Chinese Air-augmented liquid Rocket engine feasible ?

[Michel Van]

Chinese Company look into Air-augmented rocket engine
There idea is there use the Gas generator output to power Ramjet (see X post below)
They hope to get 40% increase in Performance

However is this feasible ?

https://twitter.com/raz_liu/status/1858345572635402524

[leovinus]

As it was studied in the 1960 in Europe and elsewhere, air-augmented rocket engines are probably feasible. However, it begs the question why this would be of interest now 60+ years later? Better materials? Just because they can?   

[Crispy]

Don't they already exist on some missiles? Which is where they make sense because the vehicle spends all of its flight in dense atmosphere at relatively low mach.

[Vonbrown1912]

The respective trajectories are a bit difficult to bring together no ? airbreathing engines like to hang around inside the atmosphere to get their air with the oxygen inside. Rocket engines are the exact opposite - the faster they are out of that unuseful atmosphere, the better. A rocket engine won't leave much time to a ramjet or scramjet to provide a benefit, this applies to time but also to velocity (Mach 6 for ramjets, mach 10 for scramjets).

[edzieba]

The three questions to answer are:

1) Does adding the tube around the gas-gen exhaust add more thrust than the tube weighs? If not, then the concept is dead on paper.

2) Does the total impulse added throughout the entire flight regime (surface to staging) exceed the extra impulse required to life the added tube? If not, then you're just adding dry mass.

3) If net thrust is positive, and net impulse is positive, is the added complexity worth it vs. adding (for example) an additional engine?

This may actually be viable, since the mechanism is extremely simple: add a tube. The downside is this only works on ascent, so once you bring recovery (and thus, retropropulsion) into the mix, your tube no longer does anything of value. Likewise, the longer your vehicle stays outside of the atmosphere the less benefit it can get, so upper stages and high staging velocity first stages also see less benefit - but low velocity staging is generally only preferable for recoverable stages.

It will probably work. But will it work well enough to be worth bothering with?

[Michel Van]

As it was studied in the 1960 in Europe and elsewhere, air-augmented rocket engines are probably feasible. However, it begs the question why this would be of interest now 60+ years later? Better materials? Just because they can?

i have not found any paper on this Ramjet design, expected to find something on NTRS,
Only air-breathing engine based on Ramjets/Scramjets for SSTO like 1980s NSP.   
but nothing on using Gasgenerator output for Ramjet

i want to know if this design produce more Thrust or ISP or Both ?

[Asteroza]

Wasn't the Gnom missile a solid fueled example?

Maybe NASA GTX concept?

Most air augmentation faces the weight of the ramjet inlet being a problem. Fixed means you probably have  2-3 mach range of use, variable is potentially quite heavy.

[leovinus]

As it was studied in the 1960 in Europe and elsewhere, air-augmented rocket engines are probably feasible. However, it begs the question why this would be of interest now 60+ years later? Better materials? Just because they can?

i have not found any paper on this Ramjet design, expected to find something on NTRS,
Only air-breathing engine based on Ramjets/Scramjets for SSTO like 1980s NSP.   
but nothing on using Gasgenerator output for Ramjet

i want to know if this design produce more Thrust or ISP or Both ?

Air-breathing engines for both vertical and horizontal takeoff were studied in the 1960s. A quick look at FOIAd NTRS reports from an earlier thread shows e.g.

19640046755 ,"Analytical and experimental investigation of advanced air-breathing engines mid-term technical report, 1 jul. 1962 - 31 dec. 1962", 1963,
19640049638 ,"Advanced air-breathing engine study second quarterly report, 6 jun. - 6 sep. 1963", 1964,
19640058760 ,"Air breathing booster second bimonthly progress report, 15 aug. - 1 oct. 1958", 1958,
19640059641 ,Recoverable booster study. vol. iii- airbreathing configuration studies final report, 1961,
19650060964 ,Initial Investigation of Candidate Airbreathing First Stages, 1965,
19650078377 ,Air-breathing propulsion for booster applications, 1961,

Even SR-181 had an air-breathing portion. Sometimes, this was reported as part of the "recoverable booster studies", see attached via AFHRA. Dan Sharp's book on British Secret Projects also has a lot on air-breathing trade-offs. And you are correct that is was considered for NASP as well in the 90s.

IIRC, one issue was that it seemed attractive to add (s)ramjets as a natural extension of existing jet engines. However, full development would be costly and therefore only economically viable if development of first-stage rocket boost engine and hypersonic transport engines could be combined somehow.  Also, additional air-breathing engines on the first stage booster adds weight. There was talk of "depressed trajectories" to stay in the atmosphere longer to make full use of air-breathing engines. What is worthwhile these days is an interesting question.

Another detailed discussion is e.g. here

A COMPARATIVE ANALYSIS OF SINGE-STATE-TO-ORBIT ROCKET AND AIR-BREATHING VEHICLES
Benjamin S. Orloff, Ensign, USN AFIT/GAE/ENY/06-J13
DEPARTMENT OF THE AIR FORCE AIR UNIVERSITY, 2006

with a quote from page 18-19 to provide one answer to your isp/thrust question.
 

2.5.1 Airbreathing Propulsion Advantages

Airbreathing propulsion’s most notable advantages over traditional rocket propulsion include higher specific impulse, decreased sensitivity to increases in inert mass and greater safety. By using the oxygen in the atmosphere, airbreathing propulsion does not need to carry oxidizer in the vehicle. The largest consequence of this is a larger specific impulse (Isp) than rockets. Specific impulse is a measurement of the amount of thrust produced for a given flow rate of propellant expelled. In rockets, this propellant is the sum of fuel and oxidizer, while in airbreathing engines, only the fuel is counted because the atmospheric oxygen is not considered onboard propellant. Higher specific impulses are analogous to higher efficiencies. A typical rocket will have an Isp between 300 and 500 seconds. As shown in Figure 8, airbreathing engines are capable of reaching specific impulses in excess of 7000 seconds. Because of their specific impulse, airbreathing engines can produce the same amount of thrust as a rocket engine but use less propellant. This drastically decreases the gross and inert mass of a vehicle.