Best propulsion method for a "low cost" SSTO?

[KASASpace]

Did you mean Bernoulli?

There are so many effects, it's hard to remember them all. (at least for me)

[Dwayne_Knight]

The thing about mufti-stage vehicles.

I like the Idea of ditching (or unpowered return) of used fuel storage.

I've never liked the feel of engine redundancy. It always seems it should be possible to use less mass to improve an engine you already have than include another.

I wonder if skylon or one of our hypothetical vehicles wouldn't benefit from a glide return drop tank. I think the shuttles tank was mentioned in this thread. I'm sure if I go looking this has been mentioned somewhere before.

[Exosphere]

Since turbopumps are a very expensive component of rocket engines, couldn't the cost of the vehicle be reduced by using a pressure-fed engine? It would seem like such as system would be much simpler and cheaper, since it would get rid of a lot of the complicated plumbing and the expensive engine components. Is a pressure-fed engine a possibility on a surface-to-orbit vehicle, or would the extra weight of the helium container used to pressurize the tank be prohibitive?

[KASASpace]

Since turbopumps are a very expensive component of rocket engines, couldn't the cost of the vehicle be reduced by using a pressure-fed engine? It would seem like such as system would be much simpler and cheaper, since it would get rid of a lot of the complicated plumbing and the expensive engine components. Is a pressure-fed engine a possibility on a surface-to-orbit vehicle, or would the extra weight of the helium container used to pressurize the tank be prohibitive?

Take a good look at this thing

[cantab]

It's possible, but it will hurt payload fraction by requiring stronger tankage, and it's not without its own engineering challenges. Perhaps the biggest drawback is limiting practical combustion chamber pressures since the combustion chamber pressure can't be higher than that in the tank. That means pressure-fed engines are near universal for orbital manouevering engines and the like, and quite common for upper stages (the Delta II uses a pressure-fed engine in its upper stage for example), where high combustion chamber pressures are less important, but are rare nowadays in first stages.

[Exosphere]

Take a good look at this thing

Yeah, I Wikipedia'd a list of pressure-fed rockets; the only problem is that that one could only lift one to ten metric tons to LEO. So, that article shows that it's possible to construct an orbital rocket using a pressure-fed engine. I'm wondering whether or not it's possible to construct a reusable SSTO that can carry a reasonable payload (ten to twenty tons or so) to LEO without being a massive, expensive monstrosity.

[KASASpace]

Yeah, I Wikipedia'd a list of pressure-fed rockets; the only problem is that that one could only lift one to ten metric tons to LEO. So, that article shows that it's possible to construct an orbital rocket using a pressure-fed engine. I'm wondering whether or not it's possible to construct a reusable SSTO that can carry a reasonable payload (ten to twenty tons or so) to LEO without being a massive, expensive monstrosity.

Well, than it wouldn't really be ideal for a SSTO vehicle. You would probably want an aerospike, with LH2 as its fuel.

[Exosphere]

Well, than it wouldn't really be ideal for a SSTO vehicle. You would probably want an aerospike, with LH2 as its fuel.

That's the thing. I know there are disadvantages to the design, but since turbopumps are so expensive (orbital vehicles are sometimes called "turbopumps with a rocket attached"), it would seem that pressure-fed engines would have a significant advantage in terms of cost. I was just wondering if it's possible to put the two (reusable SSTO w/ a meaningful payload capability and pressure-fed engine) together to reduce costs. Obviously, it would be difficult to do so, and obviously payload fraction will suffer (I wouldn't use a pressure-fed engine to power a super heavy lift vehicle), but I get the feeling that if it's possible to construct a reusable SSTO with pressure-fed engines, cost to orbit could be reduced. Is that feasible, or am I getting something wrong?

EDIT: It would seem like this technology would also be useful for reducing engine cost:

http://www.space.com/22568-3d-printed-rocket-engine-test-video.html

Edited April 21, 2014 by Exosphere

[KASASpace]

Well, cost wise yes. But when it comes down to it, it actually costs a great amount of money keeping the inert gas you're using at high pressure. Granted you probably could manage to get a cheapish SSTO off the ground using this concept, but your Isp will suffer greatly as well. Think about it, if you have a high pressure area forcing exhaust gases to a much lower pressure area, the speed is partially determined by the pressure. As your pressure is lower, your average Isp will go down, and as such more fuel, and thus more structural weight, and thus the curse of rocket science.

[Ikaneko]

I response to Exosphere's assertion that lower payloads means it is more expensive, what do you say about the estimate that Skylon will reduce the cost of taking 1kg to LEO from $8,500 to $650? The Big dumb booster may work, but spaceplanes work better. You also argue that shallower trajectories are less efficient. Well, do you believe that rocket fuel is more efficient than jet fuel? I think not, so the shallower trajectory is made up for by that. Also, your argument of aerodynamic stresses is invalidated by the fact that spaceplanes are planes! They are built for this kind of thing!

