SSME based SSTO’s.

[Exoscientist]

 In actuality practical SSTO’s have been feasible since the 70’s, with the development of the high performance Space Shuttle Main Engine(SSME). Here’s a SSTO that modifies the Delta IV first stage using carbon fiber tanks and two SSME’s to replace the RS-68 engine used on the Delta IV.

SSME based SSTO’s.

https://exoscientist.blogspot.com/2021/06/ssme-based-sstos.html

 

  Robert Clark

Edited June 24, 2021 by Exoscientist

[tater]

How does 10% buy reusability?

It needs full TPS, right? Then what? Wings, landing gear, etc?

Also, even 1 SSME is $100M.

[RCgothic]

And basically rebuilt between flights. That's not "cheap frequent access to space" in my book.

[wumpus]

Feasible since the 1970s?  Not with recovery.

I'm really curious how you are going to recover a Delta III with only 800kg of parts/fuel slotted for reuse.  And remember to design the computer controls with 1970s space-rated parts.  Here's what they used: https://www.history.nasa.gov/computers/Ch4-3.html

Sure, you could probably get something a little more DSP friendly than the IBM360 architecture, but don't expect to land retropropulsively.  As far as parachutes go, each Shuttle SRB had a dry mass of 91 tons and parachutes weighing 3510kg.  Scaling the parachutes down for the mass of the Delta III, so the parachute budget is slightly high.  Unfortunately, those parachutes are designed for Shuttle SRBs which are little more than thick steel tubes.  What happens when a SSME engine smacks into the water at 23 m/s (53mph.  This is lower than I remember and I'm doubting the infallible wikipedia)?

Rebuilding the SSME between flights was more expensive than building new kerolox engines for every flight.  And that was a gentle landing on a runway, something you aren't getting for 800kg.  Good luck finding anything you can rebuild after plunging into saltwater at 23 m/s (or a lot more  if memory serves).

Remember Earth isn't Kerbol.  SSTO doesn't make sense with any known chemical with 3-digit Isp.

[tater]

43 minutes ago, RCgothic said:

And basically rebuilt between flights. That's not "cheap frequent access to space" in my book.

Yeah, the refurb costs on SSMEs are nuts.

@Exoscientist can take the same concept (and illuminate what the rough recovery concept is), switch to methane, and buy Be-4 engines. The I_sp is lower (which might not make it close), but the cost for a supposedly rapidly reusable engine is under $7M. Alternately, I could imagine a hydrolox Be-4 being at least plausible to develop, sacrificing some thrust for I_sp (and Be-4 is substantially higher thrust than RS-25 so there is room there for loss).

[DDE]

5 hours ago, RCgothic said:

And basically rebuilt between flights. That's not "cheap frequent access to space" in my book.

Laughs in Space Shu... oh, wait

[magnemoe]

SSTO is an pipe dream / money sink until we get much better engines or launch systems. 
Skylon is the only one who kind of make sense but even it looses to an starship style launcher outside of flexibility if you can refuel before burn. 
Else you need beamed power or an very good fusion reactor. 
Note that planes run into the rocket equation  on long flights. An direct London to Sidney flight will be more expensive than one with Dubai as an hub. 

[jimmymcgoochie]

Fully recoverable two stage to orbit designs will trounce an SSTO rocket- the first stage engines can be optimised for operating in thick atmosphere and the second stage engines for vacuum, whereas an SSTO needs to try and do both which adds complexity.

Shorter burn times for a two-stage rocket mean less engine wear, reducing maintenance costs; returning the first stage from a fairly low speed and altitude with a smaller second stage coming back from orbit reduces the requirement for thermal shielding compared to an SSTO, assuming you’re going to recover it from orbit rather than just leaving it up there; and even if you’re using a winged SSTO that flies itself to orbit with air-breathing engines (Skylon/SABRE style) the extra mass from all that wing-related stuff, plus the larger surface area that needs shielded from re-entry heat, would reduce the payload to orbit as well as the maximum payload size as it has to fit inside the spaceplane’s payload bay, which would need to be narrow for aerodynamic reasons,

[wumpus]

Quick math note:  The original calculations assumed  that the shuttle engines would get full Isp all the way to orbit.  Fixing that leads to a delta-v of ~8500m/s.  Of course, since it launches with a thurst/weight ratio of ~1.9, you should be able to build that fuel tank to 125% of the original size.  This gives you a thrust to weight ratio of 1.5, and a payload to LEO of 10 tons.  Unfortunately, it also means that you can't use the same rocket as a TSTGeosync, which makes it far less useful.

