Why aren't turbo fan/jet engines used as boosters?

[JucheJuiceMan]

Far less fuel is consumed thus less fuel is needed to be carried which outweighs the heavier turbojet/fan engine compared to liquid and solids.

For example largest CF-6 model thst weights 5-5.5 tons can output 310kN or around 30 tons of force while consuming 8kg of fuel per second for that output.

Having 4 of these as boosters and alone they could boost a 80 ton rocket to altitude of 10-15km.

There are parachutes that are used to soft land tanks 40-50 tons.

[Steel]

https://forum.kerbalspaceprogram.com/index.php?/topic/164529-jet-powered-stage/&

[DAL59]

Why use side boosters instead of dropping from a plane?  That exists, and is called stratolaunch.  

[Steel]

Long story short: turbofans will only go up to about Mach 1. Velocity is much more important than height for a rocket and so the jets are not really worth the extra weight and (more importantly) complexity

[DDE]

Compare the dV expended for vertical ascent and dV used in horizontal acceleration. Horizontal acceleration tends to occur way above the operating envelope of scramjets, let alone turbofans. And Kerbal makes airbreathing hypersonic dash a much more credible version than it is in real life.

1 hour ago, JucheJuiceMan said:

For example largest CF-6 model thst weights 5-5.5 tons can output 310kN or around 30 tons of force while consuming 8kg of fuel per second for that output.

In which mode? Unlike rockets, turbojet thrust is really unstable and dependent on velocity and altitude. Jumpjets can barely haul themselves off the tarmac, and turbofan missiles use a solid-rocket booster.

[wumpus]

1 hour ago, DAL59 said:

Why use side boosters instead of dropping from a plane?  That exists, and is called stratolaunch.  

Er, no.  The stratolaunch plane exists, but hasn't even taxied down the runway under its own power much less flown.

There *have* been 38 successful launches of the Pegasus rocket (dropped from either a B-52 or L1011 mothership).  There are plenty of reasons why this is suboptimal: the planes used by Pegasus aren't custom and thus can't "top off" cryogenic fuel, thus forcing use of solid boosters/motors, complete inability to scale up rockets without redesigning the entire plane, and general uselessness of the concept (fuel is cheap, planes are not).

Generally speaking, this works better for "small launch" such as Pegasus than larger launches, as the square/cube law means drag isn't proportional to mass.  The other important thing to remember is that Isp gains (thanks to lower air pressure) are likely to be more significant than avoiding drag losses.  I suspect that a high TWR initial stage would also have significant advantages (presumably a SRB, although you might want to be topping off cryogenic upper stages.  Using a solid first stage would make that a lot easier).

Simply strapping jet engines on to a booster kerbal style would still have considerable issues.  Jet (or high bypass turbofan) engines each have a limited range of speeds where they are most efficient and the whole point of a rocket is to pass through each as quickly (or at least efficiently) as possible.  I'd further assume that obtaining (presumably possible for DoD/NASA) the engines from the SR-71 would at least allow efficient use from ~mach 1 to ~mach 3.  While mach 3 might be significant delta-v (often half the fuel is burnt getting to mach 3), you still have to stage and recover those engines (these are far more valuable than reusable shuttle engines), and we have learned that parachutes don't appear to be a sufficient option (although helicopter capture might just work).

A falcon 9 is said to cost $100k in fuel to launch.  This is a wildly expensive way to save $50k per launch.  Too many things to go wrong, and I really wonder if you could do a heli-capture for less than $50k per snatch, once all the costs are accounted for.  I was a big believer in air breathers before KSP.  I still am, but I suspect that the tech is much further out than you would suspect and that we could very well get a space elevator before the numbers worked for air breathers.

[DAL59]

3 hours ago, wumpus said:

air breathers.

However, you could have skylon SSTOs.  

[Katten]

How about air augmented solid fuel boosters? Is that a thing?

[NSEP]

2 hours ago, Katten said:

How about air augmented solid fuel boosters? Is that a thing?

Not exactly air breathing but:

 

[sevenperforce]

5 hours ago, Katten said:

How about air augmented solid fuel boosters? Is that a thing?

