scramjet

So a scramjet needs needs supersonic air running through it o be able to operate properly.  The problem is, it seems, is that it needs to be something on the order of mach 5 to work.  So the question is, would not it be possible to use forced induction to achieve thrust faster?  One thing that comes immediately to mind is using using the exhaust from a turbojet to help things along, the problem being excessive heat (though in all reality, when flying at hypersonic speeds the heat generated is probably worse than what a normal jet could do).  In action the idea would simply be to duct the turbojet exhaust into the scramjet unit.  Another option that came to mind when thinking of SSTO craft and the possibility of scram-assisted take off, would be to take advantage of the expansive properties of a gas when it heats up, in this case, oxygen.  Regulate the pressure release and use it to achieve scram combustion early.

The last question that comes to mind is that, it seems at some level once ignited, a scramject will keep itself ignited.  is this the case?  Is there any any change that advances in aerospike nozzles could potentially ease the barrier to entry or is a rigid de Laval nozzle essential for this particular type of jet?

Thoughts anyone?

A parasitic aircraft hoisted by a high altitude jet powerred aircraft to around 60000 feet, above the troposphere. When it is released 2 small rockets next to the fuselage under the wings are ignited to accelerate it to mach 5, as this is happenning the scramjet is heated and activated. Once the scram jet has past the operational speed needed and the scram jet is at full thrust the rockets are dropped. It will have to glide in to land like the space shuttle.

Advanced designs may have fixed rockets and future designs may replace the rockets with afterburning jets so it can reach the high altitudes needed under it's own power.

Interesting subject.

I'm wondering though, will a scramjet's propulsion override the basic air-resistance of the engine itself and the fuselage?

What is the efficiency of this system?

I know nothing about engine design and such, therefore terms like deLaval nozzles are meaningless to me.  I do wish to know more though, Know where I can find out?

Well, a scramjet needs oxygen from the air, doesn't it, so feeding it exhaust from another jet engine shouldn't be much help getting it going. And then you would need to switch from the duct to direct airflow, with the engine on, keeping more than mach 5. Good luck building robust mechanics for that.
I'm not following you on 'taking advantage of the expansive properties of gas', etc, etc. That's what jet engines do, it's how they work.

As for keeping itself ignited... Well, they certainly don't ignite the spark plugs every piston cycle. Wikipedia sez they either use a flame holder or pyrophoric fuel additives.

This is the first time I've ever seen a discussion on SCRAMjet technology other than in the academic/industry setting.  Personally, I'm happy to see it.  Here's my two cents:

As poster 4 pointed out, ducting the exhaust gas from any other engine into the SCRAMjet would prohibit any form of combustion.  There's not enough oxygen in the exhaust of any turbo-jet/turbo-fan/turbo-prop engine to use in a whole other engine.  Reheat, or after burning, is a slightly different issue; the air from the low pressure compressor (otherwise known as the fan) is ducted into the exhaust flow, mixed with a LARGE quantity of fuel and re-ignited.  This is a remarkably inefficient process, but if one needs to run away fast, it's really the only viable option. 

Combustion stability is a huge concern with SCRAMjet engines.  The flame holder and additives are no guarantee that the combustion process will continue once it began (unlike the engines in the SR71, which run beautifully if not frightfully).  This is mostly due to a lack of understanding of hypersonic flow physics at the level where they are working.  Also, normal Jet-A kerosene doesn't really cut it as a fuel in the rather extreme conditions found in a SCRAMjet combustor.  One alternative fuel is the simplest silane, SiH4, which is a lot like methane but with a silicone atom instead of carbon.  These fuels exhibit some unique and helpful properties for combustion, however storage and manufacturing processes are  nowhere near perfected, and I do not believe I've found a full combustion species list on it either (all of the products of combustion). 

There have been several "successful" test flights of SCRAMjet engines.  They have been lit, they have produced some thrust, but not enough to really do anything useful.  Poster 2 described the basic concept perfectly.  These engines can not operate under any "normal" set of conditions.  The hope with them is that the energy consumption by using a tandem and or hybrid system will be lower, much lower, than the currently accepted of cargo placement into orbit, that being solid/liquid fueled rockets motors.  When it costs, something like, $15k per pound of cargo to get something into orbit newer and cheaper methods will be sought.  Maybe we'll find it in an already established route, maybe it will be with SCRAMjet tandem systems, or maybe somebody will build a HUGE rail-gun...

I'm guessing what has happened here is that another round of budget cuts has forced NASA to disband their entire scramjet team and replace it with some guy who throws out random suggestions for discussion on 4chan.

>>6
You're closer to the truth than you know.  NASA is now one of the world champions of scrapping big, expensive projects.  I guess being "rocket scientists" didn't mean that they're actually smart enough to avoid the same projectitis that afflicts all corporate structures.

The old 'Skunk Works' came up with the SR71.  Now, NASA can't even come up with a replacement shuttle with 1000 times the project capacity behind the effort.  Hell, NASA can't even replace its launch control system despite having tried several times to do it.  I guess we should be thankful that what NASA sends up largely stays up, and when it comes down as designed, generally comes down intact.  Other than that?  BOFFO!

The best way to kill any SCRAMjet effort is to let NASA run the project.  Maybe the Chinese will learn a thing or two and run their own program by means of small, elite groups.  Chinky-chink suborbitals will be tiny points of light over America, then ... to remind us of our failures.

>>7
But RedCream, why is there a glowing hammer and sickle floating in the sky?

>>8
Post the video.

>>7

>You're closer to the truth than you know.

In before RedCream reveals the fact that he is a top-level government agent, as part of the Gilgamesh program.

>>10
No, my knowledge just requires reading various news sources instead of (say, YOU) sticking one's head up-butt to avoid hearing anything contrary to the usual feel-good crap.

>>5
Some sort of railgun + scramjet system would be wickedly awesome. And also very likely impractical. But mostly wickedly awesome.

>>11

we're already well aware of your news diet's consistency of conspiracist propaganda and communist leaflets.  you fucking anti-semetic piece of shit.

>>12
Yes, yes it would be wickedly awesome.  DISTURBINGLY huge and impractical, but damn it, I'd take the tax hike just to see it built.

The Jet engine could be configured so that there is more oxygen than needed to burn the fuel used, the remaining oxygen is used by the scramjet.

>>15
Running lean produces many pollution/combustion stability problems (I know, I know, I'm a HUGE dork).  Aside from that little gem of gas turbine combustion knowledge, there is another practical problem with ducting larger amounts of air, and that being the duct itself.  Basically, in order to accomplish what you have suggested someone would need to take a GE90, re-route the air from the fan (yeah, they still call it a fan even when it's 128 inches in diameter) into the SCRAMjet.  SCRAMjets really only kickoff at around Mach 5 or higher (go speed racer), which would DESTROY the engine which supplies the air to the SCRAMjet.  I'm sorry, man, but most aeroengines just aren't designed to withstand such a demanding/destructive environment that is produced by hypersonic flight.