An air-breathing craft does not need nearly as much fuel, and consequently it is not nearly as expensive. Sure, it has more parts than a simple rocket, but those parts are a lot less expensive than all the fuel.
Fuel is cheap. So is fuel tankage. It's rebuilding/reassembling the vehicle after every flight that costs money. Design that out of the system (can be done), and rockets get a whole lot cheaper.
An air-breathing craft can't get to space, and we have no idea how to build air-breathing craft that could even make it half way to orbit.
The arguement you make is the exact same arguement that the shuttle had. [...] The shuttle is re-useable...
No, the Shuttle is not reusable. The external tank is disposable, the solid boosters are crashed, salvaged, and parts of them overhauled and remanufactured into new boosters. You could just barely say that the Orbiter is reusable, except that it undergoes more than the equivalent of a major overhaul between every flight.
When I talk about reusable, I mean in the aircraft sense -- refuel it, kick the tires, and take off.Okay, I'm a pilot, I know the walk around is a little more than that, but a lot less than a major overhaul (only done after many flight cycles, number depending on the kind of aircraft). Think X-15 (a rocket plane) or DC-X. Note that this sort of reusability implies an ability to abort the flight at any point -- something the Shuttle with it's vertical takeoff, horizontal landing mode and 2-minute-burn solid boosters doesn't have.
When you complicate the situation, such as doing something so small as sticking the space vehicle on the side like the shuttle versus on the top, you can exponentially increase the failure rate.
That's hardly "small". That's a major design change -- changes your load paths, your aerodynamics, your flight profile, etc, etc. It's also f'cking stupid. Both Shuttle losses were attributable to this configuration. (If the Orbiter were in-line with and above the ET, it couldn't have been struck by foam (Columbia) and it might well have survived the breakup of the ET when the solids burned through it (Challenger).)
Its not that scramjets wouldn't work,
Well, we don't exactly know that yet. We do that they won't work all the way to orbit.
its that rockets DO work, and work incredibly reliably. their attitude adjustment and flight paths are infinetly simpler than something that has to calculate for air moving at such high speeds through an engine.
Exactly. Now combine whatever complex scramjet stage you have with an upper stage capable of reaching orbit, and you're likely to end up with an overall configuration that's as bad a dog's breakfast as the Shuttle stack.
No, we can't assume that. The rocket is burning a pure H2-O2 mix (in not quite stoichiometric ratios, it runs a little hydrogen rich to lower the average atomic mass of the exhaust which increases the exhaust velocity, raising the Isp).
The jet is burning an H2-O2-N2 mix, roughly 80% N2, which adds nothing to the combustion efficiency (indeed, subtracts from it because of side reactions) and increases the average exhaust weight, lowering the exhaust velocity and the Isp. Granted, some of this is offset by the not having to carry the mass of the oxidizer (although above a certain speed you do have to accelerate it or you lose the flame front, and above that speed you're better off with a rocket so as to avoid accelerating nitrogen).
I would note that from 1980-1999 that the reliability for liquid rocket launches was 97.5%. Most of the failures are inherently due to constraints caused by the low superstructure weight requirements of rockets.
Nope. Ariane 5 -- software failure. Challenger -- SRB burn through (clearly I'm excluding solids from this discussion, they should be considered ammunition only). Delta -- SRB failure. DC-X -- 100% success rate until somebody left a landing struct hydraulic line unconnected. Etc. Etc.
I don't see that we should be content with it in the space industry.
I'll agree with that, but it's not inherent to using (liquid) rockets. Part of it is the converted-ammunition disposable vehicle mentality. If you can't flight-test the vehicle (ie, the exact vehicle, not a different one of the same design), you can't shake out the bugs that you miss on inspection. Sure, designing a rocket to be reusable adds weight, which takes away from payload, but a properly reusable vehicle amortizes vehicle costs over many flights. It's this, I suspect, is what seems attractive about scramjets -- but there's no inherent reason it can't apply to rockets. The DC-X was a great proof-of-concept that rockets could be reusable (as, for that matter, was X-15).
You also have to lift the tanks, insulation, and pumps.
All of which weigh a small fraction of the propellant weight. Consider an aluminum beer can -- what is its full to empty weight ratio? Granted, rocket tankage isn't quite that good, but it's close.
