Author Topic: AJ-1E6 (Now AR-1) Progress Known?  (Read 102789 times)

Offline Prober

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Re: AJ-1E6 Progress Known?
« Reply #20 on: 08/21/2013 01:46 pm »
I don't think you're getting it. Just because 9 150K lb, extremely light, and very small engines can fit close together - does not mean that a similar amount of 300-500K lb thrust engines can be clustered like that. There are weight issues, heat issues, among others. 5.5 M is not enough.

I don't see significant heat issues for regen cooled chambers and nozzles. It's not RS-68 :)

F9 is 3.66 meter diameter.
Merlin's nozzle is 1.676 m diameter.
NK-33's nozzle is 2 m diameter (1.193 times wider than Merlin).

If NK-33s are to be mounted exactly the same way as Merlins on F9 but with all dimensions scaled by 1.2, the stage needs to be about 4.4 meters in diameter for them to fit under it.

Why do you think 5.5 m stage wouldn't be enough?

If Aerojet proposes a 5.5m wide LRB for SLS that uses four AJ-1E6's, then that's 8 nozzles in 4 pairs.  Dunno if 9 would fit, but 8 will have to fit unless Aerojet goes with only 3 engines for 6 nozzles.


All very exciting.....but 3.5 AJ-1E6's might turn out a better design.   With one Center Nozzle (open to design changes)
« Last Edit: 08/21/2013 04:54 pm by Prober »
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Offline Lobo

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Re: AJ-1E6 Progress Known?
« Reply #21 on: 08/21/2013 04:01 pm »

All very exciting.....but 5 nozzles might turn out a better design.

Assuming you mean five AJ-500's, that would only be 2500klbs of thrust, a full 1000klbs less than the 5-seg booster and Dynetics booster.  There's an older spec sheet for SLS Block II with advanced liquid booster calling out three 1 Mlb " ORSC NHE"'s which I took to mean "Oxygen Rich Staged Combustion New Hydrocarbon Engines".  So not sure if less thrust than that would work.  So give AJ-500's might not be enough thrust.  Even with the greater ISP, it still has to get off the pad with the NASA required minimum T/W ratio.
That Block II also had the extra thrust of a 5th RS-25, so the booster might even need to be a little more thrust if SLS sticks with just four on the core in a Block 2B path.

But that could be outdated information too, so not for sure...

Offline Prober

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Re: AJ-1E6 Progress Known?
« Reply #22 on: 08/21/2013 04:51 pm »

All very exciting.....but 5 nozzles might turn out a better design.

Assuming you mean five AJ-500's, that would only be 2500klbs of thrust, a full 1000klbs less than the 5-seg booster and Dynetics booster.  There's an older spec sheet for SLS Block II with advanced liquid booster calling out three 1 Mlb " ORSC NHE"'s which I took to mean "Oxygen Rich Staged Combustion New Hydrocarbon Engines".  So not sure if less thrust than that would work.  So give AJ-500's might not be enough thrust.  Even with the greater ISP, it still has to get off the pad with the NASA required minimum T/W ratio.
That Block II also had the extra thrust of a 5th RS-25, so the booster might even need to be a little more thrust if SLS sticks with just four on the core in a Block 2B path.

But that could be outdated information too, so not for sure...

I corrected my early post ....
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Offline Ronsmytheiii

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Re: AJ-1E6 Progress Known?
« Reply #23 on: 09/02/2013 07:00 pm »
« Last Edit: 09/02/2013 07:03 pm by Ronsmytheiii »

Offline Lars_J

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Re: AJ-1E6 Progress Known?
« Reply #24 on: 09/02/2013 11:50 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?

Offline baldusi

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Re: AJ-1E6 Progress Known?
« Reply #25 on: 09/03/2013 02:06 am »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?
They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

Offline Hyperion5

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Re: AJ-1E6 Progress Known?
« Reply #26 on: 09/03/2013 06:30 am »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?
They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

Even with 600lbf less thrust than an F-1B LRB-powered SLS, the Isp jump is big enough the AJ-1-E6 LRBs ought to crush them in performance.  I would bet, judging from the jump in SL Isp, that the performance jump might be 10% or more over their LRB competitor, depending on how big they make the LRBs.  Alternatively they could make the LRBs shorter, but that seems like a waste.  If you're going to offer the most complicated option and most advanced engine, I think it would pay to maximize performance to give yourself more of an edge. 

