SSTO Concept

[KASASpace]

So, I have been thinking about this topic for a while, and I came up with this:

Some sort of method of getting the craft to high enough velocities so where a ramjet provides decent thrust, perhaps a JATO like device, I don't know.

Then, the vehicle "takes off" and flies to an altitude, and a velocity, where it starts to lose thrust. Now, the ramjet is actually a bypass around the actual main engine, which is a scramjet. Since ramjets are fairly effective up to ~Mach 5 to 6, the ramjet will throttle down and the scramjet will throttle up so that the thrust remains the same. Eventually the ramjet fully shuts down and the scramjet is the only unit burning. Using the scramjet to get to fairly high velocities and altitudes, you could potentially cut a large amount of delta-v required, thanks to lift. After reaching a decent velocity and altitude, a rocket engine fires. At this point the scramjet throttles down and then cuts off thrust. The rocket engine (perhaps using liquid propane, which is in the upper 300s for Isp) will finish the trip to orbit.

So, any thoughts/improvements?

The technology for a "bypass" "ramjet" has existed since the 1960s in the SR-71, which is what this ram/scram-jet is based on.

[DerpenWolf]

Problem with having a bunch of engines is that they are dead weight when you can't use them.

[KASASpace]

Problem with having a bunch of engines is that they are dead weight when you can't use them.

Yes they are, but you will gain large amounts of cross-range capability with engines. Plus, ramjets are extremely simple, with virtually no moving parts.

[Seret]

Plus, ramjets are extremely simple, with virtually no moving parts.

Ramjets have lots of moving parts, they just have fewer than other engines.

Edited July 30, 2014 by Seret

[Ralathon]

Too much weight. You have 3 engines on that thing (4 if you include the system that brings your RAM online) wreaking havoc on your fuel fraction. I don't think you appreciate just how much dV it takes to get into earth orbit.

Try playing some KSP with the Realism Overhaul modpack and the Advanced Jet Engines mod. It is extremely difficult to make something that goes SSTO. These mods have been widely used for almost half a year now and I've seen 2 SSTO's, neither of which could haul much cargo.

[Starglider]

Even before the issue of dead weight, "taking off" at altitude violates the concept of SSTO. How about some new technologies to (somewhat) solve both these problems:

1) Provide a turbocompressor bypass to your "main" ram air intake, take off in "turbojet" mode, and switch to "ramjet" mode in-flight. The bypass will be less able to handle high airflow rates, but should still be able to go supersonic (since true turbojets can make Mach 4).

2) Provide a variable geometry combustion chamber, so when the combustion flow goes transonic, you can switch from "ramjet" mode to "scramjet" mode.

3) Since we're using something similar to a valve system to switch from "turbojet" to "ramjet", why not close both valves and supply your own oxidizer? So now you can switch from "scramjet" mode to "rocket" mode.

You'll probably need to use something like a toroidal aerospike instead of the normal exhaust nozzle, as this would operate at all altitudes. Also, you can't put turbine blades in such an exhaust stream (they'd melt), so you'll have to bleed off fuel and air to run a built-in generator (to run the turbocompressor for the air and the turbopumps for the fuel and oxidizer). Finally the variable geometry will interfere with the regenerative cooling system used to keep high-performance engines intact, so perhaps a combination of new materials (carbon nanotube - tungsten composite?) and a "shield" of unreacted air between the exhaust gasses and the chamber walls would do. Still, when I look at how technology has progressed, I think this might be doable in a few decades.

Provide a dedicated computer system to monitor and switch between the modes as needed, and you could call this a "Reflex Engine".

[Red Iron Crown]

Kerosene might be a better choice than LPG for fuel; easier handling (liquid at STP), lots of developed rocket engines for it, similar Isp.

How were you visualizing it taking off and getting to ramjet altitude/speed?

[Sky_walker]

"taking off" at altitude violates the concept of SSTO.

Is it just me, or this sentence makes no sense at all?

Too much weight. You have 3 engines on that thing (4 if you include the system that brings your RAM online) wreaking havoc on your fuel fraction. I don't think you appreciate just how much dV it takes to get into earth orbit.

This.

And also: It'll be horribly expensive to manufacture and very prone to failure. (You will most likely have to handle 6 engines (for symmetry) 4 of which are going to be a very new technologies barely going out of the experimental stage.

[Seret]

Is it just me, or this sentence makes no sense at all?

I think he's talking about the OP's:

perhaps a JATO like device, I don't know.

Where they're basically talking about using a rocket or other first stage to get up to speed for the ramjets. That wouldn't be SSTO.

[DerpenWolf]

Hybrid engines are probably the best bet, skylon's SABRE engines are a good example however I still see ways it could be improved upon.

[sojourner]

Yep, the inclusion of the JATO makes this a 2 stage vehicle.

[Rus-Evo]

Sounds to me like the OP described an SR-71 Blackbird with extra rockets added. I'm thinking a viable craft would need to be bigger.

Maybe the first near-SSTO could actually use a cheap & disposable booster rocket in some way whilst keeping the rest of the craft fully reusable. The OP doesn't sound too daft really.

[Armchair Rocket Scientist]

So, I have been thinking about this topic for a while, and I came up with this:

Some sort of method of getting the craft to high enough velocities so where a ramjet provides decent thrust, perhaps a JATO like device, I don't know.