Guess what--the Air Force isn't going to lend NASA an SR-71 so that they can blow it up when the test goes bad.
I guess the Air Force has less faith in scramjets than in rockets, then. They did lend NASA an SR-71 for some aerospike rocket tests.
NASA is doing the sensible thing by testing the engine that they've got--and consequently testing their engineering assumptions and theoretical models.
I don't have a problem with NASA doing theoretical research. Indeed, they should do more of that and get out of the space operations business, which they do badly. But nobody should kid themselves that this is part of a straightline path to cheap space travel. See subject line.
You've admirably demonstrated why high Isp rockets work better than low Isp rockets.
Now you try this one:
Try a rocket with mass M, climb it to an altitude of 100 miles, and accelerate it to Mach 25. Calculate the drag losses. (zero).
Take a scramjet with mass M/2, climb it to operational altitude, and try to accelerate it to Mach 25. Calculate the drag losses (include induced drag as well as parasitic drag).
For extra credit, calculate the maximum speed (Vmax) it reaches before it runs out of fuel, even assuming mass M with all the extra mass as fuel.
Note that Vmax considerably less than orbital velocity (Mach 25). See subject of message.
Sez you. Not a scientist, and not, quite frankly someone who has any idea.
I don't get paid to do rocket science. That doesn't mean I'm not a scientist nor does it mean I don't know rocket science.
It's about the numbers. You don't know them. Those who do know (Including me, I might add), know that it's a close thing which is unclear, even within the scientific community.
Meaning you're just bullshitting, because if it's unclear, then nobody knows the numbers. I do know the numbers for rocket tankage and oxidizer, and they just aren't that bad. (Barring some silly-ass overall vehicle design trades -- like the V-shaped tank of the X-33.)
Since when is easier a reason for stopping development?
Always, if it gets the job done. What is the job for scramjets that rockets cannot do?
Would you like more efficient rockets? If yes, then airbreathing engines are one of the available options.
No they're not. For one, an airbreathing engine is by definition not a rocket. But the main point is that an airbreathing engine is using a very dilute oxidizer, only 20%, the rest relatively inert nitrogen.
I'd point out that there's nothing saying that Scramjets: 1. must be in a lifting body 2. must be SSTO or 3. must be reusable.
All true. But (2) and (3) kill your operational efficiency (ie, add greatly to ground handling requirements) and thus raise the cost-per-flight.
As for (1), wings don't stand up to hypersonic flight very well (a tendency to burn unless actively cooled, or well-insulated with short flight times). And without wings or a lifting body, you've got a vehicle that has to lift itself entirely by thrust, and the altitude range at which scramjets are useful would be traversed in such a short time as to make using them this way downright silly.
It's hard to improve on 90-plus percent efficiency, true.
Just because their possible successors are not doing better than rockets now, doesn't mean that they won't eventually do better.
No, that alone doesn't mean that. The fact that any airbreathing vehicle wastes energy moving nitrogen around, and has to fly a high-drag profile, is what means that they won't eventually do better. And any improvment in materials technology to allow e.g. hotter burning engines, lighter weight structures, etc, can also be applied to rockets.
there's no inherent reason why a scramjet must have higher drag than a rocket of the same frontal area.
Yes, there is. Rockets can operate in vacuum, scramjets can't. There's zero drag in a vacuum.
Scramjets don't push through the air any more than any other vehicle.
Yes, they do. See above. Rockets -- after the first couple of minutes climbing mostly straight up -- don't push through any air at all.
To reiterate my original comment, scramjets might make for interesting high-speed, relatively short-range aircraft, but they won't get you to space.
Yes, fuel is cheap, but a rocket is a mountain of fuel, and reliable, lightweight superstructure for a mountain is neither cheap nor easy.
Bullshit. Reliable, lightweight structures for rocket fuel tankage is easy -- we've been doing it for nearly 50 years. Basically metal balloons, pressurizing the tank gives the ridgidity you need. Look at the mass-fraction figures for Atlas, the various Saturn V stages, or the Shuttle ET for example. The ET is in fact the 'strongback' component of the Shuttle stack. (Although overall the Shuttle stack has other problems.)
Conversely, designing a reliable, lightweight structure for a tank of fuel that has to ram through the atmosphere in excess of Mach 5 for long periods of time is hard.
