Saturday, November 2, 2013

LockMart SR-72

NASA SP-2001-4525
AvWeek has a nice marketing piece for Lockheed-Martin's Mach 6 ISR/strike concept based on Blackswift that they're calling SR-72. They have a nice set of graphics that compares the the new Mach 6-capable combined cycle engine to the old Mach 3 SR-71 cycle.

YF-12 Inlet Schematic
The SR-71 provides a nice historical example of design for high-temperature metallic structures. There is also a great deal of detail on the research NASA did on the YF-12 which includes quite a bit of declassified material on how the YF-12 family managed the inlet boundary layer and shock train across the wide range of Mach numbers at which it operated. One of the aircraft involved in that research program is at the museum so folks can go see it, and there is a virtual tour of the SR-71 cockpit available for those who can't make it to Dayton.

I thought this comment from one of the LM engineers was interesting:
It [the Blackswift conceptual design work] produced an aircraft configuration that could controllably take off, accelerate through subsonic, supersonic, transonic and hypersonic speeds. It was controllable and kept the pointy end forward.
We are not advocates of wave riders... You end up with a vehicle that is hard to take off and land, has little fuel volume and high transonic drag.
This is of course a response to their competitor who is quite a well-known advocate of wave-riders. A design technique recently demonstrated in small part (on a missile-like configuration which makes the "pointy end first" problem easier) with X-51.
LoFLYTE wind-tunnel model
The LoFlyte waverider wind-tunnel model sitting next to the YF-12 at the museum illustrates the classic waverider delta planform. In fact, this model was for low-speed handling / landing qualities testing.

D-21 Inlet Spike and Bleed Doors
Another neat item parked next to the YF-12 is the D-21. I think it is interesting to see the changes across this family of aircraft. Since the D-21 was air-launched the design did not have to accommodate take-off, acceleration and landing so it could be more of a "point design". The inlet centerbody is a variable angle compression surface to get more efficient compression as opposed to the constant angle spikes in the YF-12 and SR-71. It also exhibits pretty significant wing anhedral that is a characteristic of vehicles exploiting compression lift by getting the leading edge up on the shock (waveriding).
D-21 Front-view showing Wing Anhedral

One of the reasons that Lockheed and the Air Force were hesitant about sharing an SR-71 with NASA was changes that were made from the YF-12 inlet to make the SR-71 have a lower RCS. I'm not sure what those changes were, but from an external view you can see that the leading edge of the wing on the SR-71 is pushed right up to the inlet lip whereas there is a large gap between the inlet lip and the wing leading edge on the YF-12. This sort of little feature can act as a corner reflector and contribute significantly to the total cross section. That may be the change... or it may not ; - )

1 comment: