It’s not the height, it’s the velocity.
It’s also worth noting that a suborbit can be accurately defined as an orbit that intersects the earth or its atmosphere. So even if you have orbital speed, if there’s not a sufficient horizontal component to it, you’ll still end up back on the earth before you go around.
That’s a bit broad, it would also include escape orbits. Maybe a closed orbit with perigee below 100km?
“Escape orbit” is an oxymoron, I think. Escape is a trajectory, but not an orbit (at least not an orbit around the body being escaped).
Ah, that’s a good point. Mathematicians use the term in the wider sense, but it does sound a bit unnatural.
Escape velocity is indeed an orbit, with an eccentricity greater than 1. I’d go with ‘an orbit with eccentricity less than 1 and perigee below the Kármán line’ as a solid definition of suborbital, at least when talking about Earth.
Maybe you’d want perigee to be <0km even.
If you haven’t gotten the book yet, mine just showed up on Saturday and it’s a nice compendium of the web versions, plus more. I was very happy to see that a lot of the tongue-in-cheek superscript references were kept in place and added as footnotes. And it was attractively priced, especially for a hardcover. I was surprised at its size, honestly.
Amazingly, Amazon has been reporting it as their best-selling book for almost a week now.
Has anybody considered adding ramjets to the Lynx? They’re simple and light and would augment thrust from mach one to six using the same fuel as the rocket engines.
It doesn’t spend enough time in the atmosphere to justify the extra weight.
Probably so, but since adding speed is the issue and not height, perhaps spending more time in the atmo. is an answer? Ramjets aren’t that massive. I doubt anybody has run numbers?
The design space for spacecraft has barely been explored at all – yet another reason why I like Kerbal Space Program.
Take the SABRE engine, for instance, under development in Britain. The full-up version of the SABRE hasn’t been built in real life, but I can use it in KSP. Funny thing: although it is designed to boost a spaceplane, it works far far better on a vertically launched vehicle.
Many people have run numbers. If you want to cruise, use an airbreather. If you want to accelerate, use a rocket. The extra weight of the engine and drag of the atmosphere outweighs any trivial benefit of carrying a little less oxygen.
There is no extra weight if the airbreathing engine is a rocket. And the atmosphere where the bulk of the acceleration occurs is 100km up.
Airbreathing engines weigh more than rockets, even if they’re also rockets. And there’s no atmosphere at 100K to help the airbreather.
I misspoke. 100 thousand feet. And not having to carry 90% of the oxygen you need to get to orbit adds up to a lot more than a little extra weight on the engine. The difference is enormous. In KSP (which is only a first order approximation, but still) I’m getting a single stage to orbit, and propellant is only 30% of the initial mass.
Does KSP include drag? If not, the analysis is worthless.
Yes, drag is included (and significant). I wouldn’t bet a billion dollars on the design, but it’s worth investing a few million in some more detailed CFD.
And not having to carry 90% of the oxygen you need to get to orbit adds up to a lot more than a little extra weight on the engine.
True, it also adds up to a lot of extra weight in hydrogen tankage — which is heavier than LOX tankage. And tanks cost more per pound than LOX does.
” a lot of extra weight in hydrogen tankage”
IIRC that was an issue that ultimately sank DC-X. However, recall from the rocket equation: if there is a 90% reduction in oxidizer, then you don’t need the fuel to lift that oxidizer, either. Hydrogen tankage might weigh more than kerosene tankage, but you’re not having to carry nearly as much Hydrogen as a superficial glance would suggest.
That being said, KSP is still just a game and a crude representation of the Physics involved. Whether the SABRE engine will work as advertised is an open question. My experience playing with it suggests two things: that if the SABRE does work, then it works better for vertical launch (the key is getting it into thin atmosphere as quickly as possible, and staying there as long as possible); and that the design space has barely been explored.