The thing is that rockets are great,
Glad we agree.
unless you want to improve reliability or efficiency from current levels, then you need something else.
Hard to improve the mechanical efficiency, you'd need a denser energy source, i.e. non-chemical. And scramjets will never reach that level of efficiency, they waste too much energy shoving nitrogen around.
As for reliability -- well, scramjets are at the Goddard level compared to liquid rockets. If that. (Overall vehicle design is a whole 'nother issue, and we might well find some areas of agreement there.)
I'm also not a rocket scientist, I've been working in scramjet research for 7 years.
What a senseless waste of human life.
There are two main problems with rockets: 1. They've pretty much reached their peak efficiency.
Yeah, well, when you're at the top there's not much room for expansion. Rockets are perhaps the single most efficient machine known to man at converting fuel to motion.
2. They've pretty much reached their peak reliability.
See above. We haven't had a liquid rocket engine blow up in, what, decades? Overall vehicle reliability doesn't count -- you'd have those problems with scramjets, only moreso, since we don't have 50 years of operational experience with them, and they're operating in a tougher domain.
The comment about drag is correct, but the promise of airbreathing technology is that you lose an extra 30% in drag and gain 60% in efficiency (insert your own numbers where necessary).
In other words, pull the numbers out of your ass to make it sound good. What's the drag on a rocket in vacuum? Zero. What's the efficiency of rocket burning pure fuel and oxygen, rather than (like a jet) a fuel and 20% oxygen, 80% nitrogen mix?
Drag goes up as the square of the velocity; the amount of air you can shove down the throat of a scramjet only goes up linearly with velocity. It won't scale.
I was with you up to here: The flight profile for a long duration hypersonic craft would probably involve extended flight at altitudes where drag is less of an issue,
The Catch-22 (43?) is, it has to stay at altitudes low enough for it to get sufficient oxygen for its airbreathing engines. Too high and the engines flame out from oxygen starvation. (Too low and you either have too much drag to overcome to maintain speed, or you burn up.)
Scramjets combust the air at supersonic velocities rather than diffusing it prior to combustion the way most other engines in supersonic vehicles do.
So tell me, how much faster does the exhaust go out than the air come in?
Because that difference (times the mass of the exhaust) is the only momentum you can add to the vehicle as a whole, part of which is wasted pushing aside any air that you're not burning.
If you don't need air to run your engines, you can get out above most of it and then accelerate up to orbital speed -- and a suborbital rocket (1960s technology) will get you to the other side of the world in under an hour.
To make a practical, re-usable, passenger carrying scramjet we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part), plus the problems of designing an effective, large-scale (compared to current test models) scram-jet engine. To make a practical, re-usable, passenger carrying rocket we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part) -- but we already know how to make efficient, large-scale rocket engines.
So what were all those jets flying in the 50s? UFOs?
Both military and civilian jet aircraft were doing well in the 1950s.
And as for the Nazis developing the first jet engine, Sir Frank Whittle might have an argument with that. (Although the Germans may have had a jet -powered aircraft in the air first.)
I don't know how 'well' the DC-X did, considering that it burned itself up on one of its landings.
No, it did not. Here's the real story:
The DC-X project was initially run out of the Strategic Defense Initiative Office -- causing some turf envy at NASA. The vehicle went through a number of very successful flights (I got to see one of them) to ever higher altitudes and interesting flight profiles.
On one launch, some vented hydrogen had collected in the launch area near the base of the rocket and detonated when the engines lit. The shock blew off part of the fuselage but the DC-X just kept on climbing -- until the flight controller (I think it was Pete Conrad on that flight) and others noticed the debris falling from the vehicle and initiated the emergency abort/autoland sequence. The engines throttled back and the DC-X set itself down unharmed (aside from the initial damage). The fuselage was repaired and the DC-X flew again.
After SDIO's initial flight test sequence, the DC-X project was transferred to the control of NASA (remember that turf battle?). On the first NASA-controlled flight, a technician apparently left disconnected a hydraulic line to one of the landing legs (the rocket sat on a "milk-stool" support for launch). The flight went fine, the landing went okay until the engines shut off -- and then the unconnected leg folded up and the DC-X tipped over and fell. The impact cracked open the fuel tanks, the residual fuel caught fire, and the DC-X was destroyed.
