# High vs low pressure turbines
**URL:** https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330
**Category:** Factual Questions
**Created:** [August 20, 2015, 8:19am UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330 "2015-08-20T08:19:16Z")
**Posts on this page:** 13
**Page:** 2
### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [August 26, 2015, 8:08pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/21 "2015-08-26T20:08:24Z")
> [@MichaelEmouse](#):
>
> What kind of Isp do you think could be attained?
> Also, opinions on the roles of air-augmented rockets and turborockets?
[Here](http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node85.html) is a good primer if you want to calculate performance parameters of a ramjet.
I don’t think that an air-augmented rocket has any application. The closest thing are Russian supersonic cruise missiles like [Kh-31](https://en.wikipedia.org/wiki/Kh-31). A solid-fuel rocket is used for launch and afterwards the empty rocket becomes a combustion chamber for a ramjet.
As for rockets burning atmospheric oxygen, I have doubts as well. They are certainly technically feasible, but seem to looking for a problem to solve.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [August 28, 2015, 9:32pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/22 "2015-08-28T21:32:45Z")
> [@AdamF](#):
>
> I don’t think that an air-augmented rocket has any application. The closest thing are Russian supersonic cruise missiles like [Kh-31](https://en.wikipedia.org/wiki/Kh-31). A solid-fuel rocket is used for launch and afterwards the empty rocket becomes a combustion chamber for a ramjet.
From ground/surface launch, where weight and size are usually not major concerns, booster+ramjet seems best. However, for air launch, it may be worth it.
The Meteor missile weighs 185kg, measures 12’ by 7" and has a range of up to 300km travelling at Mach 4. That’s pretty good for such a light and compact weapon.
> **[Meteor (missile)](https://en.wikipedia.org/wiki/Meteor_(missile))**
>
> The Meteor is a European active radar guided beyond-visual-range air-to-air missile (BVRAAM) developed and manufactured by MBDA. It offers a multi-shot capability (multiple launches against multiple targets), and has the ability to engage highly maneuverable targets such as jet aircraft, and small targets such as UAVs and cruise missiles in a heavy electronic countermeasures (ECM) environment with a range far in excess of 200 kilometres (110 nmi).
> A solid-fueled ramjet motor allows the missile ...
Although now that I think about it, with air-ducting + on-board oxidizer, you might be able to get very high altitude (30-50km), long range light missiles or spy UAVs.
> [@AdamF](#):
>
> As for rockets burning atmospheric oxygen, I have doubts as well. They are certainly technically feasible, but seem to looking for a problem to solve.
The turbine is smaller, light and not exposed to the main exhaust. Specific impulse is in-between that of rocket and turbojet.
I asked about the weight and cost of the turbine because if the turbine can be smaller and made of less expensive materials, this the turborocket may be well suited to cheap cruise missiles with a range of only a few 100s of km.
> [@LSLGuy](#):
>
> All these things are pretty speculative. Some have existed as sorta-functional models. The details of them are beyond my ability to comment on.
Sure but you have the knowledge to make educated guesses. If you do not wish to, that’s fair enough.
> [@LSLGuy](#):
>
> This [SABRE (rocket engine) - Wikipedia](https://en.wikipedia.org/wiki/SABRE_(rocket_engine)) is the latest industry darling. Which may in fact have some real life in it. We’ll know for sure in about 5 years.
Yeah, unfortunately, it seems to be aimed at making orbital flight cheaper. The history of using complicated expensive tech to make orbital flight cheap and simple is not encouraging.
Anything air-breathing that’s above a turbofan is only good for missiles, missiles and missiles. Well, guided bombs that straddle the line between bombs and missiles too.
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### Author:  [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)
#### Post date: [August 28, 2015, 10:56pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/23 "2015-08-28T22:56:26Z")
The whole issue for ground based vehicles that climb above the atmosphere is that it’s bloody heavy & stupid to carry oxidizer for the trip from ground level to the top of the atmosphere when you can get all that oxidizer for free in the atmosphere.