[magnemoe]

Skylon is air breathing at lots of its trajectory, this saves weight.

Except air breathing space planes its no purpose of using single stage, far easier to do it multi stage.

Falcon 9 and heavy is planed to be reusable, first stage does not need much heat shielding.

Yes another design had been to do an larger first stage who land downrange and an returnable second stage.

[Dwayne_Knight]

Since turbopumps are a very expensive component of rocket engines, couldn't the cost of the vehicle be reduced by using a pressure-fed engine? It would seem like such as system would be much simpler and cheaper, since it would get rid of a lot of the complicated plumbing and the expensive engine components. Is a pressure-fed engine a possibility on a surface-to-orbit vehicle, or would the extra weight of the helium container used to pressurize the tank be prohibitive?

I thought the weight penalty came from having to make the whole tank withstand the high pressure needed to force the fuel out. The turbo pump acts to regulate pressure between the high and low pressure sides so the fuel container just needs enough structure to hold the mass against acceleration and most of the is done by modest over pressure.

I wonder if some other kind of pump could do the job cheaper. I could rattle off a few types of interesting pumps but they aren't cheap.

It might seem silly but what about a helical roots type pump driven thermal electrically.

Edited April 22, 2014 by Dwayne_Knight

[Dwayne_Knight]

Well, cost wise yes. But when it comes down to it, it actually costs a great amount of money keeping the inert gas you're using at high pressure. Granted you probably could manage to get a cheapish SSTO off the ground using this concept, but your Isp will suffer greatly as well. Think about it, if you have a high pressure area forcing exhaust gases to a much lower pressure area, the speed is partially determined by the pressure. As your pressure is lower, your average Isp will go down, and as such more fuel, and thus more structural weight, and thus the curse of rocket science.

Reminded me of this article I'd read once. I hope you find it interesting.

http://www.nasa.gov/centers/marshall/pdf/173616main_pulse_detonate.pdf

[Exosphere]

Reminded me of this article I'd read once. I hope you find it interesting.

http://www.nasa.gov/centers/marshall/pdf/173616main_pulse_detonate.pdf

I checked out some information about PDEs -- just wondering, would it be possible to combine that concept with something like the Skylon? Say, a PDE that operates off of a combination of atmospheric oxygen from an inlet and LH2 under atmospheric flight, and LH2 with LOX in a "rocket" mode? That would combine the qualities of a reusable SSTO space plane with an engine that gets rid of turbopumps, an air compressor (the report I read seemed to state that PDEs work somewhat like ramjets in that they don't use compressor blades/a compression system), and doesn't have to use a heavy pressure tank to hold the pressurizing gas.

What's more, it would get a pretty good Isp. The report I read on PDEs states they have a specific impulse between 2000 and 2400 seconds or so. That's higher than ramjets, scramjets, or air-augmented rockets -- it is lower than a standard turbofan/turbojet engine, though.

Here's the report:

http://ftp.rta.nato.int/public/PubFullText/RTO/EN/RTO-EN-AVT-185/EN-AVT-185-04.pdf

[sgt_flyer]

You will still need a way to bring the fuel to the engines. (And pressure fed systems is not practical in first stages / ssto - it takes too much weight fraction away from either : fuel tank structure for withstanding the pressure, or an additional tank for carrying a pressure gas (nitrogen, helium) - so you'll need a turbopump

Pressure fed systems are great for upper stages though - it limits a lot the moving parts

[Dwayne_Knight]

I checked out some information about PDEs -- just wondering, would it be possible to combine that concept with something like the Skylon? Say, a PDE that operates off of a combination of atmospheric oxygen from an inlet and LH2 under atmospheric flight, and LH2 with LOX in a "rocket" mode? That would combine the qualities of a reusable SSTO space plane with an engine that gets rid of turbopumps, an air compressor (the report I read seemed to state that PDEs work somewhat like ramjets in that they don't use compressor blades/a compression system), and doesn't have to use a heavy pressure tank to hold the pressurizing gas.

What's more, it would get a pretty good Isp. The report I read on PDEs states they have a specific impulse between 2000 and 2400 seconds or so. That's higher than ramjets, scramjets, or air-augmented rockets -- it is lower than a standard turbofan/turbojet engine, though.

Here's the report:

http://ftp.rta.nato.int/public/PubFullText/RTO/EN/RTO-EN-AVT-185/EN-AVT-185-04.pdf

I don't know. I suspect you would have to get subsonic air to get a proper air fuel mixture. Also I hear they have reliability problems.

I had a little looking around. A pump concept I initially dismissed because I was concerned about size valve switching speed and fuel pulsation may be perfectly viable.

