Click the arrow to watch an animation of a SpaceX Falcon Heavy launch. Video source: SpaceX YouTube channel.
One no longer flies.
The other has yet to leave the ground.
One of the accusations levelled by critics of the Shuttle's demise was that the United States did not have a vehicle of equivalent capability.
It's debatable whether those capabilities were “unique” and couldn't be performed by a different approach.
In any case, that starship has gone to warp, so to speak, so let's focus on the future.
NASA's third-generation booster will be the Space Launch System. Officially it will have its first uncrewed test flight by the end of 2018, although the evidence is mounting that SLS will slip to at least 2019 due to delays with the Orion crew vehicle and other critical systems.
SLS has no official missions or destinations. NASA and Congress talk about the Moon, asteroids, Mars, and solar system probes. But so far, it's all talk, which is far cheaper than any actual mission, crewed or not.
The SpaceX Falcon Heavy is poised to become the logical successor to the Shuttle. Although less powerful than the SLS, SpaceX believes it will have a test firing by the end of 2015 and be operational in 2016.
Until it leaves the pad and proves that attaching three Falcon 9 boosters to each other actually works, we need to maintain a bit of healthy skepticism.
But if you compare the numbers, Falcon Heavy — if it performs as projected — will be a significant cost savings for NASA instead of flying its own Shuttle.
Let's look at the Tale of the Tape.
Thrust — According to the SpaceX Falcon Heavy web page, FH will have 4.5 million pounds force (lbf) of thrust at launch from its 27 engines. According to the final Space Shuttle mission press kit, each of the three Space Shuttle main engines provided 490,000 lbs. when operating at 100% in a vacuum, for a total of 1.47 million lbf. Each of the two solid rocket boosters provided 3.3 million lbf, for a total of 6.6 million lbf. Add it all together, it totals about 8.1 million lbf.
Thrust advantage: Space Shuttle.
However ... Thrust doesn't mean much when you're lifting a lot more mass. The FH will have a mass of 3.2 million lbs while the Shuttle was about 4.5 million lbs. Divide mass by thrust and you get .71 for FH, .56 for Shuttle.
So the winner in mass to thrust: Falcon Heavy.
Payload — How much weight it can deliver into space is the raison d'être for any launch vehicle. One number is difficult to calculate because of the intricacies of orbital mechanics. Where is the payload going? What altitude? What inclination to the equator? Does it need to escape Earth orbit? Does it need to be stationary over one location?
Going with basic numbers, we'll look at Low Earth Orbit (LEO) which is between 100-1,200 miles and Geosynchronous Transfer Orbit (GTO), which is about 26,000 miles. The International Space Station, for example, is in LEO at about 250 miles.
According to references, FH will deliver 117,000 lbs. to LEO and 47,000 lbs. to GTO. The Shuttle delivered 53,600 lbs. to LEO and 8,390 lbs. to GTO.
Payload advantage ... Falcon Heavy.
Price — The SpaceX Falcon Heavy web page states the cost of a launch is $90 million to deliver 6.4 metric tons (about 14,000 lbs.) to GTO. According to a NASA web site, the average cost of a Shuttle launch was $450 million, although by other calculations the cost was somewhere between $1.2 to $1.5 billion.
Price advantage ... Falcon Heavy.
So what can we conclude?
With little more than half the thrust, the Falcon Heavy will deliver more than twice the payload to LEO and more than five times the payload to GTO for at least one-sixth of the cost.
The most important difference ... Falcon Heavy doesn't risk lives to deliver payload into orbit.
July 8, 2011 ... The final launch of the Space Shuttle. Video source: NASA YouTube channel.