Is it time for Booster Engines?

[twistedenginestarter]

In the world of automobiles, engine stop/start appeared now and again but didn't catch on. Now fuel is so expensive it's regularly on even upmarket vehicles. For aircraft should we be starting to think about more complex configurations?

Booster engines have appeared now and again but I don't recall anything in the western civil arena since the Trident 3. Is there now a case for having engines used for only part of the flight?

Take-off (particularly with engine failure) is a big deteriminent on what engines you have although it is a tiny portion of the whole flight. The Trident booster was to reduce runway requirements, not for example to sustain/enhance cruising speeds or levels.

I know they didn't install a booster out of choice, but maybe with fuel costs as they are now, it would make sense to have smaller, less powerful main engines supplemented by boosters. Remember the more engines you have for take-off the less power excess you need, and the less rudder moment you have to fly with.

[Intruder]

Where would you put such a booster engine? How would you offset the weight and volume penalty? Note that max cargo or passenger density is what the customer wants...

Early turbojets had a specific fuel consumption of around 1 lb/hr per lb thrust. Current high-bypass turbofans are closer to 0.3.

[spannersatcx]

engine stop/start appeared now and again but didn't catch on I think you may be mistaken on that!

[aviatorhi]

"Booster" engines have actually been applied in the past, JATOs on the 727 for instance, clearly it didn't catch on, for technical, logistical and safety reasons.

For instance... how do you "safely" abort with some rockets at full power attached to the side of your plane?

Sure you can jettison them, but then they pottentially go flying through a terminal full of people.

[cwatters]

I think the penalty of carrying engines that are more powerful than needed for cruise is small, compared to the penalty of carring extra boost engines that are only needed part of the time.

[DJ77]

What about inflight refueling for long haul flights ?

[Graybeard]

How about filling the wings with hydrogen, and the plane gets heavier as you burn it? Then you could land heavier than at takeoff.

Swissair reportedly shut down one engine at cruise in their DC-9s. It was not just flight idle, but complete shutdown. Wasn't the center engine shut down at cruise on the 727 with re-engined #1 & #3?

If you want greater efficiency, charge SLF by the pound, like normal cargo.

GB

[lomapaseo]

If you want greater efficiency, charge SLF by the pound, like normal cargo.



Yes, but with a substantial discount for the Self Loading part

[barit1]

In a typical flight cycle of a modern jet, operating from typical runways, the OEI takeoff case is NOT the most demanding operating point. Top of climb calls for more "oomph" than takeoff. This assertion is supported by the fact that a great majority of takeoffs are made either derated, or flex (ATM), or some combination of the two.

But going back to JATO - I once worked around a DC-3 fleet with JATO installed. They weren't fired until after V1.

[aviatorhi]

Shutting one engine down on a two engine airplane to conserve fuel with revenue on board isn't the most intelligent thing in the world. Though I did once overhear a rather humorous exchange about just such a thing when a Twin Otter was holding over some airport in the bush and wanted to stay overhead for a bit longer than he could have running both engines.

No sane operator would have shut an engine down on a DC-9 to do this and on the 727 we definetly do not do that.

[galaxy flyer]

D177

One thing aerial refueling is NOT, is fuel efficient. First, you have to account for the fuel burned by the tanker. Second, the receiver flies the entire segment at a higher average gross weight--it's burn is thus higher. Lastly, their would be the wasted time descending, RV'ing with the tanker burning more fuel than in cruise and wasting time, climbing back up to cruise level.

GF

[18-Wheeler]

How about just using the right aircraft for the job?
Modern engines are more than good enough to provide enough power and also good enough on fuel to give excellent range, far more than you'd ever want to sit in a passenger seat for.

[keitaidenwa]

I think it would be "challenging" to fit JATO to an airliner and still pass stage 4 noise regulation...

[twistedenginestarter]

You don't need wierd rockets.

An Airbus uses a V2500 - 27,000 lbs thrust for 2,400 Kg. You could have two main engines - CF24s at 20,000 lbs thrust - and two retractable AE3001s (9,400 lbs) for the same weight.

As a simple solution, put them at the back, around the apu (would you still need one?). I'm would guess there's enough room (although you'd have to change the tail surfaces and adjust the fuselage length to maintain CG).

Hybrid cars which use different engines for different phases, were ridiculous once for reasons of cost, weight, complexity, reliablity etc. Now things have changed completely.

[aviatorhi]

Planes and cars are different animals (I would hope this goes without saying).

