely serious thinking led me to the tongue-in-cheek line of questioning– how did the fuel economy of this space-bound Tesla compare with the fuel economies of cars that are restricted to a terrestrial existence? What about the relative carbon dioxide (CO2) emissions?
Let’s bust out that handy back-of-the-envelope to scratch out some (very) approximate estimates!
The Tesla Roadster
The car that was sent into an elliptical orbit around the Sun was Elon Musk’s personal 2008 Tesla Roadster, ‘piloted’ by a mannequin in a SpaceX flight suit named Starman. This model of Tesla electric cars weighs in at 2,723 pounds, went for a base price of $98,000, sold 2,400 units before production was stopped, and was notable as the first highway legal serial production all-electric car using lithium-ion batteries and the first all-electric car to travel more than 200 miles per charge.
The official fuel economy rating of the Tesla Roadster from the Environmental Protection Agency (EPA) is 119 miles per gallon equivalent (MPGe), being able to travel 245 miles on an eight-hour charge (the MPGe value compares the amount of electricity needed to move an electric car with the amount of gasoline needed to move a gasoline-powered car using the energy equivalence of one gallon of gas matching 33.7 kilowatt-hours of electricity).
As a comparison for the fuel economy of a Tesla Roadster:
- Freight trucks average 7.2 miles per gallon (MPG) in 2018;
- The U.S. fleet of light-duty vehicles (i.e., cars and light trucks) in 2018 averages 23.2 MPG in 2018;
- Corporate Average Fuel Economy (CAFE) standards for 2018 are projected to require new light trucks to meet a 29.4 MPG standard and new cars to meet a 39.3 MPG standard; and
- The most energy efficient vehicle available in the United States today, according to Clean Technica, is the Hyundai Ioniq Electric at 136 MPGe, while the most efficient car using gasoline is the Hyundai Ioniq Blue Hybrid at 58 MPG.
While the use of electricity when driving a Tesla (or any electric car) is indeed carbon neutral in that no CO2 is being emitted from a tailpipe, it is not entirely true to rate the CO2 emissions per mile driven as zero. The simple reason behind that is that the generation of electricity that ends up in the vehicles come tied to the CO2 emissions at the electric power generation plants. While the portion of the U.S. power sector that is driven by carbon neutral sources like wind, solar, and nuclear is growing, fossil fuels like coal, natural gas, and petroleum still accounted for over 60% of U.S. electricity generation in 2017.
As such, whenever a Tesla gets plugged into the grid it is likely receiving electricity that comes from CO2-emitting sources (not to mention the inefficiencies that come from the transmission & distribution of the electricity, the charging losses of the batteries, and the ‘vampire losses’ of charge when the car is not plugged in and not in use). Because of this, the CO2 footprint of driving a Tesla, or any electric vehicle, is intrinsically tied with the energy makeup of the particular electricity supplier.
The Nissan Leaf, another all-electric vehicle, accounts for about 200 grams of CO2 per mile (g CO2/mile) on average across the United States, while California (with one of the highest proportions of clean electricity in the country) comes in at 100 g CO2/mile and Minnesota (a state that is very dependent on fossil fuel) comes in at 300 g CO2/mile. For the sake of this exercise we’ll use these readily available Nissan Leaf numbers as the benchmark CO2 emissions per mile of an electric car, even though the Tesla Roadster is likely slightly different due to different charging rates and battery technologies.
As a comparison for this rate of CO2 emissions of an electric car:
- The average freight truck, weighing 40 tons emits about 6,472 g CO2/mile;
- The average passenger vehicle emits 411 g CO2/mile; and
- 2018 EPA standards require a projected 295 g CO2/mile for light trucks and 202 g CO2/mile for cars.
The following table summarizes this range of CO2 emissions for non-rocket fueled vehicles:
At pre-launch, Musk noted that ultimately the payload (i.e., Starman’s Tesla Roadster) would get 400,000 million kilometers (almost 250,000 million miles) away from Earth, traveling at 11 kilometers per second (almost 7 miles per second), and would orbit for hundreds of millions, or even billions of years (see below graphic of the initial orbit that Musk tweeted out after the launch). To accomplish this, the Falcon Heavy generated 5 million pounds of thrust at liftoff (making it the most powerful liftoff since Nasa’s Saturn V). Generating this amount of power is no small feat.
To estimate exactly how much fuel was used (and how much that would be in the equivalent gallons of motor gasoline) requires some estimates, but we have enough information to get at least in the ballpark.
When fueling its rockets, SpaceX uses a highly refined type of kerosene (also known as RP-1) because of its high energy per gallon, in addition to liquid oxygen (LOX) needed for combustion (the amount of LOX required is about double the amount of RP-1). The first stage of a Falcon 9 rocket (another type of rocket used by SpaceX) uses 119,100 kilograms (kg) of RP-1 and 276,600 kg of LOX, while the second stage uses 27,850 kg of RP-1 and 64,820 kg of LOX (see graphic below for what that multi-stage launch sequence looks like).