So if we can get rocket-level thrust / weight for the engine but with only having to carry fuel, not fuel + oxidizer, the whole power system gains tremendous efficiencies in all-up weight. Which drive further efficiencies in the structure and if it’s a lifting vehicle, the wing systems too. It’s a very powerful virtuous circle.
The hard part is making it work. So far.
The goal isn’t so much “cheap” like at Walmart as it is “industrially expensive, not platinum plated diamond bricks affordable by big governments in small quantities only expensive.” Which is what conventional space flight currently is.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [August 28, 2015, 11:35pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/24 "2015-08-28T23:35:04Z")
It’s bloody heavy and bloody propellant-inefficient but that doesn’t mean it’s stupid. Solid propellant is cheap, jet engines are not. Rockets tend to have much better thrust/weight ratio than jet engines. Hence concepts like the Big Dumb Booster: [Big dumb booster - Wikipedia](https://en.wikipedia.org/wiki/Big_dumb_booster)
To make a closely-related analogy: Which is most weight efficient and propellant efficient, solid propellant or cryogenic liquid propellant? Undeniably the latter. Yet ICBMs and most other forms of rocket systems used by the military overwhelmingly use solid propellant because lliquid and especially liquid cryogenic propellant is finicky as fuck.
I understand that sending something in space is never going to be Walmart cheap. When I used the word “cheap”, I was speaking in relative terms. In those relative terms, the dumb way seems to end up being smarter.
Figures the Ruskies would be better at doing things the dumb way. They have much more practice.
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### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [August 30, 2015, 2:47pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/25 "2015-08-30T14:47:46Z")
> [@MichaelEmouse](#):
>
> Although now that I think about it, with air-ducting + on-board oxidizer, you might be able to get very high altitude (30-50km), long range light missiles or spy UAVs.
Oxidizer is very heavy. 1kg of kerosene requires about 3kg of oxygen or 15kg of air to burn. There is no way that carrying oxidizer will extend the range.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [August 30, 2015, 3:11pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/26 "2015-08-30T15:11:33Z")
> [@AdamF](#):
>
> Oxidizer is very heavy. 1kg of kerosene requires about 3kg of oxygen or 15kg of air to burn. There is no way that carrying oxidizer will extend the range.
I don’t think it could extend the range compared to a jet engine or a booster+ramjet. I think it could extend the range compared to a rocket, like the Meteor missile’s propulsion does when one compares the Meteor to the AMRAAM.
You said that you didn’t think an air-augmented rocket had any application. I linked to the 300km range, Mach 4+ speed Meteor missile.
Then I speculated about the possibility of a long range\* missile that would fly at altitudes of 30-50km which may have applications for spying.
I admit that I struggle to think of many applications for a 30-50km ceiling, 100-300km range light missile aside from spying.
People who know more than me about aeronautics and orbital mechanics might also be able to deduce useful phenomena which can occur when one straddles the line between flying through the air and being in space.
\*I admit it’s an ambiguous term. I should have said a range of a few hundred kilometers.
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### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [August 30, 2015, 5:27pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/27 "2015-08-30T17:27:21Z")
> [@MichaelEmouse](#):
>
> You said that you didn’t think an air-augmented rocket had any application.
Let me clarify. I was talking about air-augmented rocket specifically. The way I understand it, it is a rocket inside a tube, with atmospheric air providing extra reaction mass and improving specific impulse. This is a cheap man’s ramjet, with the rocket engine providing the heat. So why not go with a proper ramjet?
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### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [August 30, 2015, 5:36pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/28 "2015-08-30T17:36:54Z")
> [@MichaelEmouse](#):
>
> Yeah, unfortunately, it seems to be aimed at making orbital flight cheaper. The history of using complicated expensive tech to make orbital flight cheap and simple is not encouraging.
You are absolutely correct.
> [@MichaelEmouse](#):
>
> Anything air-breathing that’s above a turbofan is only good for missiles, missiles and missiles. Well, guided bombs that straddle the line between bombs and missiles too.
Only because there is currently no good (economical) reason to cruise at Mach 2 or faster.
