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SLS / Orion / Beyond-LEO HSF - Constellation => Heavy Lift Launch Vehicle (HLV/SLS) => Topic started by: robert_d on 03/29/2013 01:30 am

Title: Delta 4 based advanced boosters
Post by: robert_d on 03/29/2013 01:30 am
Lobo's thought experiment about using Falcon cores as SLS boosters is a good way to think about how to keep development costs down. I still prefer the idea of using dual delta 4 tank sets on each side with a new thrust structure that would support 4 RS-68A engines for a total of 2.8 million pounds thrust per side.  Only eight engines, using the same LH2 as the SLS core and with a potential for a flyback version in the future.
This would seem to require only a new thrust structure and external attachment frame to put it into the running as an alternative to designing a entirely new booster.
Title: Re: Delta 4 based advanced boosters
Post by: russianhalo117 on 03/29/2013 03:00 am
Lobo's thought experiment about using Falcon cores as SLS boosters is a good way to think about how to keep development costs down. I still prefer the idea of using dual delta 4 tank sets on each side with a new thrust structure that would support 4 RS-68A engines for a total of 2.8 million pounds thrust per side.  Only eight engines, using the same LH2 as the SLS core and with a potential for a flyback version in the future.
This would seem to require only a new thrust structure and external attachment frame to put it into the running as an alternative to designing a entirely new booster.
The F-1A booster design has direct lineage to designed Saturn C-3B launcher that would have replaced the Saturn-1B first stage with Saturn-1B-2 stage, which is similar to the first stage being proposed now. The primary differences are: updated design to incorporate modern alloys, manufacturing et cetera as well as the switch to modernized F-1B engines. Other than that the booster proposal and Saturn C-3B's first stage are practically identical in its general design.

First RS-68B would be required and developed so that an oxidizer rich ignition sequence is used versus the current fuel rich ignition sequence that sets the DIV insulation on fire when free hydrogen escaping from the end engine before ignition occurs ignites partway up the CBC.

Also an RG-1/RG-2 (RP-1/RP-2) based booster can lift more than an LH2 Based booster, which is why Atlas V can lift more in the AV 551 configuration than DIV-H.
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 03/29/2013 04:53 am
Lobo's thought experiment about using Falcon cores as SLS boosters is a good way to think about how to keep development costs down. I still prefer the idea of using dual delta 4 tank sets on each side with a new thrust structure that would support 4 RS-68A engines for a total of 2.8 million pounds thrust per side.  Only eight engines, using the same LH2 as the SLS core and with a potential for a flyback version in the future.
This would seem to require only a new thrust structure and external attachment frame to put it into the running as an alternative to designing a entirely new booster.

So, you are saying basically a Delta IV core with two RS-68's on it?  It'd look a bit like a hydrolox version of the Dynetics booster?  Well, you could probably do that.   For a booster kerolox probably would be better.  More thrust for the same size engine, and smaller boosters cores, since space is limited.  But, you could have a strongback and mount these two modified Delta IV CCB's to each side of it. 
It could work, just not sure how much work it'd be to mount two RS-68's on a single Delta IV core.  Might be more trouble than it's worth.  Two RS-68A's isn't much more thrust than a single F9 v1.1. 

Interesting idea though...
Title: Re: Delta 4 based advanced boosters
Post by: Patchouli on 04/01/2013 01:11 am
Lobo's thought experiment about using Falcon cores as SLS boosters is a good way to think about how to keep development costs down. I still prefer the idea of using dual delta 4 tank sets on each side with a new thrust structure that would support 4 RS-68A engines for a total of 2.8 million pounds thrust per side.  Only eight engines, using the same LH2 as the SLS core and with a potential for a flyback version in the future.
This would seem to require only a new thrust structure and external attachment frame to put it into the running as an alternative to designing a entirely new booster.
My guess is that such an arrangement might be able to lift 90 tonnes to LEO without an upper stage.  It might lift 100 tonnes if propellant cross-feed was implemented, but that would require adding a fifth RS-25 to the core.  But eight RS-68A engines would be too much thrust, realistically, since booster weight would always be limited by volume.  Six could work and still achieve most of this performance, using either two CBC tanks per side or one new 5.5 meter diameter tank per side.

In summary, RS-68A boosters would likely not beat the performance of advanced solid or kerosene boosters, but they would also not require the development of all-new engines.

 - Ed Kyle

It would simplify pad operations since you now only need two propellants and no handling of live SRB segments.

As for too much thrust why not go with a tank stretch on the CBCs or core until the thrust to weight is back to acceptable levels?
Title: Re: Delta 4 based advanced boosters
Post by: robert_d on 04/01/2013 01:59 am
Ed,
Sorry but I don't understand the "since booster weight would always be limited by volume" part of your sentence as support for you saying 2.8 million lbs of thrust per side would be too much. The current Delta IV runs for about 4:20 with one engine so two engines per tank would run about 2:10.  The total thrust would be well under that of the new five segment solid boosters, although the performance should be similar due to the lighter overall weight (as you show with your 90 ton calculation). Why I thought this might be worth considering is that the delta cores are already designed to transfer loads to a central core, and cost sharing with existing Delta 4 production might keep costs low, even with a differrent thrust structure. There is a potential for a similar thrust structure being used for a "delta 5" heavy vehicle with two RS-25 expendables powering the two outboard cores of a three core booster.

Also if flight rates increased, a  RS-68 regenerative could be developed and ultmately a "floatback" version that would reuse the engines several times.


My guess is that such an arrangement might be able to lift 90 tonnes to LEO without an upper stage.  It might lift 100 tonnes if propellant cross-feed was implemented, but that would require adding a fifth RS-25 to the core.  But eight RS-68A engines would be too much thrust, realistically, since booster weight would always be limited by volume.  Six could work and still achieve most of this performance, using either two CBC tanks per side or one new 5.5 meter diameter tank per side.

In summary, RS-68A boosters would likely not beat the performance of advanced solid or kerosene boosters, but they would also not require the development of all-new engines.

 - Ed Kyle
Title: Re: Delta 4 based advanced boosters
Post by: sdsds on 04/01/2013 03:51 am
Just to confirm: not possible to reach the 130 tonnes goal with RS-68 powered boosters that conform to the "advanced boosters" limits?
Title: Re: Delta 4 based advanced boosters
Post by: Patchouli on 04/02/2013 01:25 am
Can the RS-68A handle the thermal environment of being clustered and near the RS-25s?
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/02/2013 05:01 pm
Just to confirm: not possible to reach the 130 tonnes goal with RS-68 powered boosters that conform to the "advanced boosters" limits?
130 tonnes to LEO would definitely be possible with a second stage, which any Block 2 SLS is going to require.  Some additional thrust might be needed at that point, but I think it would be possible to get there without having to add more engines.  RS-25 could be upgraded to 111 or 115% thrust, for example.