Also, since the topic of cars has been introduced a Prius is rated to carry somewhere around an 840 lb payload, which is incredibly low, for comparison a Mini Cooper can do almost 1000. The whole idea of carrying engines around for the sake of "boosters" is just nonsensical, if there was a way to incorporate one as an APU it MIGHT be worth looking at. Even then, the additional costs associated with an idea like that in civil aviation a) don't make any financial sense, and b) have not made financial or operational sense in the past.

[twistedenginestarter]

aviatorhi

As I explained you are not carrying any extra weight around. The weight of four engines is the same as two for the same thrust.

It probably didn't make financial sense in the past (hence the move from A340 to B777) but my whole question is does it make sense now, with fuel prices much, much higher? Does that change the equation?

barit1

If you don't need full thrust for take-off then just use two engines (lower noise?), and pull your other two out at altitude.

[GlueBall]

If you don't need full thrust for take-off then just use two engines (lower noise?), and pull your other two out at altitude.

Are you a pilot?

If you fly jets you would know that you're not saving fuel by shutting down engines in flight. Think of the drag generated by huge wind milling fans....like unfeathered, wind milling propellers.

Ever wonder why the British Airways B747-436 with an engine failure after take off at LAX didn't reach destination LHR? . . . Because of excess fuel burn on 3 engines! :ooh:

[aviatorhi]

The reason aircraft had 3+ engines in the past was because it couldn't be done with 2. We're not just talking about fuel efficiency and reliability here, we're talking about engine failure at the most critical phase of flight, taking a 707 or 727 in the air and losing more than one power plant past V1 is a good reason to put it back on the ground and take your chances if you are anywhere near MTOW.

You're talking about adding a lot of complexity to something that should be simple. The only aircraft I know of that are currently in service with the concept of booster engines you put forth are certain models of the C-119 and C-123, which, respectively, have 1 and 2 100LL powered jet engines to assist in short field takeoffs, as well as the AN-24, some of which utilize around 2000 lbs of thrust generated by the APU to assist with takeoffs... why? Because they couldn't meet performance requirements with the engines originally installed in the first place, not because it saved fuel or was easier to operate.

Also, you mention "you are not carrying any extra weight around", you're forgetting about the added weight of systems for running those two additional engines, not to mention additional maintenance, checks, inspections etc.

As far as the V2500 and CF34 (I assume that's what you meant) why would I (or anyone else) want to add the complexities of what you're suggesting when you can re-engine the aircraft with a later V2500 (like the V2533-A5) and have all the gains in power, reduction in fuel consumption and none of the headaches?

Finally, the reason this concept isn't going anywhere isn't because of what I or anyone else do or don't think, it's simply because all the aircraft on the drawing board to those nearing certifications, which are expected to be manufactures for the next 20-40 years are not employing this concept.

[barit1]

If you don't need full thrust for take-off then just use two engines (lower noise?), and pull your other two out at altitude.

This statement defines who we are dealing with. :p

[twistedenginestarter]

Hi Glueball

I hope you're not a pilot. It would be dangerous commanding a big jet if you hear voices in your ears. I never mentioned having engines windmilling in the airflow. Next time you're on Flight Simulator and you retract the undercarriage, have a look at the external view and you'll notice they're no longer sticking out. (Not my idea by the way - somebody thought of it in 1876. But I'm catching up fast)

You maybe right about the 747 but then again it might also be because it had to fly at a lower altitude than planned.

[barit1]

You maybe right about the 747 but then again it might also be because it had to fly at a lower altitude than planned.

Exactly. That's the natural consequence of OEI cruise, but I'm sure that's no surprise to you. :rolleyes:

[galaxy flyer]

Actually, the reason was NOT the fuel burn, it was the crew's lack of understanding the fuel feed and pump arrangement that lead to land short rather than continue to LHR. In fact, they would have made it, training has cleared up the issue.

GF

[barit1]

True - I had forgotten that detail. But the suboptimum cruise altitude didn't help any.

And I'm still surprised BA main base didn't advise them on the matter.

[barit1]

http://www.dilbert.com/dyn/str_strip/000000000/00000000/0000000/100000/20000/8000/000/128088/128088.strip.gif

[autoflight]

Advantage of 4 engines. Engine failure on take-off only requires consideration of loss of 25% power compared to 50% loss for 2 engine aircraft. The principle was that this allowed a higher take-off weight. I am sure there are many other issues, like 4 smaller underslung engines allowed shorter undercarriage, but offset by outboard engines further to rear on swept wing aircraft during take-off rotation and also max bank on touchdown.

