Towards a combined-cycle SSTO.

[RGClark]

This will be like the Delta Glider except the engines will not need a scramjet cycle.

Looking at the numbers I'm convinced now you can make a single stage to orbit vehicle with a combined ramjet/rocket engine, and without having to use scramjets.

The idea is to combine the turbo-ramjet/rocket into a single engine. This is what Skylon wants to do with their Sabre engine. But the Sabre will use hypersonic airbreathing propulsion up to Mach 5.5 before the rockets take over. This will require complicated air-cooling methods using heat exchangers with flowing liquid hydrogen for the Skylon.

However, just being able to get to say the Mach 3.2 reached by the SR-71 would take a significant amount off the delta-V required for orbit. Of course if the ramjet could get to Mach 5 that would be even better but key this would be doable with the existing engines of the SR-71. Note too the engines of the XB-70 Valkyrie bomber could operate at Mach 3 and as far as I know they didn't have ramjet operation mode. So it might not even be necessary for the engines to have a ramjet mode, turbojet might be sufficient.

The problem with using jets for the early part of the flight of an SSTO has been they are so heavy for the thrust they produce, generally in the T/W range of around 5 to 10, while rocket engines might have a T/W ratio in the range of 50 to 100. But a key point is the jet engine will be operating during the aerodynamic lift portion of the flight where the L/D ratio of perhaps 7. The XB-70 for instance had a L/D of about 7 during cruise at Mach 3. So if we take the T/W of the jet engine to be say 7 and the L/D to be 7, then the thrust to lift-off weight ratio might be about 50 to 1 comparable to that of rockets.

BTW, it is surprising there has been so little research on this type of combination with the jet and rocket combined into one. You hear alot about turbine-based-combined-cycle (TBCC) where it combines turbo- and scram-jets and rocket-based-combined-cycle (RBCC) , where the exhaust from a rocket is used to provide the compression for a ramjet. But not this type of combined turbojet/rocket engine. It doesn't seem to have an accepted name for example. It would not seem to be too complicated. You just use the same combustion chamber for rocket as for the jet. Probably also you would want to close off the inlets when you switch to rocket mode.

For the calculation the delta-V and propellant load would be feasible, note that for a dense propellant SSTO might require as much as 300 m/s lower delta-V than a hydrogen fueled SSTO, in the range of about 8,900 m/s, so I'll use kerosene as the fuel. Hydrogen might have an advantage though in being light-weight if what you wanted was horizontal launch. Say you were able to get to Mach 3+ with the jets, 1,000 m/s. The delta-V to be supplied by the rocket-mode is then 7,900 m/s. But note also you can get to high altitude say to 25,000 m. This might subtract another 300 m/s from the required rocket-mode delta-V, so now to 7,600 m/s.

A bigger advantage than this of the altitude is the fact that you get the full vacuum Isp during rocket-mode, call it an exhaust velocity of 3,600 m/s for kerosene rockets. Note this results in a mass-ratio for the rocket mode portion of e^(7,600/3,600) = 8.3, less than half that usually cited for a kerosene-fueled all rocket SSTO. Note the fuel required for the jet-powered portion would only be a fraction of the dry mass rather than multiples of it based on the fact the 1,000 m/s jet-powered speed is only a fraction of the 10,000 m/s or so effective exhaust speed of jet engines.

Note this brings the kerosene fuel load to be about that of hydrogen fueled SSTO's, except you still have the high density of kerosene. With modern lightweight materials this should be well doable.


Bob Clark

 

[Hlynkacg]

This would differ from Skylon how?

 

[RGClark]

This would differ from Skylon how?

The production of an operable hypersonic combustion engine is a complicated and expensive project. That's a big part of the doubt of some in the industry of the feasibility of the Skylon and of the scramjet propulsion efforts ongoing in the U.S.
However, this method would not require scramjets but only engines that have already been produced, turbo-jets perhaps with a ramjet mode.


Bob Clark

 

[Hlynkacg]

If you aren't going to take advantage of the air-breathing engines in the super/hypersonic regime why not launch vertically and ditch the wheight of wings and engines?

---------- Post added at 07:56 AM ---------- Previous post was at 07:51 AM ----------

The production of an operable hypersonic combustion engine is a complicated and expensive project. That's a big part of the doubt of some in the industry of the feasibility of the Skylon and of the scramjet propulsion efforts ongoing in the U.S.