I modeled this as follows:

Two boosters, each with three RS-68A engines, GLOW 438 tonnes each.

Core with four RS-25E engines, GLOW 995 tonnes (a bit offloaded).

Upper stage with one J-2X (ISP=437 sec assumed), GLOW 136 tonnes.

Payload 130 tonnes, ideal delta-v 9,315 meters/sec.

 - Ed Kyle

Itís interesting.  Thanks Ed.  Could you get a single hydrolox booster with three RS-68A engines, at 5.5m diameter or less?  The single D4 CCBís are already 5m diameter and 40m tall  In order to feed three RS-68Aís and not be over 5.5m wide, itíd have to be like 100mt tall or something ridiculous wouldnít it?

However, I come back to the issue if two large hydrolox boosters, each with three RS-68A, would be a new custom 1-off booster.  No different really than the Dynetics boosters, Aerojet boosters, or something similar with like three RD-180ís, or a big cluster of M1Dís.  Other than using an engine already in production. 
Maybe if this new booster would replace D4H and have launches other than on SLS or somethingÖ  Itíd be a pretty tall booster though.
I think a kerolox custom, 1-off LRB would be better than a hydrolox one if they are both new boosters.  Better thrust at liftoff and denser fuel for a more compact core, obviously.  Better attributes of a 0.5 stage booster.  And as much as it would be awesome to see the F-1 live again, using RD-180ís, or AJ-1-E6ís (if they were to get into legitimate production for a stretched Antares or something) is probably the wiser move for a kerolox booster.  Unless the F-1 powered Dynetics booster were to become the common EELV-class booster for USAF/DoD to replace Atlas and Delta.  Somehow I donít see that happening, but if something like that happened, then the Dynetics booster would be a real good option. 

So, if looking at RS-68A engines, I think seeing about developing a strongback that could mount a whole D4H to it, and use more or less off-the-shelf would be probably the better way to go, donít you think?  If that could be made to work without any deal breaking vibration or aerodynamic issues?  (As has been brought up by some on the FH booster concept thread).

Of course, doesnít a D4H cost like $400M or something?  Those might be some spendy boosters, even if they are already developed.  Unless two more D4Hís produced per year for 1 SLS flight per year brings the cost per unit down very significantly.
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/02/2013 05:04 pm
Hmmm..I wonder if there should have been just one common thread about using existing or soon to be existing boosters as SLS booster.

Ultimately, with my FH booster concept, or this D4H booster concept, that's the ultimate goal.  We really need fewer LV's/boosters in the US, not more.  To try to get the economies of scale up.
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/03/2013 12:12 am
Could you get a single hydrolox booster with three RS-68A engines, at 5.5m diameter or less?  The single D4 CCBís are already 5m diameter and 40m tall  In order to feed three RS-68Aís and not be over 5.5m wide, itíd have to be like 100mt tall or something ridiculous wouldnít it?
The Advanced Booster spec actually appears to allow up to 5.78 meters diameter and a maximum length of up to 71.646 meters.  Such dimensions should allow for a maximum of somewhere in the neighborhood of 500 tonnes of propellant in each booster, very roughly figuring.

An RS-68 is 2.43 meters diameter, so it would fit, but there isn't a lot of room.  Then again, the F-1 engines were 3.76 meters in diameter.


Ok, I wasnít aware of the 5.78m width.  I thought it was 5.5m.

Well, you are right, a new booster core with existing engines is less of a development than a new booster with new engines.  Just donít know if the RS-68Aís are the best for the boosters when you could make booster of similar power and performance, but a bit smaller, and with like three RD-180ís already in production, or three AJ-1-E6ís if those were to be a production engine outside of SLS.  RD-180ís will already be man-rated through commercial crew, and RS-68A wonít, so if NASA plans to be launching Orion on SLS, thatíd be an additional development so the RS-68A couldnít be used off-the-shelf, where the RD-180 could.  It could be man rated, but then I donít know if thereís much advantage to using the ďexistingĒ engine vs. developing the new F-1B which should be inherently man-rated. 
Man rated (at that time), off-the-shelf options will mainly be RD-180 and M1D.  A booster core made by SpaceX, or some other company who buys the M1Dís from SpaceX could be an option of using the off-the-shelf F9 boosters with a strongback isnít an option for some reason (although thatíd be my preference).  Youíd probably need about 22-23 M1Dís per booster to equal three RD-180ís.  Thatís probably less than ideal to have that man engines on a single core vs. using three F9 cores with only 9 engines each (just a matter of using several proven cores with 9 engines vs. a new core with 23 engines.)  But at least itís an American made engine.   A larger engine like a Merlin 2 would be a new engine development.  But I suppose a 5.5m methalox booster core with 5 Raptor engines would be 3.25M lbs of thrust and could be the basic core for SpaceX going forward for their heavier lift methalox plans.  And five engines is a nice geometry under a single core.  A tri-core heavy of this would give SpaceX an FX/FXH type heavy lifter of nearly 10M lbs of thrust!
ButÖthat all depends on when Raptor and the new methalox core might be available from SpaceX, as if thatíd be any type of viable option.



As for the idea of just using a pair of CBCs on each side, that would not meet NASA's Advanced Booster specification.  While a new booster tank structure would be required, the engines would already be available (largely) and since they would account for much of the booster cost, having them available and shared by another launch vehicle would be a benefit.

 - Ed Kyle

Maybe, but I suppose the entire premise of this thread, and my FH one is that NASA decides it would be cheaper and more sustainable to try to use existing boosters ďwhere practicableĒ rather than a new 1-off custom design.  A new core with existing engines is better, but I think using existing cores too is best if at all possible.  And Iíd sorta hope that NASA would consider it if there was a sound concept clustering two or three EELV-class boosters cores together to form ďclusterĒ boosters rather than monolithic boosters.
Title: Re: Delta 4 based advanced boosters
Post by: robert_d on 04/03/2013 01:33 am
Lobo,

I think your whole point is exactly correct - we cannot afford an entirely new booster development program for expected flight rates of only one or two launches per year.  I do not see the tank sizes needing to be changed for a Delta IV derived dual core system, because as I said, you get the required 2 minutes 10 seconds at the current size with 2 engines per core(or 4 per side). If Ed is right and you need only 3 engines, you would have even longer duration. I could imagine using the current outboard Delta IV boosters as is and just adding enough plumbing to connect to the third engine that would be built as part of the SLS adapter structure. Thrust transfer is already designed into the Delta boosters. And I do not think the current advanced  booster requirements would necessarily be violated as the total "span" of the system would be under the limits at 5+8.5+5 = 18.5 at the diagonal and slightly less along the major axis.

