Last week, the always-exciting Elon Musk announced a plan to launch soft, moist human passengers in his new planned rockets to fly on suborbital trips from city to city, allowing travel between almost any major city on Earth in under an hour. It’s a fascinating idea, but not a new one. In fact, Ronald Reagan mentioned the basic idea in his 1986 State of the Union address.

It’s true! Here’s what the Gipper had to say about it:

And we are going forward with research on a new Orient Express that could, by the end of the next decade, take off from Dulles Airport, accelerate up to 25 times the speed of sound, attaining low Earth orbit or flying to Tokyo within 2 hours.

That “Orient Express” was what would become the National Aerospace Plane (NASP)—a planned technology demonstrator of the passenger craft was known as the Rockwell X-30. The National Aerospace plane is actually quite different technically than what Elon made his little video about, but conceptually is the same idea: go places on Earth quicker by going through space.

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Musk is describing a two-stage rocket system (called the BFR for “Big Fucking Rocket”) that would launch and land vertically, in the exciting, ‘50s-scifi manner that SpaceX has managed to prove is feasible. The NASP was actually a lot more ambitious: a single-stage-to-orbit (SSTO) spaceplane that would launch and land like a conventional aircraft.

Both would leave the atmosphere and fly on parabolic, sub-orbital paths from one city to another. The NASP was described as being able to go from Washington, D.C. to Tokyo in two hours, a distance of 6,771 miles.

In Elon’s video, we see the example flight plan is New York City to Shanghai, with a travel time of 39 minutes to cover the 7,392 miles.

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It seems like the SpaceX rocket-based approach is faster, but when you factor in that the passengers for the SpaceX rocket are boarding and landing on floating launch platforms in the water, I suspect that total door-to-door times for both hypothetical trips would be similar, since the National Aerospace Plane didn’t require factoring in a boat trip.

Since neither system currently exists as more than just plans and data (well, the NASP project actually produced a lot of valuable research into materials, propulsion systems, a static test article, and the X-43 aircraft, essentially a scaled-down version of the X-30 demonstrator), I think we can feel free to compare the two ideas a bit.

From a logistical and comfort standpoint, I don’t think the SpaceX system can come close to what NASA was planning with the NASP. The NASP would take off and land from conventional airports. While getting onto a boat to board a rocket has a hell of a lot more world-of-tomorrow charm, if you flew on a regular basis, I can imagine that shit would get old pretty quickly.

Comfort-wise, there’s a lot that Musk left out of the video, like the boarding process, for example. Picture the worst lines to get on an airplane, but now make them vertical. There’s a reason why the video skips over that part. And, once you get in the rocket, accommodations will likely be, um, novel.

While the rocket is on the pad, a traveler can expect to be laying on their back in their seat, as is done for all vertical rocket launches. That probably rules out a snack or beverage, if you like what you happen to be wearing.

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There’s only been nine crewed suborbital spaceflights: two Mercury flights, two X-15 aircraft flights, one aborted Soyuz launch, and SpaceShipOne’s three test flights. That’s it!

The closest analogues to what Musk wants to do with his BFR are likely the Mercury flights, since they’re vertical rocket launches and, unlike the Soyuz one, intentionally sub-orbital.

On Alan Shepard’s spaceflight, America’s first human crewed spaceflight of any kind, the Redstone rocket that launched Shepard’s Mercury capsule into orbit subjected him to a maximum of 6.3g on launch, and then 11.6g at re-entry.

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Those are non-trivial numbers; a conventional airline take-off is only about 0.4g and 5g is a limit for many people before losing consciousness. I’m not saying for sure that those are the same numbers for the BFR, but a vertical launch and re-entry will subject passengers to g-loads significantly more than they’re used to.

Most rocket launches subject their crews to a maximum of about 3g of accelleration—the Space Shuttle peaked right about there—and while this is totally safe, it’s beyond what most people are used to.

A airplane/spacecraft hybrid like the NASP would be able to take a more gradual approach to reach the needed altitudes, giving comfort at the expense of speed. But, as I said before, maybe some of that time can be recovered by not having to ferry out to an ocean-based launchpad.

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Once in orbit, the typical sub-orbital flight times in this exciting microgravity part of the flight will likely be similar regardless of what hypothetical vehicle we take: about 32 minutes to go a quarter around the earth, and 42 minutes to go halfway. Not bad!

Math checks out!

If you’re smarter than me (a safe bet) then this math may be something you’d find useful:

To calculate the time of flight for a minimum-delta-v trajectory, we first find that, according to Kepler’s third law, the period for the entire orbit (if it didn’t go through the earth) would be:

Using Kepler’s second law, we multiply this by the portion of the area of the ellipse swept by the line from the centre of the earth to the projectile:

This gives about 32 minutes for going a quarter of the way around the earth, and 42 minutes for going halfway around. For short distances, this expression is asymptotic to {\displaystyle {\sqrt {2d/g}}}.

Whatever. What I’m getting at is that while Elon’s rocket-to-lunch-across-the-world plan is exciting, dramatic, and the way I’d love to travel, no question, realistically the cancelled ramjet-scramjet, shovel-shaped spaceplane that was the National Aerospace Plane actually seems like a much more practical and comfortable approach.

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I think the idea of suborbital, city-to-city transportation is fantastic, and I dearly hope it happens. But if real resources and efforts are to be put into this, I think the smarter bet is to pick up where the National Aerospace Plane left off, and come up with a transportation system that utilizes the vast network of commercial airports instead of having to trek out to a coast, take a boat, and get crammed into your seat from g-forces.

Still, that boat ride could be relaxing, I suppose.