Saturday, July 23, 2022

A Reusable SLS? UPDATED: July 31, 2022.

 Copyright 2022 Robert Clark

 The SLS is now projected to cost $4 .1 billion per flight. Because of that severe cost it is projected to only fly once per year. This can not form the basis of a sustainable Moon colonization plan. But suppose we could make the SLS reusable? It’s already known the side boosters can be made reusable as with the shuttle program. The engines on the SLS core stage were derived from the shuttle engines which were intended to be reused up to 100 times. However, since the SLS was intended to be expendable the shuttle-derived engines on the core were designed cheaper to be expendable. However, any rocket engine even an expendable in reality is reusable at least 10 times or more. This is because they have to be certified for several firings for testing purposes. This is described by the well-regarded space expert Henry Spencer:


From: Henry Spencer <>


Subject: RLV engines (was Re: X-33 Concepts: Lockheed, Mac Dac, Rockwell)

Date: Wed, 19 Jun 1996 13:03:12 GMT

In article <4q6am4$> (Andy Haber) writes:

>I think this is an area where critics can speak the loudest.  Today's 

>existing engines all leave something be be desired as true, good SSTO engines.

>This is mostly due to history.  Most engines (other than SSME's) were 

>designed for ELV's, not SSTO's.

Actually, this does not have a lot of bearing on their suitability for

RLVs.  Most ELV engines are, despite their application, reusable, because

they have to be developed and tested.  The F-1 was specified for 20 starts

and 2250s of life, the J-2 for 30 and 3750s.  Six F-1s ran over 5000s each

as part of the service-life tests.  DC-X's RL10s looked "pristine" after

20 starts; the RL10 is nominally rated for 10 starts and 4000s of firing.

>...In terms of using SSME's, sure those can used,

>although doing something to reduce the required level on maintenance on

>the existing engines is quite desirable...

Unfortunately, it probably can't go far enough.  Rocketdyne's own estimate

was that, with a *lot* of work, you could probably get SSME maintenance

costs down to $750k/engine/flight, which is unsatisfactory if you're aiming

for really large cost reductions.


If we feared danger, mankind would never           |       Henry Spencer

go to space.                  --Ellison S. Onizuka |


 Then even reusing the vehicle 10 times could result in a factor of 10 reduction of launch cost, if the maintenance cost could be kept relatively low. That quote about $750, 000 maintenance cost after a lot of work may seem low but from memory I recall it being in the range of $1 million to $2 million per engine after several years into the shuttle program.


 But how to land the SLS core? Starting the SSME’s is a complex process. Modifying them to be air-startable would not be trivial. Instead, I suggest using the method proposed for making the Centaur a lunar lander, multiple pressure-fed side thrusters for a horizontal landing. 

Robust Lunar Exploration Using an Efficient Lunar Lander Derived from Existing Upper Stages. 

 Note then that for a stage reentering to Earth broad-side almost all the reentry velocity is burned off aerodynamically just by air drag so that the stage reaches terminal velocity at approx. 100 m/s. For a stage nearly empty of fuel, this low amount of velocity could be cancelled relatively easily by pressure-fed thrusters with the thrusters running on just the residual of propellant left in the tanks.

 About the landing, there would be additional development cost for the horizontal landing thrusters.  But pressure-fed thrusters are a relatively simple technology. Compare for example the time SpaceX spent developing the Draco thrusters on the Dragon to the time developing the Merlin engine. And from discussion of the thrusters on the Starship they seem more like an afterthought compared to the cost, time, and complexity put into the Raptor engines.

 How about giving the RS-25’s on the SLS core restart capability? Again I’ll refer to the redoubtable Henry Spencer:



From: (Henry Spencer)

Subject: Re: One part Oxygen, two parts Hydrogen and BOOM!

Date: Sat, 14 Oct 2000 03:37:23 GMT

In article <>,

Pete Zaitcev <> wrote:

>> The SSMEs use "torch" igniters, little oxygen/hydrogen burners firing into

>> the preburners and chambers.  The igniters themselves are ignited by,

>> essentially, high-tech spark plugs.


>I see... obviously there cannot be a spark in a vacuum.

Not entirely true, but irrelevant -- when the igniter fires up, there's an

oxygen/hydrogen gas mixture there for the spark to travel through.

>Is the plug the reason engines cannot be restarted in orbit or

>there is more to the story?

There's nothing *fundamental* in the SSME which makes an in-space restart

impossible -- no one-shot parts or anything like that -- but it's a

complicated engine which has to be set up exactly right for a successful

start, and ground equipment (and gravity!) helps out with that.  It would

not be difficult to develop a variant which could start itself in space,

but there has been no reason to do that.


Microsoft shouldn't be broken up.       |  Henry Spencer

It should be shut down.  -- Phil Agre   |      (aka


 So likely it could be done by Aerojet, but I have no confidence they could do it in an affordable manner. Or more precisely, I have no confidence they would do it at an affordable price charged to NASA. For instance the RS-25 engine used on the SLS is derived from the SSME. It was expected to be cheaper than the SSME as it it used a lower parts counts and was not required to have the 100 times reusability of the SSME. But instead Aerojet charged more for this engine than the SSME even when accounting for inflation:

NASA will pay a staggering $146 million for each SLS rocket engine. The rocket needs four engines, and it is expendable. ERIC BERGER - 5/1/2020, 6:55 PM

 About the payload lost on reusability, a stage that goes to LEO can remain in orbit for a few orbits to come back over the landing site so minimal propellant is burned to return to launch site. 

