Author Topic: Air launched, oxygen gathered after takeoff?  (Read 8731 times)

Offline jongoff

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Re: Air launched, oxygen gathered after takeoff?
« Reply #20 on: 12/08/2009 09:43 pm »
I'll take a crack at the thermo:

Let's take our operating altitude as 40 kft (12 km).  The air at this temperature is about 220 K. 

To liquefy O2 from this temperature requires that we first remove sufficient heat to bring it down to its boiling point:
Q_cool = 29.378 [J/mol*K] * (220 K - 90 K) = 3.819 kJ/mol

Then we must liquefy it:
Q_condense = 6.82 [kJ/mol]

Assuming we separate the O2 post liquefaction, we must also cool 3.76 moles of N2 down to 90 K per mole of O2. 

Q_coolN = 3.76 * 29.124 [J/mol*K] * (220 K - 90 K) = 14.24 kJ/mol

Thus the total heat removal requirement is:
Q_tot = Q_cool + Q_condense + Q_coolN = 24.8 kJ/mol

I think you're missing one of the tricks often used in O2 liquification.  You don't need the chilled nitrogen after it's been separated off, so you can use that to prechill the incoming LOX, which cuts back quite a bit on the required mass.  I don't know if it's necessarily enough to pay for itself, but with N2 being 70% of the atmosphere, I wouldn't be surprised if it cut the required energy by at least half.

~Jon

Offline KelvinZero

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Re: Air launched, oxygen gathered after takeoff?
« Reply #21 on: 12/09/2009 08:06 am »
Hi guys,
Thanks for all these really good answers. Even if the idea itself does not pan out it is great that it can be dealt with analytically. Especially thanks for that working by Blazotron. I will have to dig up my physics books and follow it through myself.

Offline blazotron

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Re: Air launched, oxygen gathered after takeoff?
« Reply #22 on: 12/09/2009 08:39 am »
I'll take a crack at the thermo:

Let's take our operating altitude as 40 kft (12 km).  The air at this temperature is about 220 K. 

To liquefy O2 from this temperature requires that we first remove sufficient heat to bring it down to its boiling point:
Q_cool = 29.378 [J/mol*K] * (220 K - 90 K) = 3.819 kJ/mol

Then we must liquefy it:
Q_condense = 6.82 [kJ/mol]

Assuming we separate the O2 post liquefaction, we must also cool 3.76 moles of N2 down to 90 K per mole of O2. 

Q_coolN = 3.76 * 29.124 [J/mol*K] * (220 K - 90 K) = 14.24 kJ/mol

Thus the total heat removal requirement is:
Q_tot = Q_cool + Q_condense + Q_coolN = 24.8 kJ/mol

I think you're missing one of the tricks often used in O2 liquification.  You don't need the chilled nitrogen after it's been separated off, so you can use that to prechill the incoming LOX, which cuts back quite a bit on the required mass.  I don't know if it's necessarily enough to pay for itself, but with N2 being 70% of the atmosphere, I wouldn't be surprised if it cut the required energy by at least half.

~Jon

That's definately a good catch, although I am not sure if the extra weight of the required systems makes sense in such a vehicle.  My analysis considered cooling the nitrogen only to the condensation point of the oxygen, so the leftover cold N2 would still be gaseous.  In order to use it to prechill the incoming air (which is referred to as recuperation, at least when you are talking about reusing exhaust heat to preheat the incoming stream) you would (a) no longer be able to use a straight pass-through system since you would need to pass the cool N2 through a separate heat exchanger before the LOX condensing heat exchanger, and (b) need a really large heat exchanger since the volume rate would be large on both sides of hex as both streams are gaseous.  Point (a) matters much more in this system than in a ground LOX generator since efficient packaging is important and that will be easier with a straight-through system (but by no means the only way).  It's possible that it would even make sense to invest the extra energy needed to condense the nitrogen out as well so that the hardware can be much smaller and lighter, but only if the heat exchanger effectiveness is high enough and the system losses low enough to get most of the extra energy back.  This system would be even more complex, though, as you would need pumps to move around the LN2. 

If you could get everything to work out, such a system would save you about 1/2 of the fuel requirement.  So, it would be a significant reduction, but still a large energy expendature. 

Offline mlorrey

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Re: Air launched, oxygen gathered after takeoff?
« Reply #23 on: 12/09/2009 10:52 pm »
Well since N is lighter than O, you could start off with a thank of liquid air, and bubble gaseous air through it, venting pure N2 from the top, you should wind up with a tank of pure LOX. This would save you some weight without a highly complex system.
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Offline Danderman

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Re: Air launched, oxygen gathered after takeoff?
« Reply #24 on: 12/09/2009 11:40 pm »
Another approach is to use some sort of strap-on jets, which would operate for the first 60 seconds or so. I would imagine that jet engines are cheap enough to be disposable. I believe that most small SRBs have a marginal cost in the 7 figures, so they are not cheap.

Offline mlorrey

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Re: Air launched, oxygen gathered after takeoff?
« Reply #25 on: 12/10/2009 03:44 am »
Another approach is to use some sort of strap-on jets, which would operate for the first 60 seconds or so. I would imagine that jet engines are cheap enough to be disposable. I believe that most small SRBs have a marginal cost in the 7 figures, so they are not cheap.


Disposable turbine jet engines? You have to be kidding me... According to my calculations, the F100 engines in the US military inventory are valued at over $3.5 million a piece.
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Offline RanulfC

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Re: Air launched, oxygen gathered after takeoff?
« Reply #26 on: 12/14/2009 06:38 pm »
Quote
mlorrey wrote:
Disposable turbine jet engines? You have to be kidding me... According to my calculations, the F100 engines in the US military inventory are valued at over $3.5 million a piece.

