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Comment by WalterBright | Hacker News Reader

I wonder what the cost effectiveness of ballistic sub-orbital flights would be. That would solve much of the fuel efficiency problem.

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At the expense of 'Everybody dies if the slightest thing goes wrong.' And 'Everybody better be wearing space suits... Unless you want to get Souz-11'ed.

There is more then an order of magnitude of complexity between designing a vehicle that can travel at Mach 2, and one that can travel at Mach 22. Personally, I'd pass on the risks of hypersonic re-entry for my SEA->NYC flight... And on having to pee into an astronaut diaper.

Space travel (And a ballistic suborbital hop is 90% of the way there) is, as a mode of transportation, incredibly dangerous.

Why space suits? Just pressurize the cabin so everybody can breathe. As for Gs, just make sure they're pointed toward the back of the seats and you won't need G suits.

See https://en.wikipedia.org/wiki/Soyuz_11

You don't need to go all the way to space to die of hypoxia, cruising altitude for modern jets is plenty good for that: https://en.wikipedia.org/wiki/Helios_Airways_Flight_522 (and several other examples of non-airliner "ghost planes" that I'm too lazy to look up)

Didn't know about that one. Thanks for the link. Still, passenger jets are pressurized today and they rarely lose pressure. The technology is much more reliable now.

IIRC pressure loss incidents are common, but not newsworthy because nothing really bad happens. The air pressure is still high enough to allow people to breathe, so you just need to supply oxygen-enriched air while the plane descends to 10000 feet.

With suborbital flights, neither breathing nor descending more quickly would be an option.

I think you are confusing suborbital and orbital travel.

A suborbital flight that brings you all the way across the Atlantic will have almost the same speed and energy requirements as an orbital flight.

Just take a look at the typical orbit of a SpaceX second stage over the Atlantic: https://youtu.be/zBYC4f79iXc?t=32m13s - A suborbital flight that just barely goes back deep enough into the atmosphere over Africa to slow down would look almost exactly the same and wouldn't even be 100 m/s slower.

Compare that to the tiny ballistic arc of the first stage. That's how your suborbital flight would look like if you want to hit the atmosphere at a speed that doesn't require a heat shield.

There's a lot of possible trajectories, and one doesn't have use a ballistic one. Once out of the atmosphere you could leave low thrust on for the entire journey, alleviating concerns over both weighlessness and the need for excessively high speeds.

I that would be crazy. Instead of that, something like this: https://en.wikipedia.org/wiki/Reaction_Engines_A2 might be doable.

Given that many people vomit on their first zero g trip, I'm not sure how a ballistic suborbital flight works with a large passenger compartment.

Some work on this concept in the 1930's:

https://en.wikipedia.org/wiki/Silbervogel

Is it possible for an air-breathing engine to accelerate an aircraft enough for a suborbital flight?

A rocket could be fuel-efficient only if it accelerates very quickly, passengers won't survive.

On its own, no, but there are hybrid systems. For example, https://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29

SABRE is designed for orbital flights, suborbital flight requires much less velocity. Some air breathing scramjet might be enough.

Suborbital flights require almost as much velocity if you actually want to go anywhere.

Hopping up into space, and then back down to where you came from is only ~1,600 m/s.

Hopping up into space from New York, and landing in London requires closer to ~6,000 m/s.

Do you have a link with an elementary explanation of this type of numbers? I'm not doubting what you say, I'd just like to learn more about the topic.

Here's a breakdown of reaching orbital speeds, vs doing a vertical sub-orbital hop. [1]

Here are some specs for an ICBM. It has a range of 6,000 miles, and has to reach ~7 km/s (Mach 23) to do so. [2]

The mathematical reason for why suborbital flights that take you somewhere require so much velocity is because all ballistic trajectories are actually elliptic orbits. When you throw a baseball into the air, it is in an elliptic orbit - a very small orbit, with most of it passing through the space occupied by the earth. The Earth's center is one of the two centers of the ellipse.

This means that in order to send a ballistic rocket to the other side of the world, you need to 'almost get into orbit'. At this point, it takes very little energy to turn your orbit into a circle... But it also means that you had to expend orbital-levels of energy to get to the point that your suborbital hop takes you somewhere. [3]

Knowing that sub-orbital ballistic trajectories are ellipses, with one of their centers at the center of the Earth, you can calculate the velocity requirements yourself by using Kepler's second law.

[1] http://www.orbspace.com/Background-Information/Suborbital-vs...

[2] https://fas.org/nuke/guide/usa/icbm/lgm-30_3.htm

[3] http://lh3.ggpht.com/_1wtadqGaaPs/TF0zyBaNisI/AAAAAAAANLc/uU...

Great explanation, thank you. From the links you provided (all very good), I started browsing and ended up with the book "Orbital Mechanics: Theory and Applications" by Tom Logsdon [1], which appears quite good.

[1] https://www.amazon.com/Orbital-Mechanics-Applications-Tom-Lo...

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