Yes, absolutely. They are great. They have already flown, actually...though not as orbital rocket boosters. Rather, they are used for the acceleration stage of air-to-air missiles, because their low weight and high thrust/efficiency make them good for strapping to the underside of an airplane.

However, the added cost and complexity doesn't make them as useful for orbital boosters. Why add a few extra tons of metal and an expensively-developed inlet geometry when you can just make your strap-on booster a little bit bigger, or add another one altogether?

[shynung]

7 hours ago, Katten said:

How about air augmented solid fuel boosters? Is that a thing?

 

2 hours ago, sevenperforce said:

Yes, absolutely. They are great. They have already flown, actually...though not as orbital rocket boosters. Rather, they are used for the acceleration stage of air-to-air missiles, because their low weight and high thrust/efficiency make them good for strapping to the underside of an airplane.

However, the added cost and complexity doesn't make them as useful for orbital boosters. Why add a few extra tons of metal and an expensively-developed inlet geometry when you can just make your strap-on booster a little bit bigger, or add another one altogether?

Adding to this, missiles are much smaller than orbital rockets (and has to be, to fit into existing launchers or missile mounts, and these have maximum mass and dimensions limits). Squeezing ever more performance out of the same vehicle dimension means a high-efficiency engine. In the past, this is achieved by high-performance chemical propellants (which sometimes do horrible things to the human anatomy), but now, with advancements in materials and CAD technology, airbreathing turbojets or air-augmented solid rockets are the order of the day.

Though, those kinds of motors are typically reserved for anti-ship or cruise missiles, which flies for long distances at relatively low altitudes. Common air-to-air missiles such as the AIM-9 and AIM-120 still use solid motors.

[sevenperforce]

31 minutes ago, shynung said:

Adding to this, missiles are much smaller than orbital rockets (and has to be, to fit into existing launchers or missile mounts, and these have maximum mass and dimensions limits). Squeezing ever more performance out of the same vehicle dimension means a high-efficiency engine. In the past, this is achieved by high-performance chemical propellants (which sometimes do horrible things to the human anatomy), but now, with advancements in materials and CAD technology, airbreathing turbojets or air-augmented solid rockets are the order of the day.

Though, those kinds of motors are typically reserved for anti-ship or cruise missiles, which flies for long distances at relatively low altitudes. Common air-to-air missiles such as the AIM-9 and AIM-120 still use solid motors.

Longer-range A2A missiles intended for targets out of sight line are sometimes air-augmented, like the Meteor.

[wumpus]

14 minutes ago, sevenperforce said:

Longer-range A2A missiles intended for targets out of sight line are sometimes air-augmented, like the Meteor.

I'm pretty sure some full blown ramjets have been used for that.  With at least one downed aircraft.

17 hours ago, DAL59 said:

However, you could have skylon SSTOs.  

A SABRE engine should get to mach 6 (if it ever gets beyond powerpoint).  This gives you roughly the delta-v a reusable falcon 9 stage (they typically stage around mach 6 [although in space] if the booster is returning).  A Skylon SSTO would then have to deliver the Skylon orbiter into orbit (along with all SABRE equipment,wings, engine, and fuel tanks).  That's a lot of mass to have to accelerate ~7km/s.  While it is technically possible, I suspect most investors are either backing space-x or are afraid to compete with both Musk and Bezos.

Try installing Realism Overhaul and see some of the problems of spaceplanes (of course, you still miss most of the fun of maxQ and other issues).  I'd strongly suspect that a SABRE-powered rocket would still use two stages and that developing low cost reattachment after use makes wildly more sense than SSTO.

[Katten]

Aha, good points.

So, how about in ksp if you ship an air augmented booster as an ascent stage to a planet with oxygen, would that be a business case? Does any ksp planet have oxygen btw? Ive only been to duna really =)

[PB666]

On 10/30/2017 at 2:04 PM, Steel said:

Long story short: turbofans will only go up to about Mach 1. Velocity is much more important than height for a rocket and so the jets are not really worth the extra weight and (more importantly) complexity

Safely about M 0.95, higher and risk of cavitation.