No fault of the vehicle, just a technician fuck-up -- the equivalent of an airplane's gear collapsing on landing.
Scramjets don't push through the air. They suck it in for combustion.
ROFL!
That is just wrong on so many levels. You do realize, don't you, that it is impossible to suck air to a speed greater than Mach 1? (Well, unless you're sucking it into a huge vacuum chamber through a DeLaval nozzle, but only until the pressures equalize, and then only in the divergent section of the nozzle.)
Chemical fuel rockets will one day be considered as archaic as steam engines.
We all look forward to that day -- but air-breathing jets are no more advanced than rockets, and in fact they're rather Rube Goldbergish. Using jets and wings to get to space is equivalent to the "horseless carriage" era of automobiles -- or worse. It's like trying to come up with a 100MPH, 200 HP vehicle by inventing a harness that will let you hook up 200 actual horses, because they can "forage for themselves" rather than just building in a gas tank and engine.
Rockets only work better if you consider the mechanical efficiency.
Thank you, at least somebody recognizes that.
If you throw cost into the deal, rockets fall apart. They're disposable for the most part.
They don't have to be disposable. The X-15 was a fine example of a reusable rocket -- 199 flights for the 3 vehicles, several of them high enough to earn the pilots their astronaut wings. That was 40-50 year old technology. The DC-X was a great example of a reusable rocket that could take off from the ground and land under its own power.
Uh huh. THen why does the X-43 need a booster rocket?
because they can get fuel from the air, and reduce the fuel weight of the rocket
Well, one, it's oxidizer, not fuel. Two, the structural weight needed to withstand pushing through atmosphere at extreme Mach (stand up to both drag and temperatures) outweighs what you need for tanks and LOX to do it the rocket way.
Think, there's a reason that we've used rockets for 50 years instead of scramjets -- it's easier.
Also, the shuttle is basicly just a glider when landing.
What the hell does the Shuttle have to do with anything? THat is not a hypersonic-cruise vehicle, on the contrary it is designed as a high-drag vehicle so it can slow down from hypersonic on reentry. As for go-around jets, the Shuttle doesn't land supersonically, good old fashioned turbojets would work just fine. (And indeed, the old Soviet Shuttle used just such for ferrying and landing tests.)
Fuel is cheap. With a rocket, all the energy you put into lifting and accelerating that fuel you gain back when you burn it.
Burning air (as a scramjet) means (a) you're handling 400% more mass than you need to (the nitrogen) and (b) unless you add energy to it to accelerate it, you don't get as much momentum kick when you burn it.
You'll note that they accelerate the damn test article with a rocket.
For the several earlier posters who seem to think that this is the Holy Grail of Earth-to-orbit transportation -- well, maybe they're right in that it's about equally unattainable. Rockets work a hell of a lot better - as has been demonstrated by almost 47 years of orbital flight.
Any airbreathing technology suffers a couple of fundamental flaws when it comes to suborbital, let alone orbital, transport. Most obvious, the air is mighty thin up there -- so you've got to stay where the air is thicker to support combustion. (Which basically means you can't make orbit with out at least some kind of apogee kick rocket).
Secondly, pushing through all that air creates drag. Now, you either aggravate the problem by slowing the relative airspeed enough to support combustion -- meaning increasing the drag on that air (supersonic combustion alleviates this somewhat), or you don't slow it down (relatively, actually you're speeding the air up), have a harder time maintaining combustion, and more significantly, have a much lower momentum delta in the exhaust -- meaning less push to the vehicle.
Scramjets have some limited use for high speed short range flight but rockets are far more efficient and the only practical way to get to orbit.
(And while I may not be a rocket scientist, I've had long talks about just this with some very expert rocket scientists, such as Max Hunter.)
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Naw, what you got there is one of them there smoke emitting diodes.
That's not a low ID. This (19027) is a low ID.
(I know, there's still lots lower than that. That's what I get for lurking so long before registering.)
An air-breathing craft does not need nearly as much fuel, and consequently it is not nearly as expensive. Sure, it has more parts than a simple rocket, but those parts are a lot less expensive than all the fuel.
Fuel is cheap. So is fuel tankage. It's rebuilding/reassembling the vehicle after every flight that costs money. Design that out of the system (can be done), and rockets get a whole lot cheaper.