Offline MP99

Re: AJ-1E6 Progress Known?
« Reply #27 on: 09/03/2013 07:12 am »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?

They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

2x "big, dumb, lower pressure" F1s vs 3x high-pressure "first time the US has done ORSC" engines. And 6x thrust chambers.

I could see NASA coming back with "yeah, but do we want to use that on a launcher with astronauts?" NASA's LOM calculations (EG see full version of the ESAS appendixes) ratchet up pretty mercilessly as you add more engines, and those engines have worse failure modes.

I wonder how 2x 1E6 would perform, just to try to keep the complexity down?

cheers, Martin

Offline baldusi

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Re: AJ-1E6 Progress Known?
« Reply #28 on: 09/03/2013 03:43 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?

They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

2x "big, dumb, lower pressure" F1s vs 3x high-pressure "first time the US has done ORSC" engines. And 6x thrust chambers.

I could see NASA coming back with "yeah, but do we want to use that on a launcher with astronauts?" NASA's LOM calculations (EG see full version of the ESAS appendixes) ratchet up pretty mercilessly as you add more engines, and those engines have worse failure modes.

I wonder how 2x 1E6 would perform, just to try to keep the complexity down?

cheers, Martin
I've seen those appendixes and I thought those were part of the thumbs in the scale of Griffin. If they keep applying those numbers, AJ should not even bother to present their bid. Which I find quite ridiculous since even a catastrophic TP failure (think of a Zenith-2 Tselina-2 #8 type of failure), wouldn't really be that different from other failure modes on the SLS stack. LOM and almost nil chances of LOC.
Which is funny because when you let the commercial side chose a vehicle multiple chambered high pressure cycle or huge amount of engines doesn't seem to be a problem, apparently. In fact, the only GG cycle on the US fleet is not even considered.

Offline Lobo

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Re: AJ-1E6 Progress Known?
« Reply #29 on: 09/03/2013 04:01 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?

They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

2x "big, dumb, lower pressure" F1s vs 3x high-pressure "first time the US has done ORSC" engines. And 6x thrust chambers.

I could see NASA coming back with "yeah, but do we want to use that on a launcher with astronauts?" NASA's LOM calculations (EG see full version of the ESAS appendixes) ratchet up pretty mercilessly as you add more engines, and those engines have worse failure modes.

I wonder how 2x 1E6 would perform, just to try to keep the complexity down?

cheers, Martin
I've seen those appendixes and I thought those were part of the thumbs in the scale of Griffin. If they keep applying those numbers, AJ should not even bother to present their bid. Which I find quite ridiculous since even a catastrophic TP failure (think of a Zenith-2 Tselina-2 #8 type of failure), wouldn't really be that different from other failure modes on the SLS stack. LOM and almost nil chances of LOC.
Which is funny because when you let the commercial side chose a vehicle multiple chambered high pressure cycle or huge amount of engines doesn't seem to be a problem, apparently. In fact, the only GG cycle on the US fleet is not even considered.

Yea, I think NASA is moving away from some of the pre conceptions they had in the ESAS study.  Maybe not them all, but they killed all of the possibilities of putting astronauts on a D4H or A5H for various dubious reasons, but seem ok to be launching those same astronauts on Atlas now in either Dreamchaser or CST-100.

Number of engines I don't think really impact LOM all that much if there's no finger on the scale, especially if there's engine out capability. 
NASA seems to be making decisions with SLS that look to have an eye on economics, and not just being the most expensive and compliated hardware humanly possibly, which has been their hallmark for 50 years. 

I mean, I don't know that they want 80 little engines or anything.  BUt if the core has 4 engines, and the booster have 2 or 3 or 4 engines each, I don't think that's enough engines to cause anyone any -real- issue, unless there's a finger on the scale again.
But if that finger was already on the scale, they probably would have already found a way to eliminate LRB's from the advanced booster competition and pretty much just gift wrapped it for ATK, unless Aerojet could propose an alternative large solid booster.