And of course if you can recover a SSTO, you can recover a TSTO booster vastly easier.  And the mass budget in the booster recovery can be hidden in the mass of the second stage.  The final mass budget for recovering the final stage will then be  at least 1/10th of what it would mass to recover a SSTO, which is why Starship is at least in the prototype stage and nobody is building a SSTO.

I would even recommend a three-stage design for a Starship follow on.  The idea would be to add air-augmented boosters that would return to launch site.  Then the main superheavy booster would use "starship tricks" (skydiver landing, stainless steel hull) to be recoverable from the highest delta-v it could manage.

The pros:

A significant amount of mass would return  to launch site on its own.  Presumably the savings in moving the now lighter super heavy booster back home would be less than the cost of mating the extra boosters (I'm sure this isn't true now, but spacex has a lot of experience in mating final stages to used boosters).

Significant fuel savings.  Moreso if the initial booster is vertically staged, but I've always thought of these as "strap on boosters".  Not sure which is cheaper to mate.  Granted, I don't know how much Isp gain you get with air augmented methane.  Note that while this is silly thing now (spacex saves 10 times the money fishing fairings out of the ocean than it costs to fuel a Falcon 9), if you start planning now by the time you deploy such a rocket you can expect that fuel will be a more significant cost of a rocket. 

Cons:

Extra stages aren't that cheap, and integrating them is a challenge.

I'd like to think that Superheavy booster is already pretty close to as much delta-v as it can get without the active cooling tricks.  But it seems like it cuts out far shorter than you'd expect a rocket with identical engines in each stage (about halfway up in terms of delta-v is where I'd start my calculations.  Possibly closer to 2/3rds with a third stage).

I was going to say no air-augmented rocket had been made, but it appears that a prototype for Gnom was not only designed, but launched (and worked).  I'm also not sure that metholox would be that improved by air augmentation.

 

Edited June 23, 2021 by wumpus
lost "get"

[Exoscientist]

On 6/22/2021 at 11:10 AM, tater said:

How does 10% buy reusability?

It needs full TPS, right? Then what? Wings, landing gear, etc?

Also, even 1 SSME is $100M.

 The price for a SSME during the shuttle era was about $40 million. The recently quoted price of $100+ million for the RS-25 is because of costs of restarting the production line and because, frankly, for the SLS everything is inflated.

 With 100 reuses, the SSME price per engine would be $400,000 per engine per flight, a small proportion of the $100 million for a ca. 10 ton to LEO expendable launch vehicle.

 The Apollo era thermal protection was at about 15% of the landing weight. Modern materials cut this to about half that, for example with SpaceX’s PICA-X thermal protection material, or also with the Space Shuttle thermal protection, so call it 7%:

https://en.m.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system#Weight_considerations

 Commonly landing legs might be 3% of the landing weight. With composites probably can get it to about half that, call it 1%:

https://yarchive.net/space/launchers/landing_gear_weight.html

 A cylindrical body reentering broadside doesn’t even need wings. Almost all the orbital speed will still be cancelled out during reentry. Assuming a powered-final landing, you would only need to keep propellant reserve onboard to cancel out out terminal velocity, say, less than 100 m/s.

 By the way, I’m not sure that the powered-landing approach SpaceX prefers is the way to go. Using wings instead with modern composites, probably can get wing weight down to 1% to 2% of landing weight if you use short, stubby wings a la Skylon, the X-37, and the X-15.

  Note, I was using the SSME in my blog post to indicate SSTO’s actually have been doable since the 70’s. You don’t need any advanced tech to do it like nuclear engines. However, if I were to actually make an SSTO out of the Delta IV I would probably just add altitude compensation to the RS-68 already on the first stage. This way you could even get the vacuum Isp to the 465+ s range. There are many different ways of getting altitude compensation on rockets using already known tech.

 

  Robert Clark

Edited June 24, 2021 by Exoscientist
Added link to Space Shuttle thermal protection

[Exoscientist]

On 6/22/2021 at 12:57 PM, wumpus said:

Feasible since the 1970s?  Not with recovery.