An air-breathing craft can't get to space, and we have no idea how to build air-breathing craft that could even make it half way to orbit.
...when you've got duct tape?
The arguement you make is the exact same arguement that the shuttle had. [...] The shuttle is re-useable ...
No, the Shuttle is not reusable. The external tank is disposable, the solid boosters are crashed, salvaged, and parts of them overhauled and remanufactured into new boosters. You could just barely say that the Orbiter is reusable, except that it undergoes more than the equivalent of a major overhaul between every flight.
When I talk about reusable, I mean in the aircraft sense -- refuel it, kick the tires, and take off.Okay, I'm a pilot, I know the walk around is a little more than that, but a lot less than a major overhaul (only done after many flight cycles, number depending on the kind of aircraft). Think X-15 (a rocket plane) or DC-X. Note that this sort of reusability implies an ability to abort the flight at any point -- something the Shuttle with it's vertical takeoff, horizontal landing mode and 2-minute-burn solid boosters doesn't have.
When you complicate the situation, such as doing something so small as sticking the space vehicle on the side like the shuttle versus on the top, you can exponentially increase the failure rate.
That's hardly "small". That's a major design change -- changes your load paths, your aerodynamics, your flight profile, etc, etc. It's also f'cking stupid. Both Shuttle losses were attributable to this configuration. (If the Orbiter were in-line with and above the ET, it couldn't have been struck by foam (Columbia) and it might well have survived the breakup of the ET when the solids burned through it (Challenger).)
Its not that scramjets wouldn't work,
Well, we don't exactly know that yet. We do that they won't work all the way to orbit.
its that rockets DO work, and work incredibly reliably. their attitude adjustment and flight paths are infinetly simpler than something that has to calculate for air moving at such high speeds through an engine.
Exactly. Now combine whatever complex scramjet stage you have with an upper stage capable of reaching orbit, and you're likely to end up with an overall configuration that's as bad a dog's breakfast as the Shuttle stack.
Interesting. I'll have to get a copy of that book; it sounds like a lot of what I and many others have been saying for years.
then if we assume the same combustion efficiency,
No, we can't assume that. The rocket is burning a pure H2-O2 mix (in not quite stoichiometric ratios, it runs a little hydrogen rich to lower the average atomic mass of the exhaust which increases the exhaust velocity, raising the Isp).
The jet is burning an H2-O2-N2 mix, roughly 80% N2, which adds nothing to the combustion efficiency (indeed, subtracts from it because of side reactions) and increases the average exhaust weight, lowering the exhaust velocity and the Isp. Granted, some of this is offset by the not having to carry the mass of the oxidizer (although above a certain speed you do have to accelerate it or you lose the flame front, and above that speed you're better off with a rocket so as to avoid accelerating nitrogen).
I would note that from 1980-1999 that the reliability for liquid rocket launches was 97.5%. Most of the failures are inherently due to constraints caused by the low superstructure weight requirements of rockets.
Nope. Ariane 5 -- software failure. Challenger -- SRB burn through (clearly I'm excluding solids from this discussion, they should be considered ammunition only). Delta -- SRB failure. DC-X -- 100% success rate until somebody left a landing struct hydraulic line unconnected. Etc. Etc.
I don't see that we should be content with it in the space industry.
I'll agree with that, but it's not inherent to using (liquid) rockets. Part of it is the converted-ammunition disposable vehicle mentality. If you can't flight-test the vehicle (ie, the exact vehicle, not a different one of the same design), you can't shake out the bugs that you miss on inspection. Sure, designing a rocket to be reusable adds weight, which takes away from payload, but a properly reusable vehicle amortizes vehicle costs over many flights. It's this, I suspect, is what seems attractive about scramjets -- but there's no inherent reason it can't apply to rockets. The DC-X was a great proof-of-concept that rockets could be reusable (as, for that matter, was X-15).
You also have to lift the tanks, insulation, and pumps.
All of which weigh a small fraction of the propellant weight. Consider an aluminum beer can -- what is its full to empty weight ratio? Granted, rocket tankage isn't quite that good, but it's close.
Guess what--the Air Force isn't going to lend NASA an SR-71 so that they can blow it up when the test goes bad.