Offline Lobo

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Re: AJ-1E6 Progress Known?
« Reply #30 on: 09/03/2013 06:07 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?
They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

Even with 600lbf less thrust than an F-1B LRB-powered SLS, the Isp jump is big enough the AJ-1-E6 LRBs ought to crush them in performance.  I would bet, judging from the jump in SL Isp, that the performance jump might be 10% or more over their LRB competitor, depending on how big they make the LRBs.  Alternatively they could make the LRBs shorter, but that seems like a waste.  If you're going to offer the most complicated option and most advanced engine, I think it would pay to maximize performance to give yourself more of an edge. 

Well, if it has three engines, if there -was- a stand alone LV version of it (not saying there's a financial case for that), and if the AJ-1E6 could throttle down as low as the RD-180 (down to about 30% of maximum thrust...not sure if it can, but they are of fairly similar design, so maybe they can), then although they have limited fuel because they are an SLS booster and don't need a very long burn time, they could be throttled down shortly after lift off, and continue to be throttled down during ascent as applicable (to not pull too many g's, otherwise an overpowered booster like this would accelerate like crazy when not hauling up SLS's bulk).  Which could mean those 3 AJ-1E6's could burn for quite a bit longer, and maybe get enough first stage performance to not need too big of an upper stage.

Offline newpylong

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Re: AJ-1E6 Progress Known?
« Reply #31 on: 09/03/2013 06:13 pm »
How much will the better impulse over the steel casing 5 segs effect performance? The 5 casings are 3.6 millions lbs/thrust - more than any advanced booster proposals I have seen.


So is that 3 engines per booster, and they have 1 or 2 chambers per engine?
They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

Even with 600lbf less thrust than an F-1B LRB-powered SLS, the Isp jump is big enough the AJ-1-E6 LRBs ought to crush them in performance.  I would bet, judging from the jump in SL Isp, that the performance jump might be 10% or more over their LRB competitor, depending on how big they make the LRBs.  Alternatively they could make the LRBs shorter, but that seems like a waste.  If you're going to offer the most complicated option and most advanced engine, I think it would pay to maximize performance to give yourself more of an edge. 

Offline Lurker Steve

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Re: AJ-1E6 Progress Known?
« Reply #32 on: 09/03/2013 07:05 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?
They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

Even with 600lbf less thrust than an F-1B LRB-powered SLS, the Isp jump is big enough the AJ-1-E6 LRBs ought to crush them in performance.  I would bet, judging from the jump in SL Isp, that the performance jump might be 10% or more over their LRB competitor, depending on how big they make the LRBs.  Alternatively they could make the LRBs shorter, but that seems like a waste.  If you're going to offer the most complicated option and most advanced engine, I think it would pay to maximize performance to give yourself more of an edge. 

Well, if it has three engines, if there -was- a stand alone LV version of it (not saying there's a financial case for that), and if the AJ-1E6 could throttle down as low as the RD-180 (down to about 30% of maximum thrust...not sure if it can, but they are of fairly similar design, so maybe they can), then although they have limited fuel because they are an SLS booster and don't need a very long burn time, they could be throttled down shortly after lift off, and continue to be throttled down during ascent as applicable (to not pull too many g's, otherwise an overpowered booster like this would accelerate like crazy when not hauling up SLS's bulk).  Which could mean those 3 AJ-1E6's could burn for quite a bit longer, and maybe get enough first stage performance to not need too big of an upper stage.

I don't think we need to turn the SLS boosters into yet another LV. I think it would be enough to use the same engines as 2 other existing LVs (Atlas / Antares). Why split an already small volume of launches even further ?

Offline baldusi

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Re: AJ-1E6 Progress Known?
« Reply #33 on: 09/03/2013 07:21 pm »
How much will the better impulse over the steel casing 5 segs effect performance? The 5 casings are 3.6 millions lbs/thrust - more than any advanced booster proposals I have seen.
Thrust has very little bearing with performance. Specially when comparing solids vs liquids. First, you car about the whole stack T/W. And the compounded isp and propellant mass fraction.
But just look at the proposals and the advanced solids proposal with 3.6Mlbf and composite casing and improved formula (excellent pmf and improved isp). And yet they can't quite reach 130tonnes without a 5th RS-25. Yet, the F-1B boosters have an expected performance of 150tonnes with 4 RS-25. And my guess is that AeroJet proposal will have even better performance.
And the AJ-1E6 would solve the NK-33 stock problems and improve performance quite a bit on Antares. And could be a replacement on Atlas V if RD-180 was a problem. But considering that they've just extended the contract to 130 engines, unless the Russian bureaucrats do something really stupid, one SLS and Antares are real prospects.