Finally, LH2 allows for the floatback concept to be developed to make these boosters reusable.


Maybe, but I suppose the entire premise of this thread, and my FH one is that NASA decides it would be cheaper and more sustainable to try to use existing boosters ďwhere practicableĒ rather than a new 1-off custom design.  A new core with existing engines is better, but I think using existing cores too is best if at all possible.  And Iíd sorta hope that NASA would consider it if there was a sound concept clustering two or three EELV-class boosters cores together to form ďclusterĒ boosters rather than monolithic boosters.

Title: Re: Delta 4 based advanced boosters
Post by: robert_d on 04/03/2013 01:59 am
The floatback concept is a name I believe I conceived around 2001 when the delta 4 heavy was announced, and several people were discussing booster recovery and reuse options. If anyone wants to claim a precedent, that's fine. Basically it was to have a delta 4 vent all residual oxygen, and to gasify the remaining hydrogen to fill the hydrogen tank and fill an attached blimplike gas bag during the coast phase to reach a condition where the system would have neutral buoyancy. Then a fuel cell powered ducted fan system or set of small hydrogen powered thrusters would SLOWLY return to the booster to the Kennedy landing strip. 

It was felt at the time that the Delta IV boosters might be travelling too high and fast for this to work, but it dawned on me that an SLS system with additional engines would have a flight profile much closer to the STS SRB's and therefore this might be worth a remention.

In summary - NO system will be fundable for BEO missions at a flight rate that would actually accomplish something unless at least some of it is reusable. If not this or something similar the SLS will cancelled when the 5 segment solids are used up.
 
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/03/2013 05:41 pm
Lobo,

I think your whole point is exactly correct - we cannot afford an entirely new booster development program for expected flight rates of only one or two launches per year.  I do not see the tank sizes needing to be changed for a Delta IV derived dual core system, because as I said, you get the required 2 minutes 10 seconds at the current size with 2 engines per core(or 4 per side). If Ed is right and you need only 3 engines, you would have even longer duration. I could imagine using the current outboard Delta IV boosters as is and just adding enough plumbing to connect to the third engine that would be built as part of the SLS adapter structure. Thrust transfer is already designed into the Delta boosters. And I do not think the current advanced  booster requirements would necessarily be violated as the total "span" of the system would be under the limits at 5+8.5+5 = 18.5 at the diagonal and slightly less along the major axis.

 

Well, thatís a good point.  Getís me thinking.  A Delta IV standard core, with two RS-68Aís on the bottom (looking a bit like the Dynetics booster) would drain the core twice as fast.  However, the standard Delta IV has a burn time of  4 minutes, 9 seconds.  So with two RS-68Aís, they should burn for 2 minutes, 4 seconds.  Almost as long as the 5-seg and twice the thrust.  Thatís probably short of thrust though, but, if the core is stretched from 40m to 60m, then that should accommodate a 3rd RS-68 engine for the same 2 minute, 4 second burn time.  I would think such a stretch would not be too hard.  The Delta IV core is already stiff enough to handle the DCSS and 5m PLF on top, which together are probably 20m, for a 60m stack.  Actually, almost 70m.

http://www.boeing.com/assets/images/defense-space/space/delta/delta4/images/d4family.jpg

Actually, the hydrolox core itself looks close to just 30m, with the MPS and engine being another 8-10m.  So it might only need a 15m stretch to accommodate the 3rd RS-68.  Maybe add another 5m for good measure to get that burn time for all three engines up to 2 min, 10 seconds or more. 

So maybe there wouldnít even have to be a new whole booster per se, but a stretched (and modified)5m  Delta IV booster, with a triangular MPS, widened with three RS-68A engine mounts. 
Title: Re: Delta 4 based advanced boosters
Post by: a_langwich on 04/03/2013 07:04 pm
Lobo,

I think your whole point is exactly correct - we cannot afford an entirely new booster development program for expected flight rates of only one or two launches per year.  I do not see the tank sizes needing to be changed for a Delta IV derived dual core system, because as I said, you get the required 2 minutes 10 seconds at the current size with 2 engines per core(or 4 per side). If Ed is right and you need only 3 engines, you would have even longer duration. I could imagine using the current outboard Delta IV boosters as is and just adding enough plumbing to connect to the third engine that would be built as part of the SLS adapter structure. Thrust transfer is already designed into the Delta boosters. And I do not think the current advanced  booster requirements would necessarily be violated as the total "span" of the system would be under the limits at 5+8.5+5 = 18.5 at the diagonal and slightly less along the major axis.

 
...

So maybe there wouldnít even have to be a new whole booster per se, but a stretched (and modified)5m  Delta IV booster, with a triangular MPS, widened with three RS-68A engine mounts. 


This discussion has me mystified.  "We cannot afford an entirely new booster development program"...the plan is to let companies submit bids.  All you Delta IV "boosters" (sorry), knock yourself out--get some financing, and engineering support, and submit a bid for modified Delta IV cores as boosters.  IF it turns out that just adding an engine or two, and maybe stretching the tanks along with mitigation of some of the RS-68A's thin margins, adding emergency detection, reworking the fuel-rich burning at startup, and perhaps looking at a regen nozzle--IF that turns out to be so much cheaper than an "all new" resurrected F-1 booster, or solids, or whatever else is bid, then great!  Of course, it (a medium ISP hydrolox design) will also have to be at least comparable in performance to both solids and kerolox alternatives, in a task (booster) which may favor those alternatives' strengths.

The Delta IV may be the least competitive launcher in its size range, in price, in payload fraction, with safety margins not so great, without emergency detection avionics, etc.  (But great reliability--I'm not knocking it too much in the task for which it was designed, as long as USAF is willing to pay for it.)  I'm continually amazed how many people want to base decades of future launch systems on the tradeoffs it made for a different environment.

