 Suborbitals in 2010! |
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 Aug 21 2004, 07:21 PM
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Running Target  Group: Members Posts: 1,049 Joined: 24-March 03 Member No.: 4,323  |
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Aug 21 2004, 09:25 PM
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#2
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Running Target Group: Members Posts: 1,428 Joined: 9-June 02 Member No.: 2,860 |
Except, y'know, it's going to take another 25 years before those things are working well enough to stick a man on them. Scramjets have only worked in labs (if then) for the past 50 years; only recently did one manage to function - for seconds - in free flight. With only seconds of test time, it's going to be damned hard to develop scramjets.
In the mean time, it's usually easier to use rockets to get the job done, unless you really crave high speed atmospheric flight. Semi-ballistics would be much easier to develop than suborbitals. Plus, I'd prefer to ride a vehicle that didn't spend most of its time wrapped in a fireball. A semi-ballistic stays in space for most of its flight, a relatively benign environment compared to what the suborbitals try to live in. I swear, there's gotta be an "airbreathing engine mafia" behind scramjets. They're such a pain to develop and offer such limited benefits, but people keep banging their heads into the engineering barriers. |
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Aug 21 2004, 11:59 PM
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#3
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Avatar of Mediocrity Group: Dumpshocked Posts: 725 Joined: 26-February 02 From: Seattle, WA (err, UCAS) Member No.: 277 |
How much of a design factor is the "not having to carry oxidizer" thing? What sort of weight and space savings do you see with an air-breathing engine? I've heard that's a major point in favor of a (theoretical, working) scramjet, but then I'm a biologist so there's no real accurate knowledge in here. :)
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Aug 22 2004, 12:54 AM
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#4
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Running Target Group: Members Posts: 1,428 Joined: 9-June 02 Member No.: 2,860 |
Not much of a bonus. Vehicles headed to space spend most of their acceleration in a vacuum (something like 2 minutes to clear most of the atmosphere, then 4 to 6 more minutes to reach orbital velocity). If you want to get oxidizer from the atmosphere, you have to linger in the atmosphere at hypersonic speeds. That pretty much ends up destroying any benefits you gain from eliminating the need for oxidizer.
What I see is that the airbreathing engine is heavier than a rocket. You won't beat a rocket for thrust-to-weight ratios. 120:1 thrust-to-weight ratio was possible for rocket engines in the 1970s; 10:1 is good for afterburning turbojet engines. So, you're losing weight there. What I also see is that an airbreathing engine will force the vehicle to stay in the air. While rockets cannot completely ignore aerodynamics, they have a fair degree of freedom in selecting shape and form. After all, they'll be clear of the atmosphere quickly. But high-speed airbreathers...oi! They linger at hypersonic speeds, which means exterior shape is all-critical. Any tweak to aerodynamics results in a redesign of fuel tankage, heat shielding, center of gravity...everything's linked to that highly sculpted form. And vice versa: trying to shift the center of gravity or enlarge the fuel tanks will force a redesign of the exterior. And, of course, there's the heat shielding issue. Rockets are out of the atmosphere quickly. If you see one of those cameras-on-a-rocket launches for a modern Atlas or Delta, you might see a little scorching of the nose of strap-on booster rockets - a minor problem that the composite (fiberglass, carbonfiber, etc.) nose cones are designed to deal with in their brief lives. Likewise, re-entry is a brief (minutes) heat pulse that's quite solvable unless you want a lot of steering and flying at those speeds. Charring plastic has kept space probes safe when they hit atmospheres at 4x the speed of the returning Apollo capsules (Galileo's atmospheric probe hit Jupiter at 50km/s, IIRC, vs ~11km/s for an Apollo capsule returning from the moon). Heck, a thin copper shell and some water will do fine. You don't need sleek, carefully sculpted shapes - the shuttle has some astounding maneuverability during re-entry, and it has the aerodynamics of a brick (which might be insulting to bricks). But vehicles that remain at hypersonic speeds for extended periods, AND hope to benefit from the experience need sleek, carefully sculpted lines. They need heat shielding and cooling systems. Their airbreathing engines often dictate aerodynamic shaping, too. The X43 had a nose of tungsten not for heat resistance, but for controlling its balance - its aerodynamics were so screwy that it needed that heavy weight in its nose to stay balanced in flight. Anyway, here's what airbreathing engines give you: *Heavier air frame, to handle the abuse of extended hypersonic flight *Heavier heat shielding, for the same reasons *An engine with a low thrust-to-weight ratio, so it needs to be big *A lot of design problems, because airframe shape is so closely linked to everything else By the time you're done, you'll probably find the magical savings of using exterior oxidizers really aren't worth the trouble, and may have even been lost to other penalties. And what's using external oxidizer saving you, anyway? Liquid oxygen is cheaper than water or gasoline: $0.07 per liter, today. Gross liftoff weight doesn't matter much, except for the launch pad, and even then not much. It's much easier to design a lightweight, flying fuel tank (aka rocket) than a hypersonic waverider. The structure of the rocket is a lot lighter, despite the fuel it carries. The engines are lighter. The design problems are easier. Or, to put it another way: *Rocket engines have been circling payloads around the world for almost 5 decades. *Scramjets have been confirmed to operate outside the laboratory once in the same period. |
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Aug 22 2004, 05:24 AM
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Avatar of Mediocrity Group: Dumpshocked Posts: 725 Joined: 26-February 02 From: Seattle, WA (err, UCAS) Member No.: 277 |
Well, I'm convinced. :) Thanks for the excellent breakdown.