I'm really curious how you are going to recover a Delta III with only 800kg of parts/fuel slotted for reuse.  And remember to design the computer controls with 1970s space-rated parts.  Here's what they used: https://www.history.nasa.gov/computers/Ch4-3.html

Sure, you could probably get something a little more DSP friendly than the IBM360 architecture, but don't expect to land retropropulsively.  As far as parachutes go, each Shuttle SRB had a dry mass of 91 tons and parachutes weighing 3510kg.  Scaling the parachutes down for the mass of the Delta III, so the parachute budget is slightly high.  Unfortunately, those parachutes are designed for Shuttle SRBs which are little more than thick steel tubes.  What happens when a SSME engine smacks into the water at 23 m/s (53mph.  This is lower than I remember and I'm doubting the infallible wikipedia)?

Rebuilding the SSME between flights was more expensive than building new kerolox engines for every flight.  And that was a gentle landing on a runway, something you aren't getting for 800kg.  Good luck finding anything you can rebuild after plunging into saltwater at 23 m/s (or a lot more  if memory serves).

Remember Earth isn't Kerbol.  SSTO doesn't make sense with any known chemical with 3-digit Isp.

 There is an important difference between SSTO being technically impossible, and SSTO’s being uneconomical. Unfortunately, it has always been presented as the former, even when it wasn’t. During the earliest days of spaceflight it was technically impossible for chemical propulsion. But that was before we had advanced engines such as the SSME’s and lightweight materials such as carbon composites.

 Unfortunately, it continued to be said SSTO’s weren’t possible even when we had these advances.  In point of fact SSTO’s are doable now with already known, and in use tech. Commonly, an expendable multi-stage rocket will only get ca. 3% of it’s gross mass to orbit.  SSTO’s using current tech can even match that for an expendable rocket.

 Even a reusable SSTO can be cost competitive to a fully reusable TSTO. The reason is the first stage of a reusable TSTO has to be boosted back to the launch site. This means a large amount of propellant has to be reserved on the first stage to return it to the launch site. When you add on also reusability systems for both the 1st and 2nd stages, about 50% of the payload is lost for reusability for the TSTO.

 For the SSTO in contrast you just let keep it in orbit until it is above the launch site again, then land. It turns out even for a cylindrical rocket reentering broadside with no wings, almost all orbital velocity is cancelled out by reentry, requiring less than 100 m/s terminal velocity to be canceled out by the engines on landing.

 By the way, I don’t have a preference for the SSME’s or any hydrogen-fueled engine for the SSTO. They can be done using dense propellant engines also. However, altitude compensation should be used in those cases to maximize payload.

 

   Robert Clark

Edited June 24, 2021 by Exoscientist

[Exoscientist]

On 6/22/2021 at 12:59 PM, tater said:

Yeah, the refurb costs on SSMEs are nuts.

@Exoscientist can take the same concept (and illuminate what the rough recovery concept is), switch to methane, and buy Be-4 engines. The I_sp is lower (which might not make it close), but the cost for a supposedly rapidly reusable engine is under $7M. Alternately, I could imagine a hydrolox Be-4 being at least plausible to develop, sacrificing some thrust for I_sp (and Be-4 is substantially higher thrust than RS-25 so there is room there for loss).

 

 Yes. Many current or soon-upcoming rockets can be SSTO’s if you give them altitude compensation and lightweight tanks, i.e., by using already known tech.  In fact, the only ones that can’t are ones like the SLS and Ariane 5 that can’t take off without their side-boosters. Even in theses cases though they can be by cutting down the tank size or adding more engines.

  Robert Clark

[tater]

7 hours ago, Exoscientist said:

 The price for a SSME during the shuttle era was about $40 million. The recently quoted price of $100+ million for the RS-25 is because of costs of restarting the production line and because, frankly, for the SLS everything is inflated.

 With 100 reuses, the SSME price per engine would be $400,000 per engine per flight, a small proportion of the $100 million for a ca. 10 ton to LEO expendable launch vehicle.

One, you'd need to switch that to constant dollars (which is closer to $100. Two, the 40M number doesn't matter, they cost $100M now, period. That's the production number AFTER getting billions to restart production.  SSMEs are godawful expensive by an order of magnitude.