I guess the Air Force has less faith in scramjets than in rockets, then. They did lend NASA an SR-71 for some aerospike rocket tests.
NASA is doing the sensible thing by testing the engine that they've got--and consequently testing their engineering assumptions and theoretical models.
I don't have a problem with NASA doing theoretical research. Indeed, they should do more of that and get out of the space operations business, which they do badly. But nobody should kid themselves that this is part of a straightline path to cheap space travel. See subject line.
You've admirably demonstrated why high Isp rockets work better than low Isp rockets.
Now you try this one:
Try a rocket with mass M, climb it to an altitude of 100 miles, and accelerate it to Mach 25. Calculate the drag losses. (zero).
Take a scramjet with mass M/2, climb it to operational altitude, and try to accelerate it to Mach 25. Calculate the drag losses (include induced drag as well as parasitic drag).
For extra credit, calculate the maximum speed (Vmax) it reaches before it runs out of fuel, even assuming mass M with all the extra mass as fuel.
Note that Vmax considerably less than orbital velocity (Mach 25). See subject of message.
Sez you. Not a scientist, and not, quite frankly someone who has any idea.
I don't get paid to do rocket science. That doesn't mean I'm not a scientist nor does it mean I don't know rocket science.
It's about the numbers. You don't know them. Those who do know (Including me, I might add), know that it's a close thing which is unclear, even within the scientific community.
Meaning you're just bullshitting, because if it's unclear, then nobody knows the numbers. I do know the numbers for rocket tankage and oxidizer, and they just aren't that bad. (Barring some silly-ass overall vehicle design trades -- like the V-shaped tank of the X-33.)
Since when is easier a reason for stopping development?
Always, if it gets the job done. What is the job for scramjets that rockets cannot do?
Would you like more efficient rockets? If yes, then airbreathing engines are one of the available options.
No they're not. For one, an airbreathing engine is by definition not a rocket. But the main point is that an airbreathing engine is using a very dilute oxidizer, only 20%, the rest relatively inert nitrogen.
I'd point out that there's nothing saying that Scramjets:
1. must be in a lifting body
2. must be SSTO or
3. must be reusable.
All true. But (2) and (3) kill your operational efficiency (ie, add greatly to ground handling requirements) and thus raise the cost-per-flight.
As for (1), wings don't stand up to hypersonic flight very well (a tendency to burn unless actively cooled, or well-insulated with short flight times). And without wings or a lifting body, you've got a vehicle that has to lift itself entirely by thrust, and the altitude range at which scramjets are useful would be traversed in such a short time as to make using them this way downright silly.
Rockets have reached their limit.
It's hard to improve on 90-plus percent efficiency, true.
Just because their possible successors are not doing better than rockets now, doesn't mean that they won't eventually do better.
No, that alone doesn't mean that. The fact that any airbreathing vehicle wastes energy moving nitrogen around, and has to fly a high-drag profile, is what means that they won't eventually do better. And any improvment in materials technology to allow e.g. hotter burning engines, lighter weight structures, etc, can also be applied to rockets.
there's no inherent reason why a scramjet must have higher drag than a rocket of the same frontal area.
Yes, there is. Rockets can operate in vacuum, scramjets can't. There's zero drag in a vacuum.
Scramjets don't push through the air any more than any other vehicle.
Yes, they do. See above. Rockets -- after the first couple of minutes climbing mostly straight up -- don't push through any air at all.
To reiterate my original comment, scramjets might make for interesting high-speed, relatively short-range aircraft, but they won't get you to space.
Yes, fuel is cheap, but a rocket is a mountain of fuel, and reliable, lightweight superstructure for a mountain is neither cheap nor easy.
Bullshit. Reliable, lightweight structures for rocket fuel tankage is easy -- we've been doing it for nearly 50 years. Basically metal balloons, pressurizing the tank gives the ridgidity you need. Look at the mass-fraction figures for Atlas, the various Saturn V stages, or the Shuttle ET for example. The ET is in fact the 'strongback' component of the Shuttle stack. (Although overall the Shuttle stack has other problems.)
Conversely, designing a reliable, lightweight structure for a tank of fuel that has to ram through the atmosphere in excess of Mach 5 for long periods of time is hard.
The thing is that rockets are great,
Glad we agree.
unless you want to improve reliability or efficiency from current levels, then you need something else.
