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How does opening airplane bay doors (especially in the case of bombers), increase the radar cross section of the airplane and make it more visible to the radar?

These are usually combined with active measures such as carefully planning all mission maneuvers in order to minimize the aircraft's radar cross section, since common actions such as hard turns or opening bomb bay doors can more than double an otherwise stealthy aircraft's radar return

Source

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    $\begingroup$ This is too broad: What type of bay doors? How big and how do they open? How high does the aircraft fly? Is it pitching or banking to any direction? How close to the radar installation(s) is it flying? Is the radar monostatic, bistatic or multystatic? Generally, determining the RCS of a target and the factors that can affect it, is not an easy task. $\endgroup$
    – Stelios Adamantidis
    Commented Jul 14, 2015 at 10:13
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    $\begingroup$ 1. Bay doors used for dropping bombs by bombers 2. How Big? Consider any normal modern bomber 3. How high? I have not mentioned that because of the fact that wiki article mentions increase in radar cross section without mentioning altitude (and thats why planes use very high altitude to avoid detection) $\endgroup$
    – Victor Juliet
    Commented Jul 14, 2015 at 11:22
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    $\begingroup$ Your comment reference a Wiki article, yet your question does not. If you want people to take that article into consideration when answering the question, you may want to provide a link and mention the bits you feel are relevant. $\endgroup$
    – FreeMan
    Commented Jul 14, 2015 at 13:09
  • $\begingroup$ Included the source :) $\endgroup$
    – Victor Juliet
    Commented Jul 14, 2015 at 13:20

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Radar waves are sent back to the radar by two mechanisms, reflection and diffraction. Opening a bay door may present a surface that is angled such that waves will be reflected to the radar. As for diffraction, low observable (stealth) aircraft are designed so that a radar wave encountering the aircraft's skin will travel along the skin and be gradually absorbed. Opening the bay doors creates a break in the skin, causing waves on the skin to diffract. Some of the radar energy is returned to the sender.

For an amateur looking at "stealth" diffraction is often not accounted for.

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In addition to creating a surface that could better reflect radar and breaks that can diffract the radar, the interior of the bomb bay is exposed. The exterior skin of stealth aircraft is specially treated to reduce radar returns, so the untreated interior of the weapons bay and bay doors will also create a much larger radar return than even the exterior surfaces of the open doors.

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    $\begingroup$ Assuming, of course, that the bomb bay interior is not treated in radar absorbing material. Not saying that any current stealth bombers are or should be, just that it could be done. $\endgroup$
    – FreeMan
    Commented Jul 14, 2015 at 15:39
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    $\begingroup$ @FreeMan - with the associated cost of added weight of the extra layers, which would only be necessary when the doors opened and which would mitigate a tertiary effect on radar signature with the bays open. $\endgroup$
    – KeithS
    Commented Jul 14, 2015 at 20:27
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    $\begingroup$ @KeithS … and only help after the weapon has been released. The biggest RCS contribution is most likely the weapon, not the bay interior. And with weapon release the aircraft will give away its position anyway. $\endgroup$
    – Peter Kämpf
    Commented Jul 14, 2015 at 20:34
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The radar cross section (RCS) of an object does not depend on its size, but on the number and orientation of its edges. Opening weapon bays will break up what was a single surface before into several, and the need to align the doors with airflow will create a new edge orientation which will result in a new reflection spike where none was before.

Depending on the direction at which the radar is looking at the aircraft and the RCS of the plane with closed doors, the difference is between zero in the direction of the main reflection lobes and several orders of magnitude in the new edge direction, namely sideways.

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