It used to be, you had to build something to test it, or spend gazillions on mockups and models and expensive CFD. KSP replaces all that with no cost for crashing, the ability to refine and test and refine and test again for no cost, other than complete accuracy. So now,after being frustrated again and again trying to get a damned spaceplane to work, it is no cost to do something crazy like put the spaceplane engines on a VTOL, because hey, if it doesn’t work then at least the explosion will be pretty. And the damned thing “works”.
The problem is that the people developing the Sabre engine and Skylon are requesting several billion dollars.
KSP models drag, but only at the most rudimentary level (it scales in a sensible way, but a craft’s shape and orientation are mostly irrelevant to the drag experienced).
SSTO air breathers are certainly very popular among players who have options enabled that require them to care about launch costs. On the other hand, it pretty well glosses over the differences between jets, ramjets, and scramjets, and gives you engines that work in all those domains, so it’s hardly surprising.
My impression is that people rarely bother with the Cylon-style engine (stats taken mostly at face value from the development plan); it’s easy enough with the combo- jet/ramjet/scramjet and a couple lightweight rocket engines that there’s no need for it.
Hydrogen tankage might weigh more than kerosene tankage, but you’re not having to carry nearly as much Hydrogen as a superficial glance would suggest.
By “superficial glance,” you mean dozens of trade studies performed over the years.
It used to be, you had to build something to test it, or spend gazillions on mockups and models and expensive CFD.
NASA and DoD spent gazillions on the X-30 NASP, with very little to show for it.
Kerbel Space Program is one thing. Reality is another.
I’m not saying this crude simulation is reality… in fact, I believe I expressed such caveats (first order approximation, not solving n-body) with each post. What I’m saying is that if the SABRE works as advertised, then the crude analysis suggests it works far better for vertical launch than for a spaceplane. And that the design space has barely been explored; Reaction can barely get the funding to try a SABRE on the Skylon, never mind exploring it for a vertical launch. They can’t afford to have their minds that open.
As an aside, the game makes abundantly clear that guys like Jeff Greason and Rand Simberg and Paul Spudis are completely correct about space policy, at least where their ideas intersect. Players naturally gravitate toward propellant depots and orbital assembly and ISRU (rather than just bigger and bigger rockets) because it makes so damn much sense.
What about meteorites?
I think a meaningful definition has to specify a point of origin on Earth, not in space.
Meteorites are in a solar orbit, not an earth orbit, unless they get captured (e.g., by interaction with the moon).
There’s also decaying space junk. The orbit of ISS will intersect the Earth’s atmosphere, if it doesn’t receive periodic reboosts.
At that point, it is no longer in orbit.
It is still in orbit. The perigee has simply dropped to low to avoid hitting the thicker layers of the atmosphere.
(There’s also the question of how we define atmosphere. Does atomic oxygen count?)
Does atomic oxygen count?
Only after preschool.
It counts if it affects the trajectory of a spacecraft. There’s a reason the ISS needs periodic reboost, and that exosphere has a lot to do with it. The sun also has an atmosphere, the solar wind, extending all the way out to the shock front of interstellar space, over a light year away.
In practical terms, “too low to avoid hitting the thicker layers of the atmosphere” is tantamount to “no longer in orbit,” since the object on such a trajectory has no chance of returning to space permanently afterward.
I’ve seen (sci-fi-ish) proposals for high-speed trains in evacuated tubes (a la hyper loop) that would need no magnetic levitation because they would run at orbital speed…!
I must admit, I really grasped this much better once I started playing Kerbal Space Program: http://xkcd.com/1356/
And what happens when you need to slow down at the destination? You’re looking at several minutes of deceleration if you don’t kill your passengers, and for most of that time you’d be at a significant but not orbital speed – could we safely slow down from 17500 miles an hour in a tube?
You build the tube (use sci-fi tech) to follow a true straight line between your origin and destination points, burrowing through the Earh.
The minimum travel time between any two points on the Earth because 5 hours and 40 minutes.
40 minutes is the transit time for a half orbit, which this tube train does naturally because the acceleration and deceleration come for free from the slope of the tube in relation to the “g” vector. The remaining 5 hours accounts for 2 hours for the van ride at each end plus one hour to clear Security . . .