 

Quote

 The Apollo era thermal protection was at about 15% of the landing weight. Modern materials cut this to about half that, for example with SpaceX’s PICA-X thermal protection material, so call it 7%.

Both of those are expended TPS, so the vehicle would need a new heatshield each flight.

 

Quote

 Commonly landing legs might be 3% of the landing weight. With composites probably can get it to about half that, call it 1%:

https://yarchive.net/space/launchers/landing_gear_weight.html

 A cylindrical body reentering broadside doesn’t even need wings. Almost all the orbital speed will still be cancelled out during reentry. Assuming a powered-final landing, you would only need to keep propellant reserve onboard to cancel out out terminal velocity, say, less than 100 m/s.

 By the way, I’m not sure that the powered-landing approach SpaceX prefers is the way to go. Using wings instead with modern composites, probably can get wing weight down to 1% to 2% of landing weight if you use short, stubby wings a la Skylon, the X-37, and the X-15.

SSMEs cannot restart, so ever lighting them again after liftoff is not a thing. Any burns need to be done by RCS. Propulsive landing is not even on the table for anything SSME based (or any relights at all in space).

 

Quote

Note, I was using the SSME in my blog post to indicate SSTO’s actually have been doable since the 70’s. You don’t need any advanced tech to do it like nuclear engines. However, if I were to actually make an SSTO out of the Delta IV I would probably just add altitude compensation to the RS-68 already on the first stage. This way you could even get the vacuum Isp to the 465+ s range. There are many different ways of getting altitude compensation on rockets using already known tech.

You still need an upper stage since the expendable SSTO you propose (RS-68s are ablatively cooled) has to safely dispose of itself. As a result, perigee has to be kept well below space to ensure that it reenters immediately, and the payload needs a kick stage to raise that perigee.

 

You seem to sometimes refer to an SSTO that must be expendable (RS-68s, for example), and sometimes to recovering it from orbit. Which is it? If the vehicle actually reaches orbit (vs disposal), it requires some orbital engines—as RS-25s cannot restart. It requires wings/TPS/etc. We're back to some sort of space shuttle, only less capable.

Edited June 24, 2021 by tater

[wumpus]

6 hours ago, Exoscientist said:

 There is an important difference between SSTO being technically impossible, and SSTO’s being uneconomical. Unfortunately, it has always been presented as the former, even when it wasn’t. During the earliest days of spaceflight it was technically impossible for chemical propulsion. But that was before we had advanced engines such as the SSME’s and lightweight materials such as carbon composites.

 Unfortunately, it continued to be said SSTO’s weren’t possible even when we had these advances.  In point of fact SSTO’s are doable now with already known, and in use tech. Commonly, an expendable multi-stage rocket will only get ca. 3%

of it’s gross mass to orbit.  SSTO’s using current tech can even match that for an expendable rocket.

An expendable TSTO delivers a large payload to orbit.  A SSTO can deliver the  same mass to orbit, but nearly all of that is engine and fuel tanks.  Sorry, that's how the math works and why TSTO is used.

The upper stage of a TSTO will be optimized for vacuum and will always be more efficient than an altitude compensated engine of the same family, and the fuel tanks will be more efficient (as they are smaller), so they will always get more delta-v for the same fuel.  So a SSTO will simply never deliver as much total mass to orbit than the TSTO.  So instead of 3%, you may get 2.5%, where 90% of that is your SSTO.  So instead of 2% being a satellite, you have a .2% microsat.  Congratulations, and you still aren't bring the SSTO home.

6 hours ago, Exoscientist said:

 Even a reusable SSTO can be cost competitive to a fully reusable TSTO. The reason is the first stage of a reusable TSTO has to be boosted back to the launch site. This means a large amount of propellant has to be reserved on the first stage to return it to the launch site. When you add on also reusability systems for both the 1st and 2nd stages, about 50% of the payload is lost for reusability for the TSTO.

2016 called.  Spacex routinely lands boosters on barges.  It would be even easier to build a landing pad downrange of Baikonur.  Vostochny presumably has less room, but likely would still make sense.