JATO interesting except that transport, storage, handling, training and time expiry were added expense for limited advantage.

[lomapaseo]

[-quote] Advantage of 4 engines. Engine failure on take-off only requires consideration of loss of 25% power compared to 50% loss for 2 engine aircraft. The principle was that this allowed a higher take-off weight. [/quote]

Need numbers to support that argument. The take off weight is a determined at ceritification assuming a single engine failure be it one out of two or one out of 10. You get what you buy. If you want more weight buy a bigger certified two engine or 4 engine aircraft not more engines

[bearfoil]

Why carry BOOSTERS?

Bury them under the centerline, with 1km cables. The NAVY does it.

[keesje]

I put a small tail engine in a concept multirole MPA in the militairy forum. For take-off performance/ engine redundancy during heavy take-offs. More optimized main engines for long range/slower speeds.

http://i191.photobucket.com/albums/z160/keesje_pics/EuroMPAstudy_3.jpg?t=1311463771

I could be far more efficient and flexible then 4 engined aircraft like P3C & Nimrod and still be smaller & cheaper..
http://i191.photobucket.com/albums/z160/keesje_pics/orionnewconceptmerlin.jpg?t=1311463937

http://i191.photobucket.com/albums/z160/keesje_pics/EuroMultirolepatrolaircraftconceptkeesje.jpg?t=1311464000

[barit1]

I don't get it. A turboprop will always be better at takeoff than a jet or fan, in terms of thrust SFC and thrust/weight. This is because it moves a LOT more air.

[aviatorhi]

@lomapaseo

When comparing a 2, 3 or 4 engine aircraft, in which the total thrust from all engines is the same (2x50,000 lbs, 3x33,333.3 lbs and 4x25,000 lbs) the 4 engine aircraft will have the best performance (and therefore attain a higher performance certification), followed by the 3 holer and the twin. It's part of why my 727 is not performance limited in places where the competitions 737s are (as such we routinely take excess baggage and freight for the competition).

@lomapaseo

I admit that is a sleek looking aircraft, but 2 problems; (1) might want to rethink placement of the cargo door, (2) how much lost payload/additional weight etc. does putting the jet into the fuselage "cost", and how much does running it on the jet only and carrying the dead weight of the turboprops "cost"?

@barit1

There's too many engines to make effective comparison, but let's think small, a JT8D-17R vs. a PT6-67D, which do you think is gonna make more thrust at ANY altitude?

[barit1]

Not an apple-apples comparison, aviatorhi.

Pick engines with similar static thrust. e.g. For 10K lb static, the turboprop (including gears and prop) will be lighter and burn a lot less fuel than a turbofan. It's only when you get to m0.6 or so that the turbofan surpasses the turboprop.

[twistedenginestarter]

If you have more engines you have more capital and maintenance cost and more compexity in coping with technical faults in service. However the world has changed with a dramatic increase in fuel price, so we should expect to see some significant change in engineering solutions, yet 2021's 737 looks almost identical to one from 30 years ago. That doesn't seem a proportionate response to me. Someone should be coming up with more radical ideas. I know the manufacturers have their Ceeses sketching interesting concepts but they don't seem to be moving towards realizing these ideas.

[Vertical Speed]

For some years I flew the Bae 146 (Jumbolino) A strange little beast- a single engine failure didn't even require a different checklist! AND the range suddenly increased significantly. A double engine failure was a more serious situation and did happen on a few occasions, although not to me fortunately. The biggest problem with the little ALF502 engines was that they failed rather more frequently than the industry norms of the time. (Give me a B737 NG any day even if it only has two engines- but then the CFM56 is about the most reliable engine ever built!)

[twistedenginestarter]

single engine failure ... range suddenly increased significantly.
Well that's interesting (don't you think Barit?)

My assumption is internal combustion engines are more efficient when they are worked hard (as their efficiency rises with higher pressures). I can only think the increased range came from improved efficiency from the three remaining motors.

[barit1]

Well, I must agree.

This says something about the ALF502 engine. Apparently the aeroplane is overpowered to the point that at normal cruise, the engines run at very low power (thus very inefficient). OEI, the three remaining donks get pushed up to a more efficient regime.

And apparently fuel efficiency was not a primary design objective for the Bae 146. "A strange little beast" indeed!