Offline Lobo

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Re: AJ-1E6 Progress Known?
« Reply #34 on: 09/03/2013 08:01 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?
They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

Even with 600lbf less thrust than an F-1B LRB-powered SLS, the Isp jump is big enough the AJ-1-E6 LRBs ought to crush them in performance.  I would bet, judging from the jump in SL Isp, that the performance jump might be 10% or more over their LRB competitor, depending on how big they make the LRBs.  Alternatively they could make the LRBs shorter, but that seems like a waste.  If you're going to offer the most complicated option and most advanced engine, I think it would pay to maximize performance to give yourself more of an edge. 

Well, if it has three engines, if there -was- a stand alone LV version of it (not saying there's a financial case for that), and if the AJ-1E6 could throttle down as low as the RD-180 (down to about 30% of maximum thrust...not sure if it can, but they are of fairly similar design, so maybe they can), then although they have limited fuel because they are an SLS booster and don't need a very long burn time, they could be throttled down shortly after lift off, and continue to be throttled down during ascent as applicable (to not pull too many g's, otherwise an overpowered booster like this would accelerate like crazy when not hauling up SLS's bulk).  Which could mean those 3 AJ-1E6's could burn for quite a bit longer, and maybe get enough first stage performance to not need too big of an upper stage.

I don't think we need to turn the SLS boosters into yet another LV. I think it would be enough to use the same engines as 2 other existing LVs (Atlas / Antares). Why split an already small volume of launches even further ?


Agreed.  Which is why I said, "not saying there's a financial case for that", and have made that point on maybe threads talking about SLS LRB's being stand alone LV's.

And as much as I'd like to see the F-1's fly (I was only a year and a half old the last time they launched, on Skylab), I could see a stronger case made for the AJ-1E6 assuming Antares was upgraded to use it rather than two AJ-26's (I'll assume that's a likely upgrade), and particularly if Atlas were to go to it at some point...although it'd either need to be very cheap, or USAF would need to "recommend" it to ULA, otherwise I don't know what incentive ULA would have to switch Atlas to it.

However, I could even see a stronger case for a hydrolox booster powered by RS-68A...hypothetically. 
If SpaceX is able to start getting a piece of the government launch pie after 2015, then they may not need to pay ULA to maintain two EELV's.  Since Delta IV is the LV which already has the 3-core heavy flying on both coasts, there's an argument that Atlas could be retired in favor of keeping Delta.  SLS using a DCSS derived DUUS could lean in that direction too. 
So, ULA standardizes on just Delta IV with a 5m DCSS, perhaps upgrade to MB-60 on it.  DUUS is DCSS derived with MB-60.  SLS LRB's use boosters powered by 4-5 RS-68A's (2.86-3.6Mlbs at sea level respectively).  Even with 5 RS-68's, that's one less nozzle than three AJ-1E6's.
They seemed to think it was a feasible option in 2001 for an STS booster upgrade, and only 5.5m wide core.  The MPS would probably have to be fatter though, but it can be because it won't need to fit through the VAB doors on the bottom.

More Delta IV launches per year, and 10 more RS-68A's flying on every SLS launch would get that engine rate up, and as I understand, RS-68A is supposed to be pretty cheap at high volumes.
And the US LV fleet is tightened up with hopefully some better economics of scale.





Offline newpylong

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Re: AJ-1E6 Progress Known?
« Reply #35 on: 09/03/2013 08:38 pm »
That is what I thought. thank you,

How much will the better impulse over the steel casing 5 segs effect performance? The 5 casings are 3.6 millions lbs/thrust - more than any advanced booster proposals I have seen.
Thrust has very little bearing with performance. Specially when comparing solids vs liquids. First, you car about the whole stack T/W. And the compounded isp and propellant mass fraction.
But just look at the proposals and the advanced solids proposal with 3.6Mlbf and composite casing and improved formula (excellent pmf and improved isp). And yet they can't quite reach 130tonnes without a 5th RS-25. Yet, the F-1B boosters have an expected performance of 150tonnes with 4 RS-25. And my guess is that AeroJet proposal will have even better performance.
And the AJ-1E6 would solve the NK-33 stock problems and improve performance quite a bit on Antares. And could be a replacement on Atlas V if RD-180 was a problem. But considering that they've just extended the contract to 130 engines, unless the Russian bureaucrats do something really stupid, one SLS and Antares are real prospects.
« Last Edit: 09/03/2013 08:38 pm by newpylong »