 

Alright so the biggest ones are the PW4000-112, GE90 and RR Trent 8000 8105/15  that produce up 113,000 foot pounds of thrust (>500kN) at sea level. As you get over 8000 feet, although the air cools making jet engines more efficient, but the problem is the density drops. Unlike the engines on the 747 which can operate at Mach 0.93, the largest turbofans only operate to about Mach 0.9. ( GE90-115B is cabable of producing 569 kN and flying either 747 or 777 with a single engine).

Thats the biggest problem as the inlet density falls, so does fuel flow and so does thrust. It may move faster at higher altitudes but the volume is much less. 

So lets define space flight as anything with an orbit higher than 130,000 meters (80 miles). This a =  6,501,000 meters. if we use the equation u/2a to define SKE = 30656856 J/Kg . SPE = u/s - u/a = 1251104 J/Kg. The total energy added to an orbit is (if could we could warp from the ground to orbit) is  31907959. OK so we can see the problem with out too much more insight. The energy added to altitude (1.2E6 is much less than the energy added orbital energy 31.6E6. And the turbofan is not going to produce alot of thust above 15000 feet ~5000 meters). As stated above it will not move a vehicle more than say 0.95 before special engines are need. this is < 273 m/s. So our best case situation is that we get  0.086E6 j/kg of energy and we need 31.9E6 j/kg.

So that equals to 50,000 kg of lift. So now lets ask the question, how much weight do we need to life and how far.

So lets say what if. What if Space X built a launch facility Given that these powerhousse weight 8700 kg, and assuming we had an additional 3000 kg for each engine in support mountings. Could we build a launch pad and lift it to a certain altitude and launch. Lets see.

Space X rocket weighs. 549,054, lets just assume we only need a TWR of 1.1 given the efficiency of the engines and all we want to do is lift.

549054 x 9.81 = 5386220 N required. Each Engine needs 114777N just to lift itself and it and the rocket support. That means that each engine would have a net 454223N of thrust. So to get a rocket off the ground how many of these engines, running at their full intesity would you need? The answer is 12. So basically you would have a Ring with a grating and two supporting towers for the rocket and 12 rocket engines on the ring.

Would this actually work, not exactly. Vertical launching jet aircraft have the problem of exhaust reuptake into the jets, the hot CO2 gases markedely reduce compression. Instead of one harrier jet or a JSF sitting on the lauch site you have 12 of the largest jet engines built filling the entire area with hot waste gas.

The next problem is Load. Jet engines like face velocities. At 1g*mass of thrust the face velocity is all induction, the best results at sea level are with face velocities of 100 m/s. THe problem is this is not true airspeed but indicated airspeed. As altitude drops the IAS to TAS ratio drops, at 45000 feet its about 1/3. This is around coffins corner for the engine. The reason is that Mach limits faster flight and thrust generated at such a speed is insufficient to lift the aircraft when drag is factored in. At the IAS above a few thousand  meters the face pressure is dropping. So lets argue that we need 100 m/s by 5000 meters. If we assume constant acceleration (we cant) this gives a required net acceleration of 1 m/s. Therefore the amount of Net thrust needed is at least 5,935,000N requiring 13 jet engines (14 for balance).

The next problem is balance and coordination of such a structure keeping in from turning in wind, even turning it to convert some of the vertical motion into the badly needed horizontal motion. Finally you reach a point in which you need to fire your main engine, how do you protect your 14 high dollar jet engines from the heat coming from a Kerosene/Air explosion and point blank range. If you want to recover those engines you need to control the engines descent back to the launch site.

Here is the cost of those 14 jet engines. At a cost of 15,000,000 US dollars the 14 jet engines would run 210 million dollars not considering the cost of the rest of the launch pad, You are probably taking about a half of  a billion dollar launch pad that may go Poof on its first launch.

 

[kerbiloid]

Just to mention:  http://www.astronautix.com/n/n1-mok.html

Spoiler

 

7 hours ago, Katten said:

in ksp if you ship an air augmented booster as an ascent stage to a planet with oxygen, would that be a business case?