A simplified explanation of the Falcon Heavy is really that it’s composed of three Falcon 9 rockets merged into the first stage and the second stage consisting of disconnecting from the three Falcon 9 rockets and a single stage 2 rocket (along with the payload) continuing on. Making rough estimates, this means the Falcon Heavy required three times the fuel of the first stage and one times to fuel of the second stage of the Falcon 9, or a total of 385,150 kg of RP-1 and 894,620 kg of LOX (this is admittedly a simplification of the fueling process, but I’m also admittedly not a rocket scientist. In attempting to keep these estimates as rigorous as possible, see the citations and links contained here and let me know in the comments if I got something wrong– particularly if you are a rocket scientist!).
Musk, when discussing the potential dangers of the Falcon Heavy launch, noted that the fuel on board was 4 million pounds of TNT equivalent. In fact, the energy contained within looks like it could be over double that (whether this is a sign of Musk simplifying for the sake of giving the press a quote, speaking approximately without reference to the exact calculations beforehand, or missteps in my calculations, I’ll let you decide).
While the total weight of the LOX is over double the weight of the RP-1, the LOX is simply there to allow for combustion and maximize the efficiency with which the rocket is burned. As such, the energy density of RP-1 is what we care about. Using an energy density of 43.2 Megajoules (MJ) per kg, we find that the energy contained in the Falcon Heavy’s fuel tanks was over 16.6 million MJ, which is equal to about 126,000 gallons of gasoline equivalent (or over 8.7 million pounds of TNT— so while our estimate is over double Musk’s offhand remark, we can take solace that we’re in the same order of magnitude!).
In terms of the CO2 released by burning this much fuel, we can use the “well to wake” emissions number of RP-1 of 85 grams of CO2-equivalent per MJ to estimate that the total CO2 emissions were over 1.4 million kg (or 1,400 metric tons) of CO2.
Comparing Starman’s Tesla with Earth vehicles
First things first– that’s definitely the most fossil fuel used and CO2 emitted ever in getting a car from point A to point B. But that doesn’t necessarily mean that Starman’s Tesla is the least efficient or most harmful to the environment. That’s because once the fuel is burned and Starman’s Tesla was set into orbit in perpetual motion, logging millions of miles on the odometer while traveling 25,000 miles per hour, the rest of its journey was all without additional energy input.
Even the camera and communication equipment on board were attached to a battery with 12 hours of life with no other sources of energy, so after the 12 hours the equipment went dark and there was no more energy input to Starman’s Tesla– just momentum and gravity working their magic. So despite this initial abundance of fossil fuel and related CO2 emissions to set the Tesla in motion, on a per mile basis (which is how fuel economy and emissions are calculated) it will inevitably becomes the most efficient and clean car of all time!
But how long will it take for this to be true?
In terms of fuel economy, the MPGe of Starman’s Tesla improves linearly with every mile traversed through space. After 1,200 miles, the Falcon Heavy and its payload of Starman and his Tesla left Low Earth Orbit, but the massive amount of fuel means it barely even registers as a blip on this graph at about 0.0095 MPGe.
After two days when Starman’s Tesla had traveled 450,000 miles, the fuel economy had risen to a little less than half that of the freight truck. You can also note in the graph that at the point of the 36,000 mile warranty of the Tesla Roadster the fuel economy aws still less than 0.3 MPGe– you’d certainly have a lot of angry Tesla owners if that’s all they were able to recoup on gasoline costs by the end of their warranty!
Lastly, after teasing out how far Starman’s Tesla would have to travel to become the most fuel efficient car (that is or ever was) on Earth, we find that it would take:
- About 900,000 miles to beat the fuel economy of freight trucks;
- About 2.9 million miles to beat the average of the U.S. light-duty stock fuel economy;
- About 3.7 million miles to meet the 2018 light truck standards;
- About 5.0 million miles to meet the 2018 car standards;
- About 7.3 million miles to meet the most efficient gas powered car available;
- About 15 million miles to meet the efficiency of an Earthly Tesla Roadster; and
- About 17.2 million miles traveled to equal the 136 MPGe of the Hyundai Ioniq Electric, the most efficient car available.
As previously mentioned, the equipment on board Starman’s Tesla was attached to a battery that only had 12 hours of life, after which there was no functioning equipment on the Roadster. As such, there is no inherent tracking or communicating with Starman’s vehicle as it continues on its journey, making its exact tracking through space difficult.
