The bend in the SABRE engine: Why is it there?

[Themohawkninja]

One of the distinguishing features of the SABRE engine, is that the whole engine nacelle is bent. Upon doing a small bit of research the only explanation as to why it was bent was that it had something to do with getting more air into the engine, but if this is so, then why isn't such a bend incorporated in all jet engines?

[maccollo]

I think it's because the wings don't generate much lift, so the engines have to help by thrusting slightly upwards. However, the air intakes still need to be pointed into the air flow.

[Kraken tamer]

I disagree that pointing the engines in a direction other then the direction of flight would be beneficial. And if they really needed to partially support the weight of the craft with thrust, they could have just pointed the nozzle in the downwards direction and kept the pre-cooler stage strait. My theory (and it's just a theory) is that they plan to use the shape of the engine as a lifting surface, but I could be horribly wrong.

[Bunsen]

There are two parts to it: which way the inlet points, and which way the exhaust goes. I think maccollo is right about using thrust to augment the wings' lift. Most aircraft don't do that, because when you design something to spend its entire operational life in the atmosphere, it makes sense to give it efficient wings. And other planes' engines may not be close to the center of mass anyway, so pointing them away from horizontal would create large pitching torques. So Skylon's nozzles are pointed more downward than other jets.

The other half is the inlet. Again, conventional aircraft have more efficient wings, so they fly with a relatively small angle of attack (i.e. the degree to which the nose or wing is pointed above the direction of flight). Small wings and thin air necessitate a large angle of attack. Also, flying at high Mach numbers requires inlets that use shock waves as part of their mechanism of action, and those are much pickier about angle of attack than those designed for subsonic flight. So the inlet has to be aligned with the airflow during high-speed, high-altitude flight, where the angle of attack is quite large.

You can see a similar, though less pronounced, bend in the SR-71's inlets. They're pointed inward and slightly downward, since the Blackbird also maintained a fairly high angle of attack in the stratosphere and the fuselage would be pushing the air significantly outward by the time it reached the nacelles.

[Nuke]

nozzels bend down to keep them pointed into the airflow. the engine also needs to curve to line the thrust vector up with the cg. this is because most of the launch mass is fuel and it will change greately from ground to orbit. so the engines are configured to compensate.

also ninja'd

Edited January 11, 2014 by Nuke

[Kraken tamer]

NUKE: I think you may be on to something. It's difficult to say for sure because some pictures show the engines mounted inline with the low wing and other pictures show the engines mounted above the wing, inline with the center of mass.

Bunsen: I took a hard look at the SR71's engines. They inlets are angled down, as you say to face the relative airflow. I have no problem accepting this as fact.

However, I still have problems believing that they would angle the nozzles down to compensate for lift produced by the wing. In every aircraft I have flown and seen, the engines are angled the opposite way to make them parallel with the direction of flight. Indeed the SR71 also does this. It's thrust vector has a negative angle of incidence as you can see in this photo: http://lide.uhk.cz/fim/student/tazlaon1/fotoFull/SR-71%20%285%29.jpg and many others.

[Nuke]

i just read the faq on their webpage.

was going to link it but i cant seem to find it.

Edited January 12, 2014 by Nuke