 If we do use a large upper stage, then the SLS would not go to orbit and as SpaceX showed you would need minimal fuel burned if landed down range, and so minimal payload lost, rather than returning to launch site. However, there is then the cost of the upper stage. If it were the Ariane 5/6, the cost of the Ariane 6 being as low as $77 million, it should be even lower than that without the Ariane side boosters or upper stage. 

ARIANE 6 VS. SPACEX: HOW THE ROCKETS STACK UP The European Space Agency is planning to use the Ariane 6 for a variety of missions. ESA MIKE BROWN 1.24.2022 2:00 PM In January 2021, Politico reported that the Ariane 6 could launch for as little as $77 million. That’s a steep discount from the $177 million price tag for the Ariane 5.

  About the landing thrusters, I wouldn’t give a contract for it to any of the usual aerospace companies under NASA’s cost-plus contracts. Instead I would prefer doing it “in house”, so to speak. I was quite impressed by a team at Johnson Space Center led by chief NASA engineer Stephen Altemus developing an unmanned lunar lander for only $14 million development cost:

The Morpheus lunar lander as a manned lander for the Moon.

  The approach the NASA team used on saving costs was likely analogous to that used by commercial space in cutting costs.  No doubt also the pressure-fed engines being used rather than complex turbo-pump engines contributed to the low development cost.


 This report estimates the launch market as approx. $48 Billion per year by 2030:

Global Space Launch Services Market is projected to reach at a market value of US$ 47.6 Billion by 2030: Visiongain Research Inc October 05, 2021 09:33 ET | Source: Visiongain Ltd

 At a going rate of approx. $10,000 per kilo to LEO that would amount to 4,800 tons to orbit.  For a SLS lofting nearly 100 tons to orbit even in SLS 1 form, that’s quite a lot of launches it could take part in per year IF it could do it at a competitive price. If  it could do 10 reuses, that could bring the price down to $400 million per flight, or $4,000 per kilo, about the price of the reusable F9 when new, or a bit more than $3,000 per kilo of the used F9. But IF it could do 20 reuses, within the capabilities of some expendable engines, it would be $2,000 per kilo which would beat even the F9 used reusable price.

 In point of fact though the first four SLS vehicles would all use original Space Shuttle engines. Then likely each has dozens of uses left in their operational lifetimes:

Apr 6, 2021
RS-25 Rocket Engines Return to Launch NASA’s Artemis Moon Missions.


   Robert Clark



  1. "The SLS is now projected to cost $4 .1 billion per flight. Because of that severe cost it is projected to only fly once per year."
    False, the OIG report where this estimate originates puts each SLS rocket at 2.2 billion and 4.1 billion for each Artemis mission.
    The high cost is a direct product of the low flight rate
    The best way to lower SLS launch costs is to fly it more frequently, reusability makes no economic sense for a rocket that best case flies only a few times per year and results in only being a large burden on rocket performance and complexity

    "NASA will pay a staggering $146 million for each SLS rocket engine. The rocket needs four engines, and it is expendable."
    Also false, this estimate takes all the money NASA has spent on developing the new RS-25 variants, reopening the engine production line, building a new factory to increase engine production, refurbish the 16 already built Shuttle era RS-25s, build 2 new test engines AND build 24 new flight engines and divides all of that cost by the number of the new engines NASA has ordered, resulting in an incredibly bloated and incorrect estimate.
    As of now we don't know the cost of each new RS-25 engine but we do know that they are at least 30% cheaper to manufacture than the Shuttle era SSMEs.
    Considering those used to cost up to 80 million accounting for inflation the new engines are probably in the 50-60 million dollar range.
    Still very expensive but nowhere near the amount people have been misled to believe

    1. A $2 billion cost per launch makes it even easier to be profitable since at only 10 launches to bring the cost down to $2,000 per kilo.

      As I said a launch market of $48 billion amounting to ca. 4,800 tons to orbit per year could allow the SLS to make multiple launches per year, but clearly it couldn't do that if you had to build an entire new rocket every launch.

      Bob Clark

  2. If you launch one SLS vehicle per year, launch cost will probably be $4 billion. But if you launch two, the cost would probably be cut in half. With two SLS launch facilities, four SLS launches might be possible in a year possibly cutting the cost to $1 billion per launch.

    One problem with de-orbiting the core stage is that it damages the ozone layer the same way meteors reentering the atmosphere do naturally. And this is true of all space vehicles re-entering the Earth's atmosphere from orbit.

    Secondly, keeping the core stage in orbit and repurposing it could be a lot more valuable. If fact, I believe you could easily sell the core stage for $5 billion if you added docking ports and airlocks to the empty propellant tanks in orbit. This would give any nation or company that can launch even the tiniest habitat modules the ability to instantly add more than 2000 cubic meters of pressurized volume (more than twice the pressurized volume of the ISS).

    Empty SLS core stages could also be used for:

    1. Water (from Earth) and sewage (from space habitats) depots
    2. Liquid oxygen and liquid methane depots
    3. Rotating artificial gravity crop and orchard farms (much cheaper to grow food in space than to import it from the Earth's gravity well).