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?

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...