And the production of an SSTO is not?

I'm getting the feeling that your knowledge of general aeronautical engineering is even more dubious than your rockets.

 

[T.Neo]

RGClark, have you read about Skylon? It solves the issues of hypersonic flight without using scramjets. And the heat exchangers aren't filled with hydrogen, they're a helium loop, cooled by the hydrogen propellant.

As for what you're suggesting, it seemed very ramble-y to me, so I probably don't quite understand what it's actually supposed to be, but it sounds very "hey, look what johnny scribbled" and that if it would work at all, it'd be quite a kludge.

This thread on the NASASpaceflight forums contains some interesting posts about rocket/fan combinations.

 

[RGClark]

RGClark, have you read about Skylon? It solves the issues of hypersonic flight without using scramjets. And the heat exchangers aren't filled with hydrogen, they're a helium loop, cooled by the hydrogen propellant.
As for what you're suggesting, it seemed very ramble-y to me, so I probably don't quite understand what it's actually supposed to be, but it sounds very "hey, look what johnny scribbled" and that if it would work at all, it'd be quite a kludge.
This thread on the NASASpaceflight forums contains some interesting posts about rocket/fan combinations.

Thanks for the link. The development cost of the Skylon is estimated as $12 billion. That quite large amount of financing still has to be acquired. Key technical hurdles are that hypersonic combustion engine. The method by which it works is not a scramjet but it still uses a method never done before, so there is some technical uncertainty there.
In contrast the turbojet/ramjet combo engine goes back to the 60's with the SR-71 so is a well known technology. Also if based on the SR-71 it will be a smaller craft using known technology so the development costs and uncertainty will be less.


Bob Clark

 

[Urwumpe]

Thanks for the link. The development cost of the Skylon is estimated as $12 billion. That quite large amount of financing still has to be acquired.

It is a conservative estimate, which is something that I really appreciate. Just as comparison: The development costs of the Airbus A380 had been 11 million Euro or 15 billion USD.

Which is feeling pretty sane regarding the technological challenges of the slightly smaller Skylon.

 

[RGClark]

Thanks for the link. The development cost of the Skylon is estimated as $12 billion. That quite large amount of financing still has to be acquired. Key technical hurdles are that hypersonic combustion engine. The method by which it works is not a scramjet but it still uses a method never done before, so there is some technical uncertainty there.
In contrast the turbojet/ramjet combo engine goes back to the 60's with the SR-71 so is a well known technology. Also if based on the SR-71 it will be a smaller craft using known technology so the development costs and uncertainty will be less.

Bob Clark

Correction. Strictly speaking combining the rocket with the turbojet into a single engine has not been done either.


Bob Clark

 

[Urwumpe]

Correction. Strictly speaking combining the rocket with the turbojet into a single engine has not been done either.


Bob Clark

Maybe not that, but rocket engine with ramjet has already been done.

Also, the Skylon builds on the HOTOL research for the same goals, the Skylon engine is just a lot more advanced as the prototypes for HOTOL

 

[jedidia]

11 million Euro or 15 billion USD.

Now, I know the Euro has seen better times, but I don't think there ever was a point in time where this added up... :lol:

 

[Urwumpe]

Now, I know the Euro has seen better times, but I don't think there ever was a point in time where this added up... :lol:

Its the prognosis for 2020. :rofl:

 

[RGClark]

Maybe not that, but rocket engine with ramjet has already been done.

Also, the Skylon builds on the HOTOL research for the same goals, the Skylon engine is just a lot more advanced as the prototypes for HOTOL

I heard of plans but not of it being executed. Which cases are you thinking of?


Bob Clark

 

[Urwumpe]

I heard of plans but not of it being executed. Which cases are you thinking of?


Bob Clark

Take for example the boosted ramjet engines of many modern beyond-the-horizon Air-To-Air missiles. They have already solved quite many problems that you get with combined cycle engines there, especially about the nozzle and chamber geometries.

[ame="http://en.wikipedia.org/wiki/MBDA_Meteor"]MBDA Meteor - Wikipedia, the free encyclopedia[/ame]

A liquid engine of such a type could actually be simpler - you can inject oxidizer into the combustion path, but that would not be the most effective mode of operation.