As KSP orbital speeds arre several times lower that IRL, but atmosphere thiockness is the same, in KSP it can make a lesser sense. (At least, I did so in earlier KSP versions).
But irl the same additional delta-V means much less compared to the required 7800 m/s instead of 2600.
 

[p1t1o]

On 10/31/2017 at 8:44 AM, Katten said:

How about air augmented solid fuel boosters? Is that a thing?

BishBashBosh got your back

http://www.astronautix.com/g/gnom.html

A vintage Russian ICBM. Cancelled before entering service.

I presume these things did not become common because simplicity trumps the extra performance, but Im not sure.

[sevenperforce]

2 minutes ago, p1t1o said:

BishBashBosh got your back

http://www.astronautix.com/g/gnom.html

A vintage Russian ICBM. Cancelled before entering service.

I presume these things did not become common because simplicity trumps the extra performance, but Im not sure.

The GNOM was a fantastic little cruise missile that vastly outperformed its conventional counterparts but didn't enter service because of USSR politics.

Turbojets don't make much sense for orbital flight; air augmentation does. One of my pet designs is a stage-and-a-half SSTO similar to the Saturn 1-D concept, where you have a single tank feeding a single engine plus multiple engines on a jettisonable skirt. The skirt can be designed with an air-augmentation bypass without mass penalty, boosting efficiency during the initial climb.

16 hours ago, Katten said:

So, how about in ksp if you ship an air augmented booster as an ascent stage to a planet with oxygen, would that be a business case? Does any ksp planet have oxygen btw? Ive only been to duna really =)

Laythe has oxygen, so any jet engine will work there. But the nice thing about an air-augmentation approach is that you don't actually have to have oxygen. Atmospheric gas goes into the intake, is heated and expanded by mixing with the exhaust, and boosts thrust "for free", at least, when you're going at less than Mach 3 or 4. Up around Mach 5 you're going so fast that it's hard to speed up the airflow any more than your airspeed. No need for combustion.

If your atmosphere does have oxygen, you can always boost your efficiency a bit by running your engine fuel-rich so the oxygen has something to reheat with, but the gains are low.

I would love to see a "boost duct" part in KSP that would allow you to combine any rocket engine with any intake to boost efficiency in this way without requiring an oxygen atmosphere.

[p1t1o]

13 minutes ago, sevenperforce said:

The GNOM was a fantastic little cruise missile that vastly outperformed its conventional counterparts but didn't enter service because of USSR politics.

There must be a better reason that the technology hasnt been revisited since though?

[sevenperforce]

3 minutes ago, p1t1o said:

There must be a better reason that the technology hasnt been revisited since though?

No real need for it.

The GNOM was intended to minimize the size of a heavy-payload ICBM so it could fit on a mobile launcher and thus preserve retaliation ability after a US nuclear first strike. It was something like half the size of its conventional counterpart, so it worked really well for that.

The US never developed one of these because its nuclear-armed subs were even more invulnerable to a Soviet first strike and could get away with a smaller launcher by virtue of proximity. After the end of the Cold War there obviously was no more concern about minimizing the physical size of ICBMs because there was no imminent threat.

For orbital aspirations, the niche for a ducted rocket is hard to find. The in-atmosphere acceleration phase of an orbital booster is much shorter and you push out of the thick lower air very fast, so the advantage of air augmentation is more limited. You'd want to shed the shroud as soon as it was no longer useful, but that means a VERY early staging event (the Falcon 9 isn't even halfway through the first-stage burn before it clears the useful portion of the atmosphere, and it's already dV-weighted toward the upper stage). Plus, a shroud scales poorly with vehicle size, making it ill-suited for larger launch vehicles.

In most cases, it's far cheaper to simply make your rocket a little bigger than it is to design an air augmentation shroud, especially for expendables.