Well, we DO have 'throw-away' jet turbines, we use them in cruise missiles and the more "disposable" UAVs, but those are exceptions and not very usable for launch applications.
(Though I've seen some military work done on "cheap" turbojets following the hobby-level work done for converting turbochargers into jet engines. I've also seen studies on small turbojets using bypass/afterburners as "dash" ramjets, still nothing you'd be able to use for launch.)

And this goes back again to the idea of using jet-engines in a first stage/zero stage/launch assist mode. The Danni Edar concept used 10 F100s in a manner similar to the current GEM-SRB small-solid assist rockets are used. Each was a single engine in a "Pod" that contained fuel, parachute and landing gear for recovery, and they were staged at around Mach-1.5/7 at 50,000ft or so and recovered within about 10 miles of the launch site. They pods would be recovered by a two man crew and a flat-bed truck with a crane attachment and trucked back to the launch site for refurbishment and prep for the next launch.

Some basic information on the concept:
First mention found in internet Archive files of concept by Dani Eder of Boeing:

Jet Boosted/Jet Assisted Launch vehicle, posted 1995 @18 July.

Supposedly done for study by Boeing for a minimum Non-Capital Intensive Launcher concept.

•   -10 Pratt & Whitney F100-229 military afterburning turbofan jet engines (F-15 fighter engines) ‘strapped-on’ to a two-stage “Core” vehicle.
•   Total vehicle lift-off mass listed as “145,000lbs” in this entry, a later entry (dated 2 October) listed the total vehicle weight as 154,000lbs. “Core” vehicle mass is listed as 85,000lbs in the first entry and 94,000lbs in the second entry with the mass of the 10 jet engine ‘modules’ remaining constant at 6,000lbs each or 60,000lbs total.
•   Payload to orbit is listed as 6,600lbs as optimized by the POST trajectory program, and ‘assumed’ to average 6,000lb.

The original posted idea is as follows:
•   Total vehicle mass is 145,000lbs at take off consisting of:
•   10 ‘modular’ jet engine pods each weighing 6,000lbs which consist of:
-   One each P&W F100-229 engine weighing 3,700lbs
-   1300lbs of “other” equipment including module structure, landing legs, parachute, fuel tanks, mounting structure, and other equipment for operation.
-   1000lbs of jet fuel
•   A two-stage Core vehicle which consisted of three parallel tank and structure modules each powered by two Pratt & Whitney RL-10 LH2/Lox  rocket engines. The two ‘side’ modules comprised the ‘first’ stage, and were staged off as they ran dry of propellant. The center module was longer than the first stage modules and had more robust entry protection as well as mountings for a payload at the forward end. Each module was to share a common tank diameter and plumbing to reduce production costs. Each module was to be no more than 2 meters in diameter to allow return to launch site shipment using standard shipping containers.
•   On take off all 10 jet modules would be under full afterburner power and lift the entire vehicle with an acceleration of around 2 gravities. The jet modules would operate for about 60 seconds, (later changed to 80 seconds) pushing the Core vehicle to 50,000ft at Mach 1.7 (@500 meters per second) and a flight path angle of 35 degrees.
•   At this point all 6 RL-10 motors were started while the jet modules were staged off and steered away from the flight path using residual thrust. (Launch velocity at this point would be equivalent to the jet modules and/or core vehicle ballistic ally coasting up to 90,000ft if no rocket power were applied)
•   The jet modules would ‘steer’ towards a pre-determined landing point using residual thrust prior to fuel depletion and deploy parachutes and non-shock absorbing legs to land. They would then be picked up, (landing was estimated to be within 10 miles of the launch point) by a truck equipped with a crane and transported back to the launch site for inspection, repair, refueling and reuse.
•   The Core vehicle would continue on burning all 6 engines in parallel until the propellant was exhausted from the two side modules. At this point they would be ‘staged’ with the larger central module continuing to power into LEO. The first stage modules would reenter and deploy parachutes for a landing at sea. They too would be recovered, inspected, and shipped back to the launch site for refurbishment, refueling and reuse.
•   The central module would deliver 6,000lbs (net) payload to LEO and then de-orbit, reenter, deploy parachutes and land near the launch site to be refurbished, refueled and reused.

Development, production, and operations costs were based on Boeing experience and prices quoted to the author by various manufacturers.
•   Total vehicle development costs were estimated from Boeing experience up to and including the 777 aircraft with aerospace hardware development scaling as a 0.75 power of hardware weight with development cost being 6 2/3% or around $300 million dollars. It was assumed that with no need for an extensive production line, and no need to certify the vehicle to airline safety standards the overall development price could be reduced to $200 million, or by 1/3rd the cost.
•   Ground crew costs were estimated at 10 people for the rocket with another 10 for the jet engines and 10 for ‘overhead’ functions (30 people total) equaling around $2.4 million dollars per year.
•   Flights were assumed at least once per week for a ground crew cost of $50,000.00 per launch, jet and rocket fuel accounting for $40,000.00 per launch. The jet engines were assumed to be acquired surplus/used for about $2 million dollars each and the RL-10 engines new for $3.5 million each. Vehicle acquisition was projected to be around $20 million dollars per unit for small quantities and scaled from aircraft production figures.
•   Total operations costs per launch were estimated at $747,000.00 per launch including amortized development and interests costs on all over 200 launches for the vehicles, 40 for the RL-10s and effective unlimited for the jet engines.
•   Approximate total price per launch was estimated to be $ 1.5 million dollars. ($250.00 per pound/$750,000.00 per person assuming a two person launch)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

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