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Posted: Thu Oct 08, 2009 11:08 am
Its just those first few hundred meters per second dv before the ram-air effect is possible. Twelve seconds at 3gees to boost upto 350m/s. Early designs such as the Saenger Silverbird
and Soviet Keldysh
used sleds, but the efficiency of QED/ARC makes a sled superfluous. Twelve seconds of low altitude QED/ARC burn per flight doesn't seem very significant. And using one type of engine (QED/ARC-RBCC) eliminates additional complexity.
Well you don't get 3g out of those engines though.. in my current tests i calculated with 2 x 6GW Reactors. At 1500s ISP they produce a thrust of ~1631 kN, so a bit over 0.5g for a 300ton ship. They burn, or rather heat 110kg of propellant per second. Thats almost 8 tons of propellant for just accelerating to 350 m/s and you need some additional energy for a climb to 8-10km altitude to avoid too high dynamic pressures. Also you loose some energy to drag. So i guess we're more looking at 10-15 tons propellant used to get to 350 m/s.
To compare that to ducted fans, i get about the same thrust with just 300-400MW shaft power with a total fan area of 16 m^2. Also they would probably not weigh so much more than the propellant saved by using them.
Also those fans would be more like axial compressors with a pressure ratio of about 1:4. They would sit in the airflow path to the QED engine and would benefit the pressure ratio of the engine up to about M2.8.
Heres a little pic i made for roughly illustrating the concept:
Its very basic but shows the general idea.
Oh, IMO forget "Rasterscan the underbody." Instead, scan the REB inside a "standard" (sc)ramjet engine tube. The forebody/diffuser locks forward for "pure" ARC mode, but retracts to allow airflow. Place the REB generator in the engine tube like the rocket engine in a "standard" RBCC.
Yep i thought something like that too.. just use the standard QED engine and feed it with air from a seperate inlet channel.. (see pic above)
Posted: Thu Oct 08, 2009 11:36 am
As far as I know, the SABRE precooler is still expected to work. A piece has been manufactured and demonstrated, and they've solved the icing problem.
Yes i found this paper: http://www.reactionengines.co.uk/downlo ... 99-209.pdf
Thats also where i got the impressive value of up to 3000 m^2/m^3 from.
And I really think some effort should be put into trying to burn the hot hydrogen exhaust productively, while it's still in the engine and can do some good. At least it should be able to effectively scavenge ozone from the REB-heated air coming down the ram duct...
I guess this would happen anyway if you mix Air and Hydrogen and blast a multiple GW REB into it. Don't know how the reaction equilibrium would be though as the stuff gets pretty hot. But this is probably a matter of nozzle efficiency then.
What i was trying to achieve in my concept is the ability to fly a cruise speed of Mach 4-5 without using any onboard propellant. This would give the spaceplane the nice ability to do a range flight prior to starting the acceleration for orbital velocities. Would be a nice feature for launch sites that can't access certain orbital inclinations because of their longitude.. i.e. insertion into a geostationary orbit from a non equatorial launch site and such..
Posted: Thu Oct 08, 2009 5:19 pm
I don't know how suitable the SABRE precooler design is to heating a Mach 4 airflow. How much energy do you propose to bleed off with that MHD turbine?
Other than that, it seems like a decent idea to me... Dr. Bussard's SSTO design actually used conventional kerosene-fueled turbojets to get up to Mach 2...
Posted: Thu Oct 08, 2009 9:38 pm
Well the MHD Extractor would probably not be used at all in cruise flight. I got the general inspiration to use such a device from this paper: http://gltrs.grc.nasa.gov/reports/2003/ ... 212612.pdf
The idea was to use the Extractor for high mach numbers when the inlet gets more and more inefficient due to high total pressure losses. I'm not at all sure if this could be outbalanced using the MHD Extractor. The paper above speaks of a reduction of kinetic energy of the airflow by 40%, whith 20% going into generated electricity and 20% into heating the airflow. Also according to the paper the MHD Extractor would have a total pressure loss of 70% (!). So my idea was to not use it until the total pressure loss of the inlet rises above 70%. What i don't know is, what the total pressure loss of the MHD Extractor would be at higher mach numbers. The simulations in the paper where done at Mach 3 i think.
Regarding the Heat Exchanger.. yes i guess directly using the SABRE heat exchanger to dump heat into an airstream that was slowed down from M4-5 would not be possible. I just took the general figures for how much surface area per square meter might be possible and then used the thermal wizard (http://www.thermal-wizard.com/tmwiz/
) to calculate some heat transfer coefficients, saw the huge numbers and decided this might well be doable.
Posted: Mon Nov 16, 2009 10:01 am
KitemanSA wrote:With a large subsonic tow plane and a Kinetics Interchange TEther (KITE) Launcher, you can get to several mach before you turn on your motor.
That may do for bulk cargo. I think it would be harder to get such a device man rated.
Actually Kelly Space did an experiment with NASA tow launching an F-106 behind a C-141, releasing it with full fuel at 35,000 ft altitude and 400 kt.