I'm not sure how they plan on keeping the gas from bubbling and such. The key thing is it's light simple and maintains the high pressure away from the storage tank. I'd guess you could even store your first shot of fuel in the pump.

http://www.google.com/patents/US20090257888

Edited April 23, 2014 by Dwayne_Knight

[KASASpace]

Reminded me of this article I'd read once. I hope you find it interesting.

http://www.nasa.gov/centers/marshall/pdf/173616main_pulse_detonate.pdf

Reminds me of pulse jets.

Very interesting.

[Exosphere]

You will still need a way to bring the fuel to the engines. (And pressure fed systems is not practical in first stages / ssto - it takes too much weight fraction away from either : fuel tank structure for withstanding the pressure, or an additional tank for carrying a pressure gas (nitrogen, helium) - so you'll need a turbopump

Pressure fed systems are great for upper stages though - it limits a lot the moving parts

The article Dwayne_Knight linked to says:

"A major advantage is that pulse detonation rocket engines boost the fuel and oxidizer to extremely

high pressure without a turbopumpâ€â€an expensive part of conventional rocket engines. In a typical

rocket engine, complex turbopumps must push fuel and oxidizer into the engine chamber at an

extremely high pressure of about 2,000 pounds per square inch or the fuel is blown back out. The

pulse mode of pulse detonation rocket engines allows the fuel to be injected at a low pressure of

about 200 pounds per square inch."

So it would seem like it would have a much simpler fuel pumping system.

If what I've read about PDEs is correct, they're extremely simple. The engine itself doesn't have any moving parts, and the only moving part in the entire fuel tank/engine system is the valve that controls fuel supply and a pump. Basically, the engine is just an intake, fuel injector, spark plug, combustion chamber, and exhaust, all fed via fuel/oxidizer tanks hooked up by a simple valve assembly and a simple, low-cost pump. It's like the ultimate in engine simplicity -- theoretically, it seems like one could rig up a small PDE from parts obtained from a car engine and a hobby shop. Obviously, something of that size isn't going to power a Falcon 9 or any other orbital rocket, but the fact that the concept itself is that simple is promising both in terms of the cost of building orbital vehicles, the cost of maintaining those vehicles, and the safety/reliability of the engine (which is important -- customers don't like paying for rocket launches that end up dumping their multi-million dollar satellite into the ocean due to a turbopump failure).

As far as the issue of specific impulse is concerned, I've heard conflicting reports. Some state that a H2 powered PDE would have an Isp of about 3500-4000 seconds throughout their Mach regime in air-breathing mode (about the same as the Skylon's engines in air-breathing mode), whereas some have a much more optimistic value of 9,000 seconds plus. For example:

http://arc.uta.edu/research/pde_files/IspvsM.jpg

All of the sources I've seen seem to agree that PDEs in air-breathing mode are more efficient than corresponding scramjets, ramjets, or turbojets, though.

There's also some ambiguity about the Mach regime of PDEs, too. The sources I've seen have listed a PDE's maximum operating velocity as either Mach 5 or Mach 6.

So it seems like PDEs could be effective anywhere from zero airspeed to Mach 5 or zero airspeed to Mach 6, depending on the source. If PDEs could be effective up to around the Skylon's stated switch-over velocity (Mach 5.14), that would also be a huge advantage, because a space plane using PDEs wouldn't need to have the Skylon's precooler, which would reduce the cost of purchasing and maintaining the plane (Skylon-style precoolers will likely be a limited production part, which will drive up costs), as well increasing the reliability by removing one more potential point of failure in the design.

One concept I've been thinking of is a variable-cycle PDE-powered space plane. PDEs can work as either air-breathing engines or rockets, so a plane could potentially take off and accelerate to Mach 5 (ish) in air-breathing mode, then switch to rocket-mode for the rest of the trip to orbit. As an added advantage, PDEs work at low airspeeds, so you wouldn't need a takeoff assist system to accelerate the space plane up to supersonic speeds the way you do with a ramjet or scramjet.

Overall, I'd say PDEs have the potential to have a significant impact on space planes. They've experienced a lot of improvement recently due to the relatively large amount of interest in the technology (a flying prototype of a PDE-powered airplane has already flown, albeit at low speeds), and if they could be realized to their full potential, it could reduce the cost of orbital launch vehicles by at least some margin.

Edited April 23, 2014 by Exosphere

[Pds314]

Orion. Assuming cost means money and not lives or launch sites.

[Rakaydos]

Orion. Assuming cost means money and not lives or launch sites.

Cost per pound, sure, but I'm pretty sure refueling Skylon will cost less than the thousand nukes useed to launch a minimum sized orion.

Personally, I'm waiting for North Korea to give a big middle finger to the Test ban treaty and building a real Orion.

[Othuyeg]

http://en.wikipedia.org/wiki/Project_Pluto

A nuclear ramjet. Definetly the best idea ever.