Hard to improve the mechanical efficiency, you'd need a denser energy source, i.e. non-chemical. And scramjets will never reach that level of efficiency, they waste too much energy shoving nitrogen around.
As for reliability -- well, scramjets are at the Goddard level compared to liquid rockets. If that. (Overall vehicle design is a whole 'nother issue, and we might well find some areas of agreement there.)
I'm also not a rocket scientist, I've been working in scramjet research for 7 years.
What a senseless waste of human life.
There are two main problems with rockets:
1. They've pretty much reached their peak efficiency.
Yeah, well, when you're at the top there's not much room for expansion. Rockets are perhaps the single most efficient machine known to man at converting fuel to motion.
2. They've pretty much reached their peak reliability.
See above. We haven't had a liquid rocket engine blow up in, what, decades? Overall vehicle reliability doesn't count -- you'd have those problems with scramjets, only moreso, since we don't have 50 years of operational experience with them, and they're operating in a tougher domain.
The comment about drag is correct, but the promise of airbreathing technology is that you lose an extra 30% in drag and gain 60% in efficiency (insert your own numbers where necessary).
In other words, pull the numbers out of your ass to make it sound good. What's the drag on a rocket in vacuum? Zero. What's the efficiency of rocket burning pure fuel and oxygen, rather than (like a jet) a fuel and 20% oxygen, 80% nitrogen mix?
Drag goes up as the square of the velocity; the amount of air you can shove down the throat of a scramjet only goes up linearly with velocity. It won't scale.
(eg. I once ran it on a MicroVAX at an incredibly slow speed)
Heck, native code on a MicroVAX runs at an incredibly slow speed. An IA32 emulation must be positively glacial.
I was with you up to here: The flight profile for a long duration hypersonic craft would probably involve extended flight at altitudes where drag is less of an issue,
The Catch-22 (43?) is, it has to stay at altitudes low enough for it to get sufficient oxygen for its airbreathing engines. Too high and the engines flame out from oxygen starvation. (Too low and you either have too much drag to overcome to maintain speed, or you burn up.)
Scramjets combust the air at supersonic velocities rather than diffusing it prior to combustion the way most other engines in supersonic vehicles do.
So tell me, how much faster does the exhaust go out than the air come in?
Because that difference (times the mass of the exhaust) is the only momentum you can add to the vehicle as a whole, part of which is wasted pushing aside any air that you're not burning.
If you don't need air to run your engines, you can get out above most of it and then accelerate up to orbital speed -- and a suborbital rocket (1960s technology) will get you to the other side of the world in under an hour.
To make a practical, re-usable, passenger carrying scramjet we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part), plus the problems of designing an effective, large-scale (compared to current test models) scram-jet engine. To make a practical, re-usable, passenger carrying rocket we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part) -- but we already know how to make efficient, large-scale rocket engines.
Which seems easier?
So what were all those jets flying in the 50s? UFOs?
Both military and civilian jet aircraft were doing well in the 1950s.
And as for the Nazis developing the first jet engine, Sir Frank Whittle might have an argument with that. (Although the Germans may have had a jet -powered aircraft in the air first.)
I don't know how 'well' the DC-X did, considering that it burned itself up on one of its landings.
No, it did not. Here's the real story:
The DC-X project was initially run out of the Strategic Defense Initiative Office -- causing some turf envy at NASA. The vehicle went through a number of very successful flights (I got to see one of them) to ever higher altitudes and interesting flight profiles.
On one launch, some vented hydrogen had collected in the launch area near the base of the rocket and detonated when the engines lit. The shock blew off part of the fuselage but the DC-X just kept on climbing -- until the flight controller (I think it was Pete Conrad on that flight) and others noticed the debris falling from the vehicle and initiated the emergency abort/autoland sequence. The engines throttled back and the DC-X set itself down unharmed (aside from the initial damage). The fuselage was repaired and the DC-X flew again.
After SDIO's initial flight test sequence, the DC-X project was transferred to the control of NASA (remember that turf battle?). On the first NASA-controlled flight, a technician apparently left disconnected a hydraulic line to one of the landing legs (the rocket sat on a "milk-stool" support for launch). The flight went fine, the landing went okay until the engines shut off -- and then the unconnected leg folded up and the DC-X tipped over and fell. The impact cracked open the fuel tanks, the residual fuel caught fire, and the DC-X was destroyed.