But fear not– a great tool was launched after the Roadster was launched into orbit called ‘Where is Roadster?‘ Using the knowledge available regarding the position, orbit, and speed of the Tesla, this tool shows approximately where in its orbit the Roadster is and how far it has traveled in aggregate. This tool does not allow going back to see when exactly certain distances were passed, but from watching the site myself I can attest that Starman’s Roadster passed 17.2 million miles on the afternoon of February 14, 2018– meaning it only took eight days for this Tesla Roadster to become the most efficient car ever!
Any distance it continues to travel will only increase the overall fuel economy (if you want to calculate this for yourself at any given moment, divide the current miles from ‘Where is Roadster?‘ by 126,279 gallons of gasoline equivalent).
In terms of CO2 emissions per mile, Starman’s Tesla improves according to a power equation– meaning in this case that there are drastic improvements in CO2 emissions per mile initially that flatten out over time. By the time Starman’s Tesla leaves Low Earth Orbit, not nearly enough miles have been traveled to offset the massive amount of CO2 emissions from the rocket launch, with Starman’s Tesla coming in at a mindblowing 1.2 million g CO2/mile at 1,200 miles– the equivalent of 182 freight trucks moving a mile at a time.
After two days and 450,000 miles traveled, the CO2 emissions per mile had dropped to 3,143 g CO2/mile, blowing way past the average freight truck emissions after about 219,000 miles. After the 36,000 mile warranty, the emissions still averaged over 39,000 g CO2/mile– another tidbit that would enrage an environmentally conscious electric car owner if it happened to them.
Again projecting out how far Starman’s Tesla would have to travel to become the cleanest car in existence, we find that it would take:
- About 3.4 million miles to be cleaner than the average passenger vehicle;
- About 4.7 million miles to be cleaner than an electric vehicle charged in fossil-fuel-dependent Minnesota;
- About 5.0 million miles to meet the emissions standards for light trucks in 2018;
- About 7.0 million miles to meet the emissions standards for cars in 2018;
- About 7.1 million miles to be cleaner than the average electric vehicle in the United States; and
- About 14.1 million miles to be cleaner than an electric vehicle charged in renewable-energy-heavy California.
Again by watching the ‘Where is the Roadster?‘ tool, I found that Starman’s Tesla also became the cleanest car ever (on a g CO2/mile basis) on February 14, only 8 days after launch. As with the fuel economy, this figure will only get better and better as Starman racks up the limitless miles circling the Sun for millions or billions of years (to calculate an updated emissions per mile, divide 1,414,270,800 grams of CO2 emissions by the updated miles traveled from ‘Where is Roadster?‘).
So there you have it, despite the massive amounts of fuel and resultant CO2 emissions required to launch the Tesla Roadster in space, it only took eight days of traveling faster than any car ever before to become the most fuel efficient and least CO2-emitting (on a per mile basis) ever made. But that fact was inevitable given that it’s in orbit around the Sun and will likely be for the rest of humanity’s existence– so what really is the point of crunching the numbers like this? Hopefully you’ll come away from this article with a handful of takeaways and topics/issues on which to do some more reading and learning:
- The impressiveness of this feat accomplished by Musk adn the whole team at SpaceX cannot be overstated. The Tesla Roadster weighs just 2,723 pounds, but this launch was testing a rocket system whose ultimate payload capacity extends to almost 141,000 pounds sent to Low Earth Orbit, 37,000 pounds sent to Mars, and 7,700 pounds sent to Pluto– all at decreased cost compared with historical launches that really opens up doors. That is the most important takeaway from the Falcon Heavy launch, a huge step towards what Musk hopes to be the next great space race.
- Beyond that, running through these tongue-in-cheek calculations should hopefully serve to pique your interest and give some information on the relative fuel efficiency electric cars are able to achieve, but also some of the current shortcomings in terms of using them as a way to reduce CO2 emissions. A lot of interesting pieces have been written on the true environmental impact of electric cars, as well as how that might evolve in the future. I’ll recommend a couple (from Green Car Reports, Wired, The Union of Concerned Scientists, and Scientific American, just to name a few), but it’s an important topic with much more out there to be read and debated.
- In addition, given the relative fuel economies and CO2 emissions of various vehicles (as wella s regulations covering these measurements), let that be a reason to look more into the efficiencies and emissions of your vehicles. In particular, you’ll note the average passenger vehicle has twice the emissions per mile as a new Model Year 2018 car that complies with EPA regulations, while the new cars will also get up to 74% more MPG compared with the average for the U.S. fleet of light-duty vehicles. Keep these types of figures in mind the next time you’re in the market for a vehicle, and consider how much fuel and emissions savings are being protected and increased by these existing regulations (both fuel economy and car emissions regulations are being considered for rollbacks by the Trump administration) as automotive regulations and policies continue to make the news.
Sources and additional reading