One potential use for air augmentation might be improved air-launch. With air-launch, you definitely want your rocket to be as small as possible for the sake of your carrier plane, so cutting down on mass and size with a shroud is useful. Plus, you're already moving horizontally with significant airspeed at launch, so you get more benefit out of your shroud, and you can design it for a much smaller range of air pressures and densities.

 

[wumpus]

2 minutes ago, sevenperforce said:

One potential use for air augmentation might be improved air-launch. With air-launch, you definitely want your rocket to be as small as possible for the sake of your carrier plane, so cutting down on mass and size with a shroud is useful. Plus, you're already moving horizontally with significant airspeed at launch, so you get more benefit out of your shroud, and you can design it for a much smaller range of air pressures and densities.

I'm fairly surprised this was never developed for sub-launch.  Presumably the Navy was confident in subs to hide sufficiently close to their targets and that the range was "good enough".

As for air-launch, just this Halloween Orbital showed that it is cheaper to use surplus ICBMs to launch your "air launch" vehicles than to redesign them with air augmentation and air launch (Minotaur C is essentially a Pegasus on top of a Peacekeeper booster).

It really looks to me like "nearly all the benefits of SABRE (~2km/s delta-v), but much less cost".  As much as I'd love to see it on monsters like the BFR and New Glenn, I' fairly sure that "much less cost" isn't remotely "less" enough.

[winged]

On 30.10.2017 at 7:22 PM, JucheJuiceMan said:

Having 4 of these as boosters and alone they could boost a 80 ton rocket to altitude of 10-15km.

10-15 km? Not gonna happen because jet engines siginificantly lose thrust with altitude - so after just few kilometers TWR will drop below 1.

Why bother with this if you can launch a rocket from airplane at 10-12 km of altitude and 250 m/s of horizontal velocity?

Or even better: you can build reusable rocket stage.

[sevenperforce]

2 minutes ago, wumpus said:

I'm fairly surprised this was never developed for sub-launch.  Presumably the Navy was confident in subs to hide sufficiently close to their targets and that the range was "good enough".

Nuclear stealth subs can camp out just off the coast, surface and fire nukes, and sink again in a matter of minutes. The target gets no warning whatsoever because the dV and flight time is so low.

2 minutes ago, wumpus said:

As for air-launch, just this Halloween Orbital showed that it is cheaper to use surplus ICBMs to launch your "air launch" vehicles than to redesign them with air augmentation and air launch (Minotaur C is essentially a Pegasus on top of a Peacekeeper booster).

The air-augmentation advantage for air-launch would be if you can get an orbital launcher small enough that you could load several of them into a conventional heavy bomber with no major changes to the aircraft itself.

2 minutes ago, wumpus said:

It really looks to me like "nearly all the benefits of SABRE (~2km/s delta-v), but much less cost".  As much as I'd love to see it on monsters like the BFR and New Glenn, I' fairly sure that "much less cost" isn't remotely "less" enough.

It definitely has all the benefits of SABRE with less cost. In fact, if you took a vehicle half the size of Skylon, filled it up with kerolox, and slapped a pair of Merlins on it with air augmentation shrouds, it could basically do everything Skylon can do. The shrouds even double as altitude-compensation nozzles.

But like I said, it doesn't scale well at all. The length of the shroud is a function of the diameter of the shroud, so mass growth gets pretty bad pretty quick.

[wumpus]

11 minutes ago, sevenperforce said:

But like I said, it doesn't scale well at all. The length of the shroud is a function of the diameter of the shroud, so mass growth gets pretty bad pretty quick.

I suspect that will be the reason it never takes off (unless they are side boosters).  This screams of something you want more in larger boosters where reduction of total mass (and related structural issues) is a big thing.  On the other hand, you would think microsat launchers would be desperate for any more efficiency  through the atmosphere.

[sevenperforce]

1 hour ago, wumpus said:

I suspect that will be the reason it never takes off (unless they are side boosters).  This screams of something you want more in larger boosters where reduction of total mass (and related structural issues) is a big thing.  On the other hand, you would think microsat launchers would be desperate for any more efficiency  through the atmosphere.

And microsat launchers can always just slap on a COTS SRB for extra dV.