Electric fan to get ramjet airborne?

[rhb]

TL;DR Summary

Use of electric fan to run a ramjet until it reaches running speed.

Does anyone know of prior work on the use of an electric fan to supply input air to a ramjet until the air speed is sufficient (~0.2 M) for normal operation?

It's obviously possible, but might not be practical. The objective being the simplicity of a pulse jet without the vibration issues.

 

[rhb]

FWIW I've just looked at some "related" threads suggested by the forum program. Gad!

I've been reading texts by Hesse and Mumford, Coates and some others for a couple of weeks. While jet propulsion and design is a new topic for me, I have a 5000+ volume technical library and the education to go with it. Just in geology and geophysics, not engineering. Aside from a full machine shop, I've got a *very* complete electronics lab.

BTW The 6th ed of Anderson's "Fundamentals of Aerodynamics" proved ot be just what I wanted. I alos picked up Designing Unmanned Aircaft Systems" by Gundlach and "Subsonic Combustion Ramjet Design" by Ingenito. The latter is *not* very good. In fact, when I tried to return it, Amazon gave it to me. Go figure! I'm waiting for "Combustion Processes in Propulsion: Control, Noise and Pulse Detonation" by Gabriel to arrive.

 

[boneh3ad]

I've never heard of a ramjet that will operate at $M\approx 0.2$. Generally, I think you need something more like $M\geq 0.5$, and really they are terribly inefficient until $M\approx 2$, at which point they surpass a standard jet engine in performance.

So I would suggest that, no, the use of an electric fan to get up to speed for a ramjet is not feasible and likely not even possible. Even if you could, what would you do with the "fan," which would have a very large frontal area (i.e., drag) when you get up to higher speeds?

 

[Benjies]

I've never heard of a ramjet that will operate at $M\approx 0.2$. Generally, I think you need something more like $M\geq 0.5$, and really they are terribly inefficient until $M\approx 2$, at which point they surpass a standard jet engine in performance.

So I would suggest that, no, the use of an electric fan to get up to speed for a ramjet is not feasible and likely not even possible. Even if you could, what would you do with the "fan," which would have a very large frontal area (i.e., drag) when you get up to higher speeds?

I'm looking at the pressure ratio of Mach 1.5 inlets to be less than 2- I've never heard of a subsonic Ramjet. Am I missing a piece of history here with the ramjet? I'd thought it had to be supersonic.

Source on inlet pressure ratio: https://apps.dtic.mil/sti/pdfs/ADA596056.pdf

 

[boneh3ad]

I'm looking at the pressure ratio of Mach 1.5 inlets to be less than 2- I've never heard of a subsonic Ramjet. Am I missing a piece of history here with the ramjet? I'd thought it had to be supersonic.

Source on inlet pressure ratio: https://apps.dtic.mil/sti/pdfs/ADA596056.pdf

I believe they technically work at high subsonic Mach numbers but they're so inefficient no one ever actually does that. They're always rocket or turbojet boosted up to something around Mach 2 or 3 and then they switch over to ramjet mode.

 

[rhb]

"Usually a Mach number of at least 0.2 is required for any operation at all..."
from "Jet Propulsion", Hesse & Mumford, 2nd ed. p. 28

Ramjets were extensively used in early cruise missiles which were launched by rockets. The efficiency and thrust at M < 1 are horrible as shown in the attached figures from "Aerothermodynamics of Gas Turbine and Rocket Propulsion", Coates, pp 134-135.

I have in mind sliding shutters that open the ram inlet and seal off the fan once up to operating speed. The thermodynamic obstacles to the concept are obviously quite severe and would require a substantial engineering effort to produce. It is not something I have any interest in attempting to build. Even if I built it, I have no place to test such a thing. My interest is simply "can it be done?" as an alternative to pulse jet propulsion. Practicality depends upon the thrust from a ramjet exceeding the thrust from the fan mode operation at the transition to ramjet operation.

I'm interested enough to write a program to do ideal operation calculations to determine how big a fan is required to bring such a contraption up to starting speed and accelerate to 0.7 M or faster, determine the total weight-thrust relationships, plausible total weight, etc. An almost, but not entirely, idle speculative exercise driven by technical curiosity.

Yeah, I ask myself weird questions sometimes. I'm a retired research scientist/programmer with an interest in pretty much anything physics related. I view such exercises as a good excuse to learn a little bit about some topic of which I am generally fairly ignorant. There have been many instances in which such follies paid off because I knew a little bit about something no one else at work knew anything about.

 

[boneh3ad]

I just don't understand the motivation here. An electric "fan" won't get you very high in speed so your ramjet will have to take over when it is still very inefficient. The ramjet itself is going to require hydrocarbon fuels as well, so now you are committing to carrying both a battery bank and a traditional jet fuel on board. Further, you are going to hand off power to the ramjet while it is still so inefficient that it will likely suck up more fuel than just using a turbojet/fan would have, especially when factoring in the SWAP concerns of batteries.

This doesn't seem economical or "green," even if it's technically possible.

 

[rhb]

The design objective was a propulsion system which is both *very* cheap to manufacture and avoids the vibration of a pulse jet.

While suitable small turbines are now widely available, they are not cheap. For the unstated application the pulse jet is more suitable.

Finding the references cited in the document @Benjies linked are more than worth the reading I've been doing on this while suffering from covid. I was very bored. Aside from causing severe fatigue, Covid scrambles thought processes pretty thoroughly. Sufficiently so that even the ideal gas law was difficult to remember :-(

This started simply as something to do while I'm unable to do anything else. Now I've got a bunch of OXCART documents to read which is a much better use of my time. Mission accomplished.

 

[anorlunda]

Starting the ramjet is one thing, keeping it running is another. Even if you started it with the help of a fan, it would not stay running after removing the fan unless the velocity of the vehicle is large enough.

A ramjet with a full time fan is called a turbojet.

 

[berkeman]

The design objective was a propulsion system which is both *very* cheap to manufacture and avoids the vibration of a pulse jet.

Could you maybe just use a large weather baloon arrangement to lift it to altitude, then drop it like a bomb in a nose-down attitude so it gets to >0.5M before hitting the ground and fire up the ramjet then to climb back up? Seems lot cheaper (although kind of slow in the first ascent phase)...

 

[cjl]

Really, the cheapest and simplest option is to just rocket boost it, or carry it aloft with another plane. There's a reason ramjet powered drones and missiles nearly invariably use one of these two options.

 

[Rive]

I have in mind sliding shutters that open the ram inlet and seal off the fan once up to operating speed.

I think at low speed a simple LOx tank could provide more 'air' than the equivalent mass of battery.