It's not like SLS is planned to launch at some high rate which will appreciably change the economics for Delta IV, nor is a core with one or more additional engines, probably stretched, and modified in various other ways, going to roll off a standard Delta IV core production line.  Perhaps by adding a Chinese army of cores--six?  eight?--to SLS, even one launch a year would be a lot of production.  But it adds a lot of extra complexity to the launch vehicle.
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/03/2013 09:40 pm

This discussion has me mystified.  "We cannot afford an entirely new booster development program"...the plan is to let companies submit bids.  All you Delta IV "boosters" (sorry), knock yourself out--get some financing, and engineering support, and submit a bid for modified Delta IV cores as boosters.  IF it turns out that just adding an engine or two, and maybe stretching the tanks along with mitigation of some of the RS-68A's thin margins, adding emergency detection, reworking the fuel-rich burning at startup, and perhaps looking at a regen nozzle--IF that turns out to be so much cheaper than an "all new" resurrected F-1 booster, or solids, or whatever else is bid, then great!  Of course, it (a medium ISP hydrolox design) will also have to be at least comparable in performance to both solids and kerolox alternatives, in a task (booster) which may favor those alternatives' strengths.


The thought is that using existing hardware will be cheaper to develop than creating brand new hardware, even if the per unit price isnít necessarily better.  And it could at least Ėhelp- the economy of scale of the LV the booster is based off of.  Where a dedicated purpose built booster will only fly with SLS, and you run into the problem that STS had with the ATK booster.  A sole booster provider, and never launched enough to really get the cost down.

However, to your point about just letting the chips fall where they may, and maybe a new Dynetics booster would be cheaper than a D4 derived booster.  I donít think anyone has a problem with that.  But NASA would need to change their bid solicitation so that some of the existing LV booster concepts could be considered and compete.  As it is not, I donít know that they could.  Basically NASAís requiring a single core purpose built to lift on the upper thrust beam, etc etc.  So the purpose of this thread and the Falcon Heavy booster one is to tug on this thread a little and see if there are even ways to provide SLS boosters out of existing LVís if NASA were to loosen their boundaries a bit.


The Delta IV may be the least competitive launcher in its size range, in price, in payload fraction, with safety margins not so great, without emergency detection avionics, etc.  (But great reliability--I'm not knocking it too much in the task for which it was designed, as long as USAF is willing to pay for it.)  I'm continually amazed how many people want to base decades of future launch systems on the tradeoffs it made for a different environment.

It's not like SLS is planned to launch at some high rate which will appreciably change the economics for Delta IV, nor is a core with one or more additional engines, probably stretched, and modified in various other ways, going to roll off a standard Delta IV core production line. 


And this is the point Iíve made here.  I think Falcon 9 will the best option to look at for this concept, because it would likely be the cheapest LV that will exist, with the highest flight rate.  And it will already be man-rated for commercial crew.  Itíll have better thrust and smaller size due to being kerolox.  Delta IV is already very expensive, so Delta IV boosters could be very expensive, and I donít know if the SLS flight rate will be such to change that much.
M1D is a US-built and designed engine, and should get built in a very high quantity if 9 are used on every core.  That works in itís favor.

However, in D4ís defense, RS-68A is  about the only existing US-built large booster engine out there.  RD-180 is Russian built, which might have some political resistance (or maybe not, who knows in todayís day and age?).  And supposedly, Iíve heard RS-68A has the ability to be a very cheap engine if the production rate was high enough.   I donít think the high number of M1D engine would be a problem with Falcon boosters, but some people apparently do.  So RS-68A get away from that.

But if Dynetics could propose a brand new booster, with a new engine, that will only fly on SLS (so a low production rate), with all that new development, for cheaper than a booster based on flying hardware already in production, then by all means, go with it.  But I think existing LV derived SLS boosters should be given consideration if the were technically feasible and affordable. 
Title: Re: Delta 4 based advanced boosters
Post by: robert_d on 04/03/2013 11:11 pm
Quote from: a_langwich link=topic=31491.msg1034509#msg1034509 daPluste=1365015844

This discussion has me mystified...

>> and<<

It's not like SLS is planned to launch at some high rate ...


What Lobo said.
Plus my point about reusibility. I am as deeply mystified that you would accept this projected low flight rate as a given. A BEO program will accomplish very little if it can only launch 130 tons or so per year for 3 billion in launch costs.  Boostback for the Falcon 9's or delta 4's or my (speculative) floatback idea would be ways to start constructing a sustainable program. 
Title: Re: Delta 4 based advanced boosters
Post by: a_langwich on 04/04/2013 03:25 am

The thought is that using existing hardware will be cheaper to develop than creating brand new hardware, even if the per unit price isnít necessarily better.  And it could at least Ėhelp- the economy of scale of the LV the booster is based off of.  Where a dedicated purpose built booster will only fly with SLS, and you run into the problem that STS had with the ATK booster.  A sole booster provider, and never launched enough to really get the cost down.

But it is not existing hardware.  Adding another engine, lengthening the core, changing the load environment, vibration environment, aero environment, possibly trajectory, adding in margins for human cargo--those aren't simple changes, they pretty much require re-analysis of every component, and pretty substantial redesigns.

When you are done, it won't help with economies of scale of the LV it was based on, except at the part supplier level perhaps, and in the sense that ULA will have other business to help towards overhead. 

The RS-68 will probably be common, granted, and it may lower its marginal cost, but it will also require some redesign and testing.
The pressure to do substantial redesign will be high, because putting 6-8 of these around any core will introduce base heating issues, which will make a regen nozzle desirable or necessary, which is the start of the slippery slope.

I agree, it would be nice to get the launch costs down, and desirable to not have a sole-provider, low launch rate booster.  But wishing that basing something on Delta IV would fix that problem won't make it so.


Quote
However, to your point about just letting the chips fall where they may, and maybe a new Dynetics booster would be cheaper than a D4 derived booster.  I donít think anyone has a problem with that.  But NASA would need to change their bid solicitation so that some of the existing LV booster concepts could be considered and compete.  As it is not, I donít know that they could.  Basically NASAís requiring a single core purpose built to lift on the upper thrust beam, etc etc.  So the purpose of this thread and the Falcon Heavy booster one is to tug on this thread a little and see if there are even ways to provide SLS boosters out of existing LVís if NASA were to loosen their boundaries a bit.


Does the solicitation really require purpose-built?  Lifting on the upper thrust beam seems reasonable to me, given that they aren't going to redesign their core specifically for new liquid boosters.  I think you have to live with that requirement, and redesign your booster if necessary.  Again, you couldn't have avoided a substantial redesign anyway. 

Quote
And this is the point Iíve made here.  I think Falcon 9 will the best option to look at for this concept, because it would likely be the cheapest LV that will exist, with the highest flight rate.  And it will already be man-rated for commercial crew.  Itíll have better thrust and smaller size due to being kerolox.  Delta IV is already very expensive, so Delta IV boosters could be very expensive, and I donít know if the SLS flight rate will be such to change that much.
M1D is a US-built and designed engine, and should get built in a very high quantity if 9 are used on every core.  That works in itís favor.