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### Author:  [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)
#### Post date: [August 31, 2015, 1:34am UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/29 "2015-08-31T01:34:49Z")
> [@AdamF](#):
>
> Let me clarify. I was talking about air-augmented rocket specifically. The way I understand it, it is a rocket inside a tube, with atmospheric air providing extra reaction mass and improving specific impulse. This is a cheap man’s ramjet, with the rocket engine providing the heat. So why not go with a proper ramjet?
First, the fact that it’s a cheap man’s ramjet. Lower price means more widely available and more readily used. I think it can safely be speculated that missiles are only going to grow in importance an that bringing down the price can have major benefits.
Second, carrying onboard oxidizer means it can fly at higher altitude than a ramjet could. If the propellant is mixed properly, it can be oxidizer rich for the initial boost, oxidizer poor for the climb and then medium for the very high altitude, very high velocity flight.
In what ways do you think it could be useful to fly higher than a ramjet can?
> [@AdamF](#):
>
> Only because there is currently no good (economical) reason to cruise at Mach 2 or faster.
What reasons are there that might become economical?
> [@AdamF](#):
>
> You are absolutely correct.
“It’ll be cheaper because we’ll only have to refurbish it”, they said.
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### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [August 31, 2015, 4:42pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/30 "2015-08-31T16:42:37Z")
> [@MichaelEmouse](#):
>
> First, the fact that it’s a cheap man’s ramjet. Lower price means more widely available and more readily used. I think it can safely be speculated that missiles are only going to grow in importance an that bringing down the price can have major benefits.
> Second, carrying onboard oxidizer means it can fly at higher altitude than a ramjet could. If the propellant is mixed properly, it can be oxidizer rich for the initial boost, oxidizer poor for the climb and then medium for the very high altitude, very high velocity flight.
When saying “cheap” I was being disparaging. I don’t see why it should be any cheaper than a proper ramjet. Liquid-fuel rocket by itself is more complex than a ramjet, and getting one to operate inside a tube will create problems. Heat management in particular.
And there is no real upper altitude limit for a ramjet. As you go higher, the thrust decreases, but so does the air drag.
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### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [August 31, 2015, 7:05pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/31 "2015-08-31T19:05:12Z")
> [@MichaelEmouse](#):
>
> “It’ll be cheaper because we’ll only have to refurbish it”, they said.
In part, but also because of the extra weight. A reusable vehicle is heavier than a disposable one, driving up the cost. For example, you could have launched a 20 tonne satellite with both the Space Shuttle and Delta IV Heavy. In addition to the satellite, Delta needs to lift 4 tonne upper stage into orbit. The Shuttle would have to lift 80 tonne orbiter.
That said, the Shuttle was designed to launch spy satellites into the polar orbit, repair them in space and recover them at the end of their useful life. And in this mission, the Shuttle would have been peerless. If only the Air Force hadn’t bailed on it at the last moment.
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### Author:  [@swampspruce](https://boards.straightdope.com/u/swampspruce)
#### Post date: [September 1, 2015, 3:29pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/32 "2015-09-01T15:29:19Z")
Food for thought: While a solid fuel rocket is cheaper, it’s also non-controllable. That is another reason why the military uses liquid fuels.
[Aerospike engines](https://en.wikipedia.org/wiki/Aerospike_engine) would allow you to get more controllability wrt flying in a variety of regimes. I seem to recall one proposal was a near space ramjet airliner that would allow you to fly from somewhere like NY to Sydney in little over an hour but while we could build something like that now, you’ll never see it due to the litigatory and financial constraints.
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### Author:  [@AdamF](https://boards.straightdope.com/u/AdamF)
#### Post date: [September 2, 2015, 6:03pm UTC](https://boards.straightdope.com/t/high-vs-low-pressure-turbines/728330/33 "2015-09-02T18:03:25Z")
> [@swampspruce](#):
>
> Food for thought: While a solid fuel rocket is cheaper, it’s also non-controllable. That is another reason why the military uses liquid fuels.
Military seems to be staying away from liquid-fuel rocket engines.
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