# Jet engine propulsed rocket?

### Help Support The Rocketry Forum:

#### sross19

##### Member
Hi,

I don't know if this has been done or attempted before but I was thinking about building a rocket propulsed by a jet engine. I know there are already model jet engines on the market used in model planes... but I was more thinking about building myself the jet engine so that it can be built directly into the rocket so that the rocket itself would be the center of the jet engine, the compression chamber would be surrounding it and the combustion chamber would be at the end of the rocket where the solid motor is...

Here's what my rocket would look like:

Code:
    /\
||    <--- Nose cone, payload and recovery system
||
|-||-|
|-||-|  <--- Compression Chamber
|-||-|
\ || /
/|!!|\   <--- Solid motor (center)
/_|\/|_\  <--- Fins (left & right) and combustion chamber (center)
The rocket at the center could be detachable from the jet engine so that I could install the solid motor and prepare the rocket for launch...

So what do you think? Would it be feasible? Would it perform better than a standard rocket or would the jet engine add too much drag?

Thanks

Stéphane Ross

#### Stones

##### Well-Known Member
So...is there a jet engine AND a solid (BP/AP) motor involved or am I not reading this right?

#### MarkABrown

##### Well-Known Member
Originally posted by sross19
So what do you think? Would it be feasible? Would it perform better than a standard rocket or would the jet engine add too much drag?
It could work but I seriously doubt that it would perform even close to a conventional rocket design. With your jet engine in there, the added weight alone will kill your performance.

#### sross19

##### Member
Originally posted by Stones
So...is there a jet engine AND a solid (BP/AP) motor involved or am I not reading this right?
Well what I mean is that the jet engine would use a solid propellant ( a regular model rocket solid motor ) to ignit the compressed air in the combustion chamber... instead of using kerosene like in normal jet engines... so its just a sort of modified jet engine... in fact all i'm doing in my design is compress the air and bring it at the exhaust of my solid motor to have a greater boost ( in theory hehe )

It could work but I seriously doubt that it would perform even close to a conventional rocket design. With your jet engine in there, the added weight alone will kill your performance.
Yeah probably, but i'll have to get more info on how to build a jet engine to see how could it be achieved by adding the less weight possible.

Maybe by modifying a bit my design I can have less drag and weight added...

EDIT:

Also one thing to consider is that once my solid motor will stop to burn, I think that the jet engine will slow the rocket and alter a lot the coast phase ( since the compressed air is no more ignited, the jet engine will probably act like a parachute ) ... to solve this problem I could do a second stage to my rocket... once the solid engine in my jet engine stop to burn, it activates the second stage and the jet engine is detached from the rocket so that it does not slow down the rocket... but like I earlier said that would only be useful if the jet engine can achieve better performance than a regular rocket ( although it would still be very interesting to build such a rocket )

#### wwattles

##### Well-Known Member
*begin lecture on modern airplane propulsion systems*

Basically what you're proposing is a modified ramjet, where the solid motor provides the fuel.

The key to jet engines is that the compression phase boosts air pressure (thus temperature) to the point where the fuel injected will burn easily. To do so for modern turbojet/turbofan/turboprop engines requires a multi-stage compressor fan system to boost the pressure sufficiently beyond what normal ram pressure would be. They work best at subsonic speeds.

Ramjets, on the other hand, use airspeed alone to provide the pressure, which ignites the fuel on its own, which then propels the airframe forward, with enough thrust to overcome drag/gravity AND sustain the ram air combustion. Typically they operate at high supersonic speeds. (several times higher than Mach 1)

*end lecture on propulsion systems*

The problems with using these in a rocket are manifold:
1) You'd need to either use metal (a no-no) or high-temp ceramic (very hard to create with the right tolerances)
2) You'd be adding so much weight to the rear of your rocket that you'd end up with a model that is impossibly hard to get balanced. The more weight you add to the rear, the more you'd have to add to the nose for stability. The more you add to the nose, the more you have to add to the rear to propel it! The cycle repeats until you've gotten way beyond what you'll be able to build easily.
3) solid motors don't produce propulsion the same as liquid fuels do. Solids rely on burning a propellant which is expelled, using the "equal and opposite reaction" principle. Liquids superheat the air, causing it to expand, thus creating the overpressurization inside, which results in the pressures inside and outside seeking to equalize, which then forces the inside gasses out, providing the "equal and opposite reaction".
4) The design you illustrated would only serve to cool the rocket's exhausts and quite possibly increase the Krushnik effect inside the combustion area, reducing thrust, not increase it.

All in all, an interesting idea, but unfortunately, one which works better for larger scale systems (like modern cruise missiles) and liquid propellants.

WW

#### sross19

##### Member
Thanks a lot for that info wwattles!

I see I still have a lot to learn hehe... I didn't know liquid propellant had such a role in jet engines...

Although if i used a liquid engine and a fan system to compress the air like you mentionned for the jet engines would it be a viable system? Also would it be possible to build such a design with a liquid motor certified by NAR or this would enter the realm of amateur rocketry?

#### shrox

##### Well-Known Member
Look up "pulse jet" on Google.

shrox

#### sross19

##### Member
I came across some interesting info about ramjet engine... I read the the minimum speed requiered for a ramjet engine to work was around 400 mph (although it has low effiency at that speed )... my question is: is it possible to attain this speed with a model rocket? (I heard some model rocket could acchieve mach 1 so in this case a ramjet would work )... I'm asking this cause supposing I use a first stage solid motor to propel my rocket at mach 1 then I could switch to a 2nd stage ramjet engine afterward... what you think?

Look up "pulse jet" on Google.
I'll look for info on pulse jet, I think some have used such engine to propel model planes...

#### sross19

##### Member
I came accross this interesting page : https://www.ukrocketman.com/digital/pulsejets.shtml

It had info on pulsejets and ramjets for rocketry... although they said that the most efficient approach for model rockets was using what is called a ramrocket or ducted rocket

#### Justy

##### Well-Known Member
Look up the Jetex (Jet-X) Jetmaster -- a solid fuel pellet powered reloadable motor system from the 1950's -- and its "thrust augmenter tube" for an example of a mass produced ducted rocket. The thrust augmenter was funnel, wider than the motor itself, closing down to a narrow tube 4" long.

The best schematic I've found after a few minutes searching is actually the box art...

#### sross19

##### Member
cool!

https://jetex.home.att.net/motors.html#augmenter

It said that it could increase thrust from 20 to 30%... very nice indeed

I'll try to find more info on ducted rocket and try to come up with my own design

thanks for the info

#### Hospital_Rocket

##### Well-Known Member
Would someone please tell the Navy that shore duty doesn't mean I should be out at sea for a month?
Deal, Then you can try my two shore stations

Port Clarence, Alaska - About equidistant between Nome and the old USSR.

Marcus Island - 2000 miles from anywhere!

A