I would agree.  SpaceX might be able to produce a very competitive booster, if they chose.  I don't believe they could just strap together a handful of F9s, but perhaps with a fair amount of engineering justification they could come close.  Economies of scale, and mass production, require producing substantially identical items.  Variations ramp up the cost pretty quickly to lose economy of scale.


Quote
However, in D4ís defense, RS-68A is  about the only existing US-built large booster engine out there. 

That, I think, is exactly why it comes up again and again, despite being poorly matched for this purpose.  I don't think you will be able to use RS-68 in its A config.  It will have to have modifications, in margins, in initial startup or seal design, in instrumentation and detecting emergency conditions, and possibly in handling the thermal environment around the nozzle.  The question is whether, when you've done all the necessary engineering, you would wish you had picked a better starting design.


Quote
But if Dynetics could propose a brand new booster, with a new engine, that will only fly on SLS (so a low production rate), with all that new development, for cheaper than a booster based on flying hardware already in production, then by all means, go with it.  But I think existing LV derived SLS boosters should be given consideration if the were technically feasible and affordable. 

I agree existing LV-based boosters should be given consideration,* but I get the sense that this thread has pre-judged the engineering to be negligible and the affordability to be automatically superior. 

*but I don't agree that SLS should have its core redesigned to make them more competitive.  And I would not agree that "consideration" would include assuming there would be economies of scale down the road, if the bidders were not willing to make that wager themselves in their pricing.
Title: Re: Delta 4 based advanced boosters
Post by: a_langwich on 04/04/2013 04:43 am
Quote from: a_langwich link=topic=31491.msg1034509#msg1034509 daPluste=1365015844

This discussion has me mystified...

>> and<<

It's not like SLS is planned to launch at some high rate ...


What Lobo said.
Plus my point about reusibility. I am as deeply mystified that you would accept this projected low flight rate as a given. A BEO program will accomplish very little if it can only launch 130 tons or so per year for 3 billion in launch costs.  Boostback for the Falcon 9's or delta 4's or my (speculative) floatback idea would be ways to start constructing a sustainable program. 


Why would you assume a higher flight rate as a given, and then use that to explain how your costs would come down?  That little sleight-of-hand didn't work for the shuttle.  I am hopeful the flight rate and schedule for SLS are both conservative, but I don't see any way a 100+ tonne launcher would crank out enough launches to gain appreciable economies of scale.  Don't forget, if you reduce the price for SLS, you will have enough money to do more mission work OR launch more, but probably not both. 

You can't just strap on Delta IVs or Falcon 9s.  A substantial amount of engineering work must be done, and I don't think the end result has much chance of rolling off the same production line and being performance-competitive at the same time.  Maybe for the Falcon 9, but adding engines and/or stretching Delta IV cores represents a completely different design.

Again, this is a bid competition.  If ULA is willing to propose a boostback or floatback booster, and price it lower based on a gamble about reusability and commonality, then that should help their bid.  If they are not willing, then it can't happen.  Sadly, little Armadillo Aerospace has done quite a bit more booster flyback testing on flight hardware than mighty ULA, despite ULA proposing such a scheme years ago. 

SpaceX has a serious advantage in reusability, in that they are committed from Elon Musk down toward accomplishing it.  So if they propose a c'mon-back design in a year or two, they will have existing hardware and design experience and expertise.  However, the cost savings associated with reusability are still hypothetical at this point. 
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/04/2013 06:01 am

But it is not existing hardware.  Adding another engine, lengthening the core, changing the load environment, vibration environment, aero environment, possibly trajectory, adding in margins for human cargo--those aren't simple changes, they pretty much require re-analysis of every component, and pretty substantial redesigns.

When you are done, it won't help with economies of scale of the LV it was based on, except at the part supplier level perhaps, and in the sense that ULA will have other business to help towards overhead. 

The RS-68 will probably be common, granted, and it may lower its marginal cost, but it will also require some redesign and testing.
The pressure to do substantial redesign will be high, because putting 6-8 of these around any core will introduce base heating issues, which will make a regen nozzle desirable or necessary, which is the start of the slippery slope.

I agree, it would be nice to get the launch costs down, and desirable to not have a sole-provider, low launch rate booster.  But wishing that basing something on Delta IV would fix that problem won't make it so.


First, Delta IV isn't my favorite existing launcher to try to make a booster from.  Falcon 9 is.  Better thrust, all-US built, and should be the lowest cost of any existing LV.  IF F9/FH can't be made to work, it's unlikely Delta IV or Atlas V could be made to work. 

http://forum.nasaspaceflight.com/index.php?topic=31455.0

However, Delta IV is also an interesting intellectual exercise.
But, it would require some extensive mods that F9 -might- not need.  The shorter diameter, larger thrust, and more compact design lends itself better to try to bundle 3 of them together to try to form a booster within the limitations set out by NASA.  Really, if the 3 cores could just be cross braced together, they could use the existing booster interfaces that they'll have anyway to mount to the FH central core.  Except they'd mount to some sort of strongback adaptor, which would then properly transfer the thrust force to the upper thrust beam so that the SLS core wouldn't have to be redesigned. 
That strongback/adaptor element WOULD be a new development.  Like you said, you can't just strap existing boosters to SLS (although that would have been the better way to go, if SLS had been designed like AJAX).
But...it really shouldn't be too much to develop.  it's just a structural element.  There's not tankage or plumbing on it. 
The same could be done for the Atlas V core to do a similar configuration.

Delta IV runs into problems though, because that confirguration with three Delta IV's is too wide, but two Delta IV's is probably not enough power. 
And the whole concept hinges on using as much existing hardware possible, as close to off-the-shelf as possible.

And obviously if engineering such a strongback proved too expensive and not cost effective, then the idea would loose out to purpose built boosters.

But, I don't think using F9's or Atlas V's would be as difficult as you think.  The strongback adaptor shouldn't be all that expensive to develop  I'd think certainly cheaper than a brand new booster from scratch, with all the plumbing, structure, and tanks associated with that.  It's certainly not to be dismissed as negligable, but I don't think they'd be deal breakers either.  Be interesting to see some PDR's anyway.  :-)
Title: Re: Delta 4 based advanced boosters
Post by: hkultala on 04/05/2013 05:24 am
This seems to be the most stupid "SLS improvement idea" I've yet seen.