Offline baldusi

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Re: AJ-1E6 Progress Known?
« Reply #36 on: 09/03/2013 08:55 pm »
However, I could even see a stronger case for a hydrolox booster powered by RS-68A...hypothetically. 
If SpaceX is able to start getting a piece of the government launch pie after 2015, then they may not need to pay ULA to maintain two EELV's.  Since Delta IV is the LV which already has the 3-core heavy flying on both coasts, there's an argument that Atlas could be retired in favor of keeping Delta.  SLS using a DCSS derived DUUS could lean in that direction too. 
So, ULA standardizes on just Delta IV with a 5m DCSS, perhaps upgrade to MB-60 on it.  DUUS is DCSS derived with MB-60.  SLS LRB's use boosters powered by 4-5 RS-68A's (2.86-3.6Mlbs at sea level respectively).  Even with 5 RS-68's, that's one less nozzle than three AJ-1E6's.
They seemed to think it was a feasible option in 2001 for an STS booster upgrade, and only 5.5m wide core.  The MPS would probably have to be fatter though, but it can be because it won't need to fit through the VAB doors on the bottom.
More Delta IV launches per year, and 10 more RS-68A's flying on every SLS launch would get that engine rate up, and as I understand, RS-68A is supposed to be pretty cheap at high volumes.
And the US LV fleet is tightened up with hopefully some better economics of scale.
H2 is a horrendous fuel for a LEO booster. The tanks and the engines T/W is horrible. Look at the Delta IV LEO numbers. Specially the Medium. And the RS-68A is heavy even for an hydrogen engine. Plus, the ablative nozzle has very limited lifespan. But the main issue is that the boosters are volume limited. They can't be bigger than 5.5m, due to VAB size. So, you couldn't really put a booster much bigger than a Delta IV. You could put about 80% more propellant than a Delta IV core stage. And let's say that you add four engines (you can't fit more), that's 3Mlbf per booster. But you'd only have fuel for around 115s (back of the envelop calculation), that's even less than the SRB! I don't think you'd get much performance.
Now, if they had gone with an AJAX-like rocket, of course that the RS-68A boosters of the Heavy could have done wonders. With six cores I guess they'd had amazing performance. But that path was (regrettably) nor chosen.

Offline MP99

Re: AJ-1E6 Progress Known?
« Reply #37 on: 09/03/2013 09:01 pm »
So is that 3 engines per booster, and they have 1 or 2 chambers per engine?

They say 2 x 550klbf chambers per engine. That's 3.3Mlbf per booster, with at least 297s of SL isp and 331s isp. That must give some amazing numbers.

2x "big, dumb, lower pressure" F1s vs 3x high-pressure "first time the US has done ORSC" engines. And 6x thrust chambers.

I could see NASA coming back with "yeah, but do we want to use that on a launcher with astronauts?" NASA's LOM calculations (EG see full version of the ESAS appendixes) ratchet up pretty mercilessly as you add more engines, and those engines have worse failure modes.

I wonder how 2x 1E6 would perform, just to try to keep the complexity down?

cheers, Martin
I've seen those appendixes and I thought those were part of the thumbs in the scale of Griffin. If they keep applying those numbers, AJ should not even bother to present their bid. Which I find quite ridiculous since even a catastrophic TP failure (think of a Zenith-2 Tselina-2 #8 type of failure), wouldn't really be that different from other failure modes on the SLS stack. LOM and almost nil chances of LOC.
Which is funny because when you let the commercial side chose a vehicle multiple chambered high pressure cycle or huge amount of engines doesn't seem to be a problem, apparently. In fact, the only GG cycle on the US fleet is not even considered.

One thing I would say is that the calculations appeared to be consistent with the pLOM/pLOC calculations they were quoting for Shuttle.