No fault of the vehicle, just a technician fuck-up -- the equivalent of an airplane's gear collapsing on landing.
Scramjets don't push through the air. They suck it in for combustion.
ROFL!
That is just wrong on so many levels. You do realize, don't you, that it is impossible to suck air to a speed greater than Mach 1? (Well, unless you're sucking it into a huge vacuum chamber through a DeLaval nozzle, but only until the pressures equalize, and then only in the divergent section of the nozzle.)
Chemical fuel rockets will one day be considered as archaic as steam engines.
We all look forward to that day -- but air-breathing jets are no more advanced than rockets, and in fact they're rather Rube Goldbergish. Using jets and wings to get to space is equivalent to the "horseless carriage" era of automobiles -- or worse. It's like trying to come up with a 100MPH, 200 HP vehicle by inventing a harness that will let you hook up 200 actual horses, because they can "forage for themselves" rather than just building in a gas tank and engine.
Rockets only work better if you consider the mechanical efficiency.
Thank you, at least somebody recognizes that.
If you throw cost into the deal, rockets fall apart. They're disposable for the most part.
They don't have to be disposable. The X-15 was a fine example of a reusable rocket -- 199 flights for the 3 vehicles, several of them high enough to earn the pilots their astronaut wings. That was 40-50 year old technology. The DC-X was a great example of a reusable rocket that could take off from the ground and land under its own power.
Scramjets are good for launch vehicles
Uh huh. THen why does the X-43 need a booster rocket?
because they can get fuel from the air, and reduce the fuel weight of the rocket
Well, one, it's oxidizer, not fuel. Two, the structural weight needed to withstand pushing through atmosphere at extreme Mach (stand up to both drag and temperatures) outweighs what you need for tanks and LOX to do it the rocket way.
Think, there's a reason that we've used rockets for 50 years instead of scramjets -- it's easier.
Also, the shuttle is basicly just a glider when landing.
What the hell does the Shuttle have to do with anything? THat is not a hypersonic-cruise vehicle, on the contrary it is designed as a high-drag vehicle so it can slow down from hypersonic on reentry. As for go-around jets, the Shuttle doesn't land supersonically, good old fashioned turbojets would work just fine. (And indeed, the old Soviet Shuttle used just such for ferrying and landing tests.)
But they will get you through the most energy demanding part of the trip without having to carry the oxygen.
No, the most energy demanding part of the trip is not the first 100 miles, it's the accelerating up to Mach 25 to maintain orbit.
LOX is cheap, tankage is cheap. Don't get hung up on current stupid designs like Shuttle.
Do the math, people, the rocket equation isn't that difficult.
Do the math.
Fuel is cheap. With a rocket, all the energy you put into lifting and accelerating that fuel you gain back when you burn it.
Burning air (as a scramjet) means (a) you're handling 400% more mass than you need to (the nitrogen) and (b) unless you add energy to it to accelerate it, you don't get as much momentum kick when you burn it.
You'll note that they accelerate the damn test article with a rocket.
For the several earlier posters who seem to think that this is the Holy Grail of Earth-to-orbit transportation -- well, maybe they're right in that it's about equally unattainable. Rockets work a hell of a lot better - as has been demonstrated by almost 47 years of orbital flight.
Any airbreathing technology suffers a couple of fundamental flaws when it comes to suborbital, let alone orbital, transport. Most obvious, the air is mighty thin up there -- so you've got to stay where the air is thicker to support combustion. (Which basically means you can't make orbit with out at least some kind of apogee kick rocket).
Secondly, pushing through all that air creates drag. Now, you either aggravate the problem by slowing the relative airspeed enough to support combustion -- meaning increasing the drag on that air (supersonic combustion alleviates this somewhat), or you don't slow it down (relatively, actually you're speeding the air up), have a harder time maintaining combustion, and more significantly, have a much lower momentum delta in the exhaust -- meaning less push to the vehicle.
Scramjets have some limited use for high speed short range flight but rockets are far more efficient and the only practical way to get to orbit.
(And while I may not be a rocket scientist, I've had long talks about just this with some very expert rocket scientists, such as Max Hunter.)