Hydrolox for boosters?

Hydrolox has a very good isp, but loses to kerolox on practically all other aspects (thrust/weight, price/weight, tankage size).

In boosters lots of thrust(for as cheap as possible) is needed, and the boosters itself can be volume constrained. And isp is not very important, even though it has slight effect.

So, all the pitfalls of hydrolox and gaining only little from the strong points of hydrolox. Kerolox booster with higher propellant mass but less engine mass, less tankage mass is just cheaper and lifts more.


And it seems now you are no longer proposing a using delta iv as booster but lenghtened one with three engines which means it's a completely different thing than the original, and you have to spend the big R&D costs.


And about flyback. ULA has NO PLANS of recovering delta iv first stages, it's designed to be always expendable, and the engines are not designed to be used multiple times.

Some other(kerolox-based) options however have been developed to be reusable(thought they have not yet been recovered); The RD-170/RD-180/RD-190 family is designed to be reusable, SpaceX develops it's engines to be reusable.
Title: Re: Delta 4 based advanced boosters
Post by: FinalFrontier on 04/05/2013 05:31 am
Lobo's thought experiment about using Falcon cores as SLS boosters is a good way to think about how to keep development costs down. I still prefer the idea of using dual delta 4 tank sets on each side with a new thrust structure that would support 4 RS-68A engines for a total of 2.8 million pounds thrust per side.  Only eight engines, using the same LH2 as the SLS core and with a potential for a flyback version in the future.
This would seem to require only a new thrust structure and external attachment frame to put it into the running as an alternative to designing a entirely new booster.

No its not that would never work. Horrible suggestion.
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/05/2013 04:34 pm
Lobo's thought experiment about using Falcon cores as SLS boosters is a good way to think about how to keep development costs down. I still prefer the idea of using dual delta 4 tank sets on each side with a new thrust structure that would support 4 RS-68A engines for a total of 2.8 million pounds thrust per side.  Only eight engines, using the same LH2 as the SLS core and with a potential for a flyback version in the future.
This would seem to require only a new thrust structure and external attachment frame to put it into the running as an alternative to designing a entirely new booster.

No its not that would never work. Horrible suggestion.

Yea, I think Delta IV would be much harder to make into boosters than Atlas V or Falcon 9.  It's a fun exercise nonetheless.

I think probably the only real way to have Delta IV derived boosters for SLS is to directly mount multiple cores to the SLS core directly like AJAX (but obviously that won't happen, as NASA won't redesign the SLS core as has been discussed on these hypothetical theads).
OR...widen the core up to 10m, and put a jettisonable engine ring of RS-68A's around the bottom like Saturn V-B.  Keep RS-25's under the core in a four-engine tight cluster, but have like six RS-68A's outboard that all fall away during ascent.  Then you get rid of the cheap RS-68's early, but keep the high performance RS-25's all the way to suborbit.
Keep the RS-68A's pushed to the outter dimension so they aren't trapped under the core exposed to hot gas buildup. 
Maybe it a 1.5 stage to orbit core.
That's a completly different core than current SLS though...although kind of a cool concept.  So probably best to steer clear of hydrolox "boosters" for it.
Title: Re: Delta 4 based advanced boosters
Post by: edkyle99 on 04/05/2013 07:37 pm
This seems to be the most stupid "SLS improvement idea" I've yet seen.

Hydrolox for boosters?

Hydrolox has a very good isp, but loses to kerolox on practically all other aspects (thrust/weight, price/weight, tankage size).
LOX/RP beats LOX/LH2 on size and, all other things being equal, on cost - but not by a large margin. 

Shuttle LRB studies by Martin Marietta and GD during the late 1980s showed that LOX/RP boosters had a less than 5% operational cost advantage over LOX/LH2 boosters.  GD nonetheless recommend LH2 (Martin Marietta went with RP). 

The difference today is that RS-68 exists, while the RP engine does not, which would make a big difference in development costs.  RD-180 exists, but would need to be built in the U.S., which would make it $$$$.  A gas-generator RS-68 might even beat a U.S.-built staged combustion RP engine on production cost.

RS-68 is a booster engine, so merits consideration - though I would suggest only for a new 5.5+ meter diameter booster.  T/W is not an issue, because an LH2 booster will only weigh 70% as much as an RP booster.  LH2 requires LESS thrust, and thrust costs money. 

RS-68 is good enough for NRO and its multi-billion dollar satellites.  Why not SLS?

 - Ed Kyle 
Title: Re: Delta 4 based advanced boosters
Post by: sdsds on 04/06/2013 04:39 am
Maybe it's worth stating the obvious: this would be a version of SLS powered by Rocketdyne engines. (SSME, RS-68, J-2X.) One would expect that for this design to get much consideration Rocketdyne would need to champion it. That might be difficult for them right now. The acquisition of Rocketdyne by GenCorp was expected to close in the first half of 2013. Any word on that?
Title: Re: Delta 4 based advanced boosters
Post by: a_langwich on 04/07/2013 02:50 am
This seems to be the most stupid "SLS improvement idea" I've yet seen.

Hydrolox for boosters?

Hydrolox has a very good isp, but loses to kerolox on practically all other aspects (thrust/weight, price/weight, tankage size).
LOX/RP beats LOX/LH2 on size and, all other things being equal, on cost - but not by a large margin. 

Shuttle LRB studies by Martin Marietta and GD during the late 1980s showed that LOX/RP boosters had a less than 5% operational cost advantage over LOX/LH2 boosters.  GD nonetheless recommend LH2 (Martin Marietta went with RP). 

The difference today is that RS-68 exists, while the RP engine does not, which would make a big difference in development costs.  RD-180 exists, but would need to be built in the U.S., which would make it $$$$.  A gas-generator RS-68 might even beat a U.S.-built staged combustion RP engine on production cost.

RS-68 is a booster engine, so merits consideration - though I would suggest only for a new 5.5+ meter diameter booster.  T/W is not an issue, because an LH2 booster will only weigh 70% as much as an RP booster.  LH2 requires LESS thrust, and thrust costs money. 

RS-68 is good enough for NRO and its multi-billion dollar satellites.  Why not SLS?

 - Ed Kyle 

RS-68A exists, but does not have the safety margins or instrumentation NASA would demand to launch people.  In study after study where RS-68s were clustered together, the inability of the existing nozzles to withstand those thermal conditions have been highlighted.  The fuel rich startup also highlights a condition which would need redesign.  There are a handful of prototypish (RS-84), past-production (F-1), or variations of current production (AJ-26) kerolox hardware out there.  (SpaceX of course has M1D in production now.)  I suspect some of those would not be too different in development cost than modifying the RS-68.