Cheers, Martin

Offline baldusi

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Re: AJ-1E6 Progress Known?
« Reply #38 on: 09/03/2013 10:10 pm »
Of course they would. They were derived the same way.
Yet, by those calculations, the Shuttle should have lost crew due to catastrophic turbopump failure. Only issue with engines was actually an instrumentation failure, and the solids, supposedly the safest item, actually killed one crew. Second crew was sort of a whole system problem.
But it was true that the only LOC due to LV in the whole history of space travel were on shuttle. And the 78% of dead astronauts while on mission, non the less. Soyuz ended up being much safer. I'm not discussing that.

Offline Lobo

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Re: AJ-1E6 Progress Known?
« Reply #39 on: 09/03/2013 10:12 pm »
H2 is a horrendous fuel for a LEO booster. The tanks and the engines T/W is horrible. Look at the Delta IV LEO numbers. Specially the Medium. And the RS-68A is heavy even for an hydrogen engine. Plus, the ablative nozzle has very limited lifespan. But the main issue is that the boosters are volume limited. They can't be bigger than 5.5m, due to VAB size. So, you couldn't really put a booster much bigger than a Delta IV. You could put about 80% more propellant than a Delta IV core stage. And let's say that you add four engines (you can't fit more), that's 3Mlbf per booster. But you'd only have fuel for around 115s (back of the envelop calculation), that's even less than the SRB! I don't think you'd get much performance.


http://forum.nasaspaceflight.com/index.php?topic=32705.msg1091427#msg1091427

See my post here, which references a 2001 study into LRB upgrade options for STS.  The -only- drawback they had with a five RS-68 powered LRB, which was only 5.5m wider, was the length would be taller than the ET.  But I believe the height they cite for the booster is actually less than the max booster height NASA is saying now for SLS.  So the one issue with it for STS was removed for SLS. 
However, the 5XRS-68 MPS on it flairs out form the 5.5m core.  not a problem for the STS ET because there's nothing there, but that could be an issue with the SLS MPS sitting right next to it.  However, if these are just to be booster only, and not stand alone LV's, then the engines could flair out out away from the core and not be quote centerline.  As long as the other booster was the same, it should be fine.  There may be other ways to address that too.

I think "horrible" is not a great statement for a hydrolox booster.  Delta IV does work pretty well.  The performance for the single stick Delta IV medium without SRB's to GTO is pretty close to the Atlas V-401 without SRB's, as well as Falcon 9v1.1.  All are right around 4.5mt to GTO, give or take a few hundred kg's. 
To LEO there's a little more difference, but even then note, the Delta IV medium without SRB's is about 4mt less to LEO than F9v1.1 (70% LEO capability), despite having less than 60% of the thrust at sea level. 

I wouldn't recommend building a brand new hydrolox booster engine, just there's one that already exists that -could- be fairly inexpensive if it was made in enough quanity.  And SLS will already be sharing Delta IV systems.
And NASA is already considering the engine along with the others...that's them, not me.

As far as the booster burn time, I think it would be wider, but also taller than the Delta IV core.  But the main difference is the Delta IV core feeds a single RS-68A at full throttle (on a medium) for about 250s.  Boosters only need to burn for 120-150s.  I think 150 is what Dynetics is planning on and 120 is about what the SRB burn for.  And what might be a factor is if a booster had 4 or 5 RS-68's, they'd all be full throttle at lift off, but SLS will only need that max thrust to get off the ground.  In short order, it can start to throttle down engines, and/or shut some off.  So a booster for 4-5 RS-68's doesn't need to be 4-5 times the size of the Delta IV core.  The engines don't need to be burning at full throttle the whole time.  And you get a very large ISP boost over even staged combustion hydrolox.  I think that's where the difference gets made up. 

But, I'm no expert.  Just saying NASA looked at the idea in 2001 and didn't see any issues with it, and they seem to be considering it again for SLS advanced boosters.  Although, they may only be considering it because it's a US made engine in production.
But then, that's -main- reason to consider it, I think, is it's an existing production engine, and cost shared with USAF.  Something that's not the case with F-1B or AJ-1E6.  Not because hydrolox is the best booster fuel.


Now, if they had gone with an AJAX-like rocket, of course that the RS-68A boosters of the Heavy could have done wonders. With six cores I guess they'd had amazing performance. But that path was (regrettably) nor chosen.

We are in agreement here.  :-)




 

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