# Re-engineering a turbo-ramjet

**URL:** <https://boards.straightdope.com/t/re-engineering-a-turbo-ramjet/978755>\
**Category:** Factual Questions\
**Created:** [January 25, 2023, 1:11am UTC](https://boards.straightdope.com/t/re-engineering-a-turbo-ramjet/978755 "2023-01-25T01:11:59Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\
**Post date:** [January 25, 2023, 1:11am UTC](https://boards.straightdope.com/t/re-engineering-a-turbo-ramjet/978755/1 "2023-01-25T01:11:59Z")

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Look at this image of a Pratt and Whitney J58 turbo-ramjet:  
[63ae17a2b5962683ab44f80b9934b6d5.jpg (1180×831) (pinimg.com)](https://i.pinimg.com/originals/63/ae/17/63ae17a2b5962683ab44f80b9934b6d5.jpg)

Specifically, the large tubes which bypass the compressor and turbine sections during supersonic velocities.

The thing about ramjet operation is…it only works at velocities above the speed of sound: supersonic air being “rammed” into the intake is compressed/heated by the speed of the air so it can sustain combustion.

What if we preheat these bypass tubes?

Re-channeling exhaust thrust, electric elements, nuclear, magic unicorn farts…  
could artificially heating the incoming charge of air sustain a ramjet light-off lower limit at a more feasible sub-sonic velocity?

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**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\
**Post date:** [January 25, 2023, 1:40am UTC](https://boards.straightdope.com/t/re-engineering-a-turbo-ramjet/978755/2 "2023-01-25T01:40:59Z")

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Heat is not the point. You need the ram pressure to raise the pressure in the combustion chamber and to provide a “cork” that make the results of combustion expansion go out the back producing thrust, not out the front producing anti-thrust.

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**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\
**Post date:** [January 25, 2023, 5:50am UTC](https://boards.straightdope.com/t/re-engineering-a-turbo-ramjet/978755/3 "2023-01-25T05:50:55Z")

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Indeed.

The ram effect doesn’t replace the combustor; it replaces the compressor stage.