Rather than arguing about what will cost how much, that's a question easily decided by the bidders on an advanced booster contract.  If, that is, the bidder can muster the credibility they can execute said design for the bid amount--that's probably why NASA has paid(?) Dynetics to do some exploratory work.  If some enterprise can win the bid with an RS-68 booster, I'm all for it. 
 
Production cost is not likely to be a factor here, the cost of infrastructure to allow continued production will continue to be the cost driver IMO.  On production cost, (expendable) solids would clean up, would they not?  RS-68's wonderful production cost hasn't helped Delta IV be very competitive on cost grounds, has it? 

Why isn't thrust-to-weight an issue?  Doesn't the booster have to help lift not just its own weight, but help with the core and up?  And true, LH2 is lighter, but it's larger, and doesn't that structure carry weight?  Doesn't an Atlas 5 core have a large T/W than a Delta IV core?  Thrust costs money, so why is Atlas 5 the preferred single-core ULA offering?  I'm not arguing these in terms of hypothetical hydrolox vs kerolox, we are actually talking about Delta IV hardware, and an AJ-26 based kerolox booster might be similar to Atlas hardware.
Title: Re: Delta 4 based advanced boosters
Post by: robert_d on 04/07/2013 03:41 am

The further alternative would be to use the STS main engine reusable version.  Production could be restarted in parallel to the expendable version for the core. It was a tragedy that just as this engine was mature enough to justify reuse vs. disposal the space shuttle was cancelled. The additional gain in ISP trumps any of the arguments for kerolox, IMO.  About six would be needed per side and could be built into the new adapter that would replace the current thrust structures and interface with the SLS core. This would eliminate ANY cost for upgrades and manrating beyond what will be incurred by the RS-25E program.

Also note that what is considered a detriment fo hydrolox by some (the larger tank size) actually becomes a benefit if the floatback concept could be implemented, in that the large volume of the H2 tanks would be filled with hydrogen gas, thus reducing the size of the external inflated blimp-like gas bag.  Floatback would be slow and gracefull with airbags deployed so that the booster would land horizontally.  Once the hydrogen was purged, it could be towed back to a horizontal integration facility.  No fast deceleration, landing legs, or other time critical events required once the gas bag is inflated.
   
Title: Re: Delta 4 based advanced boosters
Post by: edkyle99 on 04/07/2013 05:28 am
Why isn't thrust-to-weight an issue?  Doesn't the booster have to help lift not just its own weight, but help with the core and up?  And true, LH2 is lighter, but it's larger, and doesn't that structure carry weight?  Doesn't an Atlas 5 core have a large T/W than a Delta IV core? 
Liftoff thrust to weight ratio is the same for Delta 4M and Atlas 5-401.
Quote
Thrust costs money, so why is Atlas 5 the preferred single-core ULA offering?
In part because it uses Russian-built engines that cost less than U.S. built engines. 

 - Ed Kyle
Title: Re: Delta 4 based advanced boosters
Post by: MATTBLAK on 04/07/2013 05:54 am
In our many forays into 'Rocket Lego' I don't remember but has anyone ever crunched the numbers for a Delta IV core with 2 and/or 4 Atlas V cores as strap-ons? ('3 stick' & ''5 stick' configurations, with various upper stage options).
Title: Re: Delta 4 based advanced boosters
Post by: Proponent on 04/07/2013 03:23 pm
Liftoff thrust to weight ratio is the same for Delta 4M and Atlas 5-401.

On the other hand, the thrust-to-weight ratio of a lower-Isp booster like the Atlas necessarily increases faster.
Title: Re: Delta 4 based advanced boosters
Post by: edkyle99 on 04/07/2013 04:11 pm
Liftoff thrust to weight ratio is the same for Delta 4M and Atlas 5-401.

On the other hand, the thrust-to-weight ratio of a lower-Isp booster like the Atlas necessarily increases faster.
Yes, resulting in a bit less gravity loss, which the LH2 rocket makes up for in part by working at higher ISP. 

Upper stage differences account for much of the performance differences between the two EELV Mediums.  Centaur weighs several hundred kilograms less than the 4-meter DCSS at burnout, despite carrying more propellant, which translates directly into more payload.

 - Ed Kyle
Title: Re: Delta 4 based advanced boosters
Post by: edkyle99 on 04/10/2013 04:31 pm
This article says that RS-68 costs have dropped 30% as a result of the EELV block buy.

http://www.spacenews.com/article/launch-report/34773pwr-sees-better-days-ahead-after-post-shuttle-dropoff#.UWWTnUq1Uyd

 - Ed Kyle
Title: Re: Delta 4 based advanced boosters
Post by: edkyle99 on 04/11/2013 04:36 pm
Here's a look back at Advanced Booster concepts circa 2001, as reported by MSFC.  The RS-68 booster then envisioned used five engines for ATO, etc., which is more than needed for SLS.  The height constraint mentioned for the Shuttle application does not apply to SLS.

 - Ed Kyle
Title: Re: Delta 4 based advanced boosters
Post by: a_langwich on 04/11/2013 05:01 pm
Why isn't thrust-to-weight an issue?  Doesn't the booster have to help lift not just its own weight, but help with the core and up?  And true, LH2 is lighter, but it's larger, and doesn't that structure carry weight?  Doesn't an Atlas 5 core have a large T/W than a Delta IV core? 
Liftoff thrust to weight ratio is the same for Delta 4M and Atlas 5-401.

Ah, I see.  I had looked up the datasheets on your website, but didn't notice the Atlas 5 CCB thrust figures were vacuum, while I had used Delta IV CBC thrust figures for SL.
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/11/2013 05:09 pm
This article says that RS-68 costs have dropped 30% as a result of the EELV block buy.

http://www.spacenews.com/article/launch-report/34773pwr-sees-better-days-ahead-after-post-shuttle-dropoff#.UWWTnUq1Uyd

 - Ed Kyle

Ed,
I think you've made the case for the RS-68A of being a viable booster engine (and NASA is evaluating it from the sounds of things, along with the F-1B and a large ORSC kerolox engine).
But, unless we could mount the entire D4H to each side of the SLS core (which I don't think we can based on the width in that direction of the ML opening per the Advanced booster solocitation, that dimension is only like 9.3m, and D4H will be over 15m in that direction...even FH will be too long that way), and since I don't see NASA redesigning the SLS core to mount the booster directly like AJAX, I just think any monolithic hydrolox core would have to be too wide to mount the three RS-68A's required.  I think that's the problem with going with hydrolox boosters using RS-68A's.  (although maybe I'm wrong on that).

