The other day, Steven made a post comparing solid-fuel rockets to liquid-fueled. I can’t link it, because although it seemed fairly benign, it apparently angered someone he disagreed with, so Steven pulled it. However, the vanishing act prompted Fledge to comment here that it was probably because engineers are really, really picky about things. (They have to be. I don’t want to fly in an airplane designed by someone who said “I guess the wings will stay on.”)
That’s an interesting perspective and highlights something I noticed, but never really thought about — it’s the engineering posts that cause all the fuss on Steven’s site. (Well, since he rarely mentions politics nowadays, and always locks those posts.) However, I disagree with Fledge. I don’t think it’s engineers, per se that are arguing with Steven — it’s non-engineers (or engineers outside their expertise) whose beliefs could be described as “religious” rather than “scientific.” In short, people who believe things like “we can solve all of our energy problems with solar power!” regardless of what the science or engineering has to say on the matter.
However, it triggered an e-mail discussion regarding the difficulty in reaching space, and I did a bit of research that backed up Steven’s assertation that there is a big difference between reaching the edge of space, and staying there (in low earth orbit). The difference in energy required is something like thirty times as much, because you don’t just have to loft a payload straight up, you have to impart enough lateral motion that it will stay up. (Then there’s increased atmoshpheric drag…) Suprisingly, the extra energy to reach the moon? Only twice what’s necessary to reach orbit. Getting off this dirt-ball is the hard part.
the Earth’s surface
|speed||period/time in space||specific orbital energy|
|minimum sub-orbital spaceflight (vertical)||6500 km||100 km||0.0 km/s||just reaching space||1.0 MJ/kg|
|ICBM||up to 7600 km||up to 1200 km||6 to 7 km/s||time in space: 25 min||27 MJ/kg|
|LEO||6,600 to 8,400 km||200 to 2000 km||circular orbit: 6.9 to 7.8 km/s
elliptic orbit: 6.5 to 8.2 km/s
|89 to 128 min||32.1 to 38.6 MJ/kg|
|Molniya orbit||6,900 to 46,300 km||500 to 39,900 km||1.5 to 10.0 km/s||11 h 58 min||54.8 MJ/kg|
|GEO||42,000 km||35,786 km||3.1 km/s||23 h 56 min||57.5 MJ/kg|
|Orbit of the Moon||363,000 to 406,000 km||357,000 to 399,000 km||0.97 to 1.08 km/s||27.3 days||61.8 MJ/kg|
No wonder the Abh don’t land on planets. The real reason is they can’t afford it in the budget!
But what this tells me is that Burt Rutan is no closer to replacing our government-funded space program than the Wright Brothers and their first airplane were to flying the Atlantic with Lindbergh.
In real terms, he’s probably much further away, because no fundamental tech breakthroughs were required for Lindbergh’s flight; it was just a matter of the engineering advances in building planes and engines, and figuring out what worked. But we’ve already done that with the Russian and American space programs. What everyone thinks Burt is up to is trying to do it cheaper and without governmental waste.
In reality, he’s up against a hard scientific limit: it just isn’t possible to cram that much energy into something that can lift itself and the fuel to finish the job, without making something very, very big and expensive. Energy density bites us in the ass again. It’s a problem with fueling cars, and it’s a problem with fueling rockets: every pound of fuel you need at the end of the trip is one you have to carry from the very beginning. Try to drive a car all the way from Prudehoe Bay to the Cape Verde islands — without stopping for gas. How big of a trailer would you have to tow? Could your car even pull it? Could a pickup truck pull it? If it takes a semi (less fuel effecient) to pull the load, how much more gas or diesel do you need now, how much bigger did the trailer get, and can the semi pull that increased amount?
The whole point of NASA using multi-stage rockets, detachable boosters, and droppable tanks was to manage that problem. Add fuel at the beginning, and then shed the dead (container) weight at the first possible moment. So what about Burt’s plans? SpaceShipTwo will reach a speed of 4,200 km/h (2.02 kps), using a single hybrid rocket motor. It will launch in midair at 15,200m (50,000ft) from its mother ship, WhiteKnightTwo. So his “first stage booster” gets just under 5% of the distance (measured in altitude only), and significantly less than Mach 1 (~.33 kps, no speed was given but it’s subsonic) and just under 5% of the velocity. That’s not really impressive, but it does carry a major gain: he gets out of the densest region of the atmosphere, which has not been included anywhere in the energy budget. His second stage, SpaceShip Two, will reach 112km in height (of a minimum 200 needed for temporary orbit), but I don’t know what the max speed is. (Don’t forget, this is on a vertical flight profile, which is minimum-distance, minimum-time, minimum-energy.)
This is not a system that’s going to get anywhere near orbit. The engineering won’t support it. Burt Rutan knows that, which is why he designated it as Scaled Composites “Tier 1b program.” The third system is supposedly planned as part of the “Tier 2 program.” At least, it’s planned in the sense of, “we’d really like to do that.” Given the resources needed to make SS2 fly as a paying concern for Virgin Galactic, I doubt Burt’s putting much effort into a completely new design yet. “Tier 2,” suggests that’s exactly what it is; the energy figures above state unequivocally that it has to be radically different. If it’s for real, and not just marketing hype, that is.
If White Knight 3 were the size of a 777, but built with modern composites, optimized for speed and altitude, how big of a payload could SS3 carry? And would it be any cheaper than the current $100 per kilo cost of the shuttle? Important questions, which Burt’s in no position to answer yet.
I’ve got some other things to do right now, so I’ll be back later with some alternatives….
(Edited for clarity and spelling at 6:40)