Then, the vehicle "takes off" and flies to an altitude, and a velocity, where it starts to lose thrust. Now, the ramjet is actually a bypass around the actual main engine, which is a scramjet. Since ramjets are fairly effective up to ~Mach 5 to 6, the ramjet will throttle down and the scramjet will throttle up so that the thrust remains the same. Eventually the ramjet fully shuts down and the scramjet is the only unit burning. Using the scramjet to get to fairly high velocities and altitudes, you could potentially cut a large amount of delta-v required, thanks to lift. After reaching a decent velocity and altitude, a rocket engine fires. At this point the scramjet throttles down and then cuts off thrust. The rocket engine (perhaps using liquid propane, which is in the upper 300s for Isp) will finish the trip to orbit.

So, any thoughts/improvements?

The technology for a "bypass" "ramjet" has existed since the 1960s in the SR-71, which is what this ram/scram-jet is based on.

The problem with scramjets is they have horrible TWRs compared to other jet engines, and once you get significantly above mach 5 the aerodynamic heating will destroy most materials, so you need a heavy TPS. Also, as mentioned you need to get to mach 5 just to run the scramjet. With conventional jets this means your vehicle needs to have 4 engine systems: a scramjet, a ramjet to get the vehicle up to mach 5 so the scramjet can work, a turbojet to get the vehicle up to mach 2-3 so the ramjet can work, and a rocket to get from the scramjet's top speed to orbit.

Having four separate propulsion systems on an SSTO is ludicrously impractical. This is why most of the design proposals out there use some sort of advanced engine that can operate in at least two of the speed regimes. For example, Skylon's SABRE engines are supposed to combine a rocket engine with a pre-cooled turbojet (mach 0-5) and they don't bother with a scramjet. Another promising concept is the Pulse Detonation engine. The technology is in its infancy, but they're supposed to be usable from mach 0 to mach 5 as well. I'm not sure how well precoolers would work with a pulsedet, but it's possible they could allow even faster atmospheric speeds at the expense of requiring a separate rocket engine.

[KASASpace]

Too much weight. You have 3 engines on that thing (4 if you include the system that brings your RAM online) wreaking havoc on your fuel fraction. I don't think you appreciate just how much dV it takes to get into earth orbit.

Too much weight? Do you understand how a ramjet works? It's practically a tube with an injector, although that's a gross exaggeration. A ramjet's biggest difference from a scramjet is geometry, so not much increase of weight there. The heaviest engine will most likely be the rocket engine itself, although it could be pressure-fed, but that's not that efficient.

"taking off" at altitude violates the concept of SSTO

I never said that, I said:

"takes off" and flies to an altitude

A turbo-compressor is complicated, and that's why I didn't mention having one onboard.

Edited July 30, 2014 by KASASpace

[KASASpace]

Kerosene might be a better choice than LPG for fuel; easier handling (liquid at STP), lots of developed rocket engines for it, similar Isp.

How were you visualizing it taking off and getting to ramjet altitude/speed?

I chose propane because it's more efficient, almost 30 to 50 increased Isp over kerosene, and it's a lot easier to acquire propane.

I don't know, perhaps some sort of magnetic acceleration? A hydraulic system akin to the systems onboard carriers?

Where they're basically talking about using a rocket or other first stage to get up to speed for the ramjets. That wouldn't be SSTO.

I never said it was final. There are other methods.

[Armchair Rocket Scientist]

Too much weight? Do you understand how a ramjet works? It's practically a tube with an injector, although that's a gross exaggeration. A ramjet's biggest difference from a scramjet is geometry, so not much increase of weight there. The heaviest engine will most likely be the rocket engine itself, although it could be pressure-fed, but that's not that efficient.

It's a really big, heavy tube that doesn't produce very much thrust. Here.

A good rocket engine will have a TWR of 50-150. Turbojets and turbofans? 5-10. Scramjets? 2. I'm guessing ramjets are somewhere between normal jets and scramjets.

Granted, the vehicle doesn't need to have a total TWR above 1 on its jet engines, but you'd still need 0.3-0.5. The rocket engine would actually be the lightest (and if you count the extra tank mass required by pressure-fed engines, a pump-fed engine is lighter. Pressure-fed engines are used because they're cheap and easy to make restart many times).

A turbo-compressor is complicated, and that's why I didn't mention having one onboard.

You'll still need a turbopump to pump the fuel for your jet engines, although if you have an active cooling system, precooler, or both, you can transfer heat from the air to fuel and cause the fuel to expand, then use the energy of the expanding fuel to drive the turbopump.

[Red Iron Crown]

I chose propane because it's more efficient, almost 30 to 50 increased Isp over kerosene, and it's a lot easier to acquire propane.

I think ease of acquisition may vary by region. Kerosene is basically aviation fuel, which is not overly difficult to acquire in most parts of the world.

I've never seen specs for any kind of propane rocket, can you share where you got your info? Intuitively I would think specific impulse for any hydrocarbon fuel would be pretty similar because the actual propellants are always the same mass (CO2 and H2O), but that's just a guess on my part.

I don't know, perhaps some sort of magnetic acceleration? A hydraulic system akin to the systems onboard carriers?

Actually you answered my question in your previous post, if I understand you the craft would also operate in a more traditional turbofan/turbojet mode to get up to speeds where ramjets start making sense.

I think multimode engines like the JT-11D or proposed SABRE would probably mass less than separate engines for each mode, at the cost of some complexity.