Not to go OT, but your arguments did get me thinking about other hypothetical options...very hypothetical...

Maybe if the SLS core was shrunk down to about 6.5m wide.  Then it could mount two 6.5m wide hydrolox boosters.  (NASA seems adverse to having more than two boosters period, so I'm exploring this route).
So you have basically a D4H on steroids, each using RS-68A.  If they were long enough, maybe they could have a four RS-68A MPS, but the central core would only mount two RS-68A's when launching with boosters, so it would burn longer.  The outboard cores would have four engines, and burn out faster and be jettisoned.  That'd be about 6.4Mlbs of thrust at liftoff. 
RS-68A's aren't as efficient when used as a sustainer, I understand, but maybe the "Block 1B" upper stage, which would be 6.5m wide now, could be longer with like six RL-10's, so they can do more of the ascent boost than current Block 1B upper stage would, and stage sooner.  Maybe more like a JUS.  (A J2X could be an option on it too)
Such a system could launch as a single stick too, with the same upper stage, and four RS-68A's on the core, and stage earlier with the upper stage doing more of the ascent. 
It'd have a modified (shorter) upper thrust beam like STS/SLS, but that would be left off for a single stick launch.
This would require RS-68A to be man-rated of course, unless this was the cargo-only heavy lifter and Orion was launched on Atlas V-55x with a new 5m common upper stage (Jim said once that that LV could lift a fully fueled Orion to LEO) and return to the 1.5 launch system.  Then this HLV would not need to be man-rated. 
Iíll assume if this could get around 110mt or so to LEO, then with Orion launching on Atlas V, the combined LEO capacity could be the mandated 130mt.
If the SLS will be about 200ft tall, then Iíd assume a similar length for the 6.5m cores.  Hopefully thatís enough for four RS-68A engines.  (They could even mount some GEM-60ís each.  More synergy with EELV and again, itís a cargo launcher).

Yea, I know itís probably too much divergence form the baseline SLS core to be in the realm of ďreasonableĒ hypothetical, but really, with no metal bent, itíd just be redesigning to make about 2m skinnier.  The booster mounts and upper thrust beam would be pretty similar.  Height for the tower would be about the same.  Itís just shrinking the core and fattening up the boosters, and making all 3 cores common.  Delta IV booster mounts could be used on the bottom, for more synergy.  Most things would actually remain the same.  Those RS-68A prices would probably drop even more.

Any idea of the capacity of something like that?
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 04/11/2013 05:22 pm
Here's a look back at Advanced Booster concepts circa 2001, as reported by MSFC.  The RS-68 booster then envisioned used five engines for ATO, etc., which is more than needed for SLS.  The height constraint mentioned for the Shuttle application does not apply to SLS.

 - Ed Kyle

Ahhh...I posted mine before I saw you just posted this.

Well, 5.8m wide and 200ft tall could feed five RS-68's?  Is that with a burn time similar to the STS 4-seg SRB's or something?  I wouldn't think that'd be enough volume to feed five engines for every long. 
But, I suppose you really only need a burn time similar to SRB's, the three RS-25's on the orbiter are designed to take over the ascent from SRB sep.  Although, would hydrolox boosters separating at the same time as the STS 4-segs accelerate STS to the same speeds?  Or do they need a longer burn to get the same speed before separation?

Today, with maybe four RS-68A's on each, and making them just a bit narrower to about 5.5 or 5.6m (5.76 the absolutely max?), and keeping the same height, that could be a viable booster for SLS.  The four RS-25's on the core are designed to take over ascent after SRB separation at about 124 seconds into launch, although again, would the core be going the same speed with 124 seconds of burn from hydrolox boosters as it will be from 5-seg SRB's? 

Doesn't the extra SRB thrust give it better accelleration during those first two minutes?

Anyway, to dovetail on my earlier post, fatten these up to about 6.5m, and launch two of them on an identical 6.5m core all with RS-68A's.
Maybe five engines on the boosters, and 2 on the core.  The core won't go all the way to orbit probably with the low ISP RS-68A's, but it could burn for longer than the boosters.  Maybe burn for 6 minutes and let the upper stage do the rest?
Title: Re: Delta 4 based advanced boosters
Post by: Lobo on 07/15/2013 10:24 pm
Here's a look back at Advanced Booster concepts circa 2001, as reported by MSFC.  The RS-68 booster then envisioned used five engines for ATO, etc., which is more than needed for SLS.  The height constraint mentioned for the Shuttle application does not apply to SLS.

 - Ed Kyle

Ed,
How many RS-68's would be needed for SLS?

Five RS-68A's would produce about as much take off thrust as the 5-seg booster or two F-1B's.

Would three be enough?  Or not enough thrust?

The five RS-68 version would be interesting, especially if the four outter engines could be jettisoned like the S-1D or Atlas.  Not so much as a booster advantage, but if say ULA were to want to bid this "Delta derivative" CCB for the SLS Advanced booster competition, and if successful, they were to retire Delta IV entirely, let Atlas V take over the Delta IV-medium work, and let this booster replace the D4H for that 25mt range.

The outter four RS-68's are jettionsed at maybe 120-150 seconds into flight, with the core burning for perhaps another 150-200 seconds with the single RS-68A.  Then a 5m DCSS or probably better yet, ACES on top.

A variation could be having an SLS RS-25E in the central mount, and letting that engine burn the core almost to orbit.  (Especially if the central RS-68A would have base heating problems) Maybe not the cheapest 25mt class vehicle, but it'd use existing engines, an existing core (if ULA were to compete and win the booster competition), and an existing upper stage.  Even the DCSS would give good BLEO capability with a sustainer core burning almost to orbit with an RS-25E.

The booster version could omit the central mount entirely, and run the RS-68A's at full throttle for longer until the booster are burnt out. They could omit the hardware needed to jettison the ring too for the booster version.

Seems pretty unlikely that ULA would retire the 5m D4 CCB for this, but in a hypothetical world where they would be interesting in getting that SLS booster work, it's not completely inplausible as D4-medium is redundant with Atlas V, and this CCB would be redundand with D4H. (Since we've established that a tri-core D4H is inplausible to use as a booster itself...as was the genesis of this thread).
It would add two engines to the LV over D4H, but also remove two cores.

A smaller version could be three RS-68's in line on that aft skirt, but I'm not sure if only three would be enough thrust for SLS in the booster configuration?  Be fine I think for a D4H class replacement.