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Aug 22 2004, 07:10 AM
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#6
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Running Target Group: Members Posts: 1,049 Joined: 24-March 03 Member No.: 4,323 |
I have to say that i really do take issue with this part:
certainly, you dont mean to imply that its pointless to try to figure out different ways of doing things just because our current methods work well enough? just because rockets do what we want them to do doesnt mean some other method cant do it better. while scramjets might not have proven themselves yet as a worthy replacement for rockets, i wouldnt be so quick to dismiss the possibility that they might yet do so. while i respect your opinion as someone far more knowledgeable in the field than i am, i am hesitant to ignore the fact that there are quite a few people, all exceptionally knowledgeable concerning this specific technology, who are very excited about the prospects. |
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Aug 22 2004, 12:35 PM
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Running Target Group: Members Posts: 1,428 Joined: 9-June 02 Member No.: 2,860 |
That's absolutely correct. I was using poor logic there.
There are also very knowledgeable people who are unimpressed with scramjets, and do not share the optimistic assumptions of what's referred to as, "the airbreathing engine mafia." That's not a term I coined. While scramjets are not working well yet, the problems of designing a vehicle to use them can and have been analyzed. Flying in the atmosphere at hypersonic speeds generates known engineering problems, ones well enough known that vehicles can be designed. The performance of scramjets has been estimated from wind tunnel tests, and the weight penalties of scramjets can be estimated for designing said vehicle. Wit happy-fuzzy assumptions, scramjets look good. With less happy-fuzzy assumptions, rockets tend to look better except for certain narrow military applications. For further input on the subject, wander over to sci.space.tech and google for "scramjet mafia" or just "scramjet." I recommend looking at only the past 2 years' of posts. |
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Aug 22 2004, 05:03 PM
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Running Target Group: Members Posts: 1,428 Joined: 9-June 02 Member No.: 2,860 |
http://yarchive.net/space/launchers/airbreathe.html
A rather wandering thread that highlights some of the issues of scramjets. Posts over #70 start deliberately scramjet problems in depth, while others may be generally informative. Useful quotes: "Hypersonic travel combines all the disadvantages of airplanes with all the disadvantages of rocket flight and all the disadvantages of re-entry --- continuously." And: "...there's the carrot of not carrying your oxidizer along. The price, of course, is taking it in at increasing Mach numbers, and the drag imposed thereby, likely a heavier engine for the same thrust, major thermal issues, etc. ... And LOX is cheap." And: ">Basically my calculations show that you can save the entire first >stage if you build an air breathing booster. That's plausible. But so what? You've turned a rocket first stage into a jet first stage. In the process, you've made it harder to build and more difficult to develop. For what? To save *LOX*? WHY???" |
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Aug 23 2004, 01:16 PM
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Moving Target Group: Members Posts: 269 Joined: 26-February 02 Member No.: 752 |
so is the scramjet religion related to the Aurora religion?
Cray74, seriously, guys like you and raygun bring this forum above the usual monkeys throwing feces level of internet forums. |
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Aug 23 2004, 09:24 PM
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#10
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Neophyte Runner Group: Members Posts: 2,073 Joined: 23-August 04 Member No.: 6,587 |
Cray74
I found your post very interesting. In looking at the article it listed 3 uses. Missiles, military aircraft and orbital access. The last seems a very poor place to use an air-breathing engine, as it will need a second propulsion system for operation outside of the atmosphere. I know nothing about the specifics of missiles but I suspect my suggestions about military aircraft may apply. In the arena of military aircraft that must fly in the atmosphere due to there purpose thus incurring all the penalties of high mach flight such as high temperature and aerodynamic design. The only problem that you rased that still applies is the thrust-to-weight ratio at witch point you have a competition between the light rocket engine with its heavy fuel/oxidiser and the heavy Scramjet engine with its lighter fuel load. I doubt it will ever be used for commercial transport of people or cargo. Edward |
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