6 hours ago, Exoscientist said:

 For the SSTO in contrast you just let keep it in orbit until it is above the launch site again, then land. It turns out even for a cylindrical rocket reentering broadside with no wings, almost all orbital velocity is cancelled out by reentry, requiring less than 100 m/s terminal velocity to be canceled out by the engines on landing.

 By the way, I don’t have a preference for the SSME’s or any hydrogen-fueled engine for the SSTO. They can be done using dense propellant engines also. However, altitude compensation should be used in those cases to maximize payload.

 

   Robert Clark

You still have all the fun of building a heat shield the size of a full rocket (and a much bigger rocket if you want any payload at all).  The 1970s solution was to hand-apply individual tiles after every mission.  It was almost as expensive as rebuilding the SSMEs, and that was only for the orbiter (the surface area of the fuel tank would have been worse).  The X-37 budget is conveniently classified, and Starship requires some fancy active cooling pushing LOX(? some coolant) out the heatshield.  It is a non-trivial thing to do.

If you can handwave the recovery of a full-scale SSTO from orbit as such a trivial matter, why have the only recovered boosters been the Shuttle SRBs and Falcon 9, and that only 5 years after the initial launch?  The shuttle proved how expensive it was to recover even 27% of the total dry mass from orbit.  And don't forget the only reason the shuttle managed to deliver the orbiter plus a small (23 tons less than 1/3rd the orbiter's mass) into LEO was that they ditched 380 tons of booster dry mass.

[tater]

I should add that I actually see a use case for an operationally reusable SSTO.

To define terms, operationally reusable means something akin to military aircraft operations (vs civilian). Might not be pull up to the gate, then fly again in 30 minutes, it might take some time, but still quick (rearming and refueling a military aircraft, for example, in a peacetime cadence). So some number of HOURS to refly, not days. Nearly zero cost other than propellants and checkouts. Maintenance on the order of specialty aircraft, no more (maybe whatever the cadence is on a U-2 or SR-71?).

If that could be a thing, delivering crew to orbit would be a desirable use case. Fly to space daily, drop some crew off at a station, bring some home.

Short of that? Meh.

Edited June 24, 2021 by tater

[RealKerbal3x]

27 minutes ago, wumpus said:

Starship requires some fancy active cooling pushing LOX(? some coolant) out the heatshield.

Correction, Starship uses mechanically attached ceramic tiles. They dropped the active 'sweating' heat shield a while back.

(Shuttle also used ceramic tiles, but they were glued on rather than attached mechanically)

[DDE]

1 hour ago, wumpus said:

Spacex routinely lands boosters on barges.  It would be even easier to build a landing pad downrange of Baikonur.  Vostochny presumably has less room, but likely would still make sense.

There are no adequate transportation options for Baikonur. Vostochnyi might even be able to use the barge trick, it has river access.

[sevenperforce]

6 hours ago, wumpus said:

You still have all the fun of building a heat shield the size of a full rocket (and a much bigger rocket if you want any payload at all).  The 1970s solution was to hand-apply individual tiles after every mission.  It was almost as expensive as rebuilding the SSMEs, and that was only for the orbiter (the surface area of the fuel tank would have been worse).  The X-37 budget is conveniently classified, and Starship requires...

The X-37 uses TUFROC, a single-piece ceramic shield. It is repeatedly reusable from LEO entries, but it is very heavy (at least, compared to lightweight silica tiles).

There's just no way you can get a heat shield into that mass fraction.

[Exoscientist]

14 hours ago, wumpus said:

An expendable TSTO delivers a large payload to orbit.  A SSTO can deliver the  same mass to orbit, but nearly all of that is engine and fuel tanks.  Sorry, that's how the math works and why TSTO is used.

No. That’s the point I’m making. By using high performance engines such as the SSME’s and lightweight tanks such as with carbon fiber, then an expendable SSTO can even match the ca. 3% payload fraction of current expendable multistage rockets.

 This is because the greatly improved Isp of the high performance engine radically improves total mass to orbit. Then when you add onto that the dry mass is greatly reduced by lightweight tanks, the expendable SSTO matches or even exceeds the payload fraction for the current multi-stage expendable.

 Note I said it would match payload  for the current  multi-stage expendable. That is because none of them use both high performance engines and lightweight tanks.  BUT if you did, then even your multi-stage payload would be greatly improved. For instance in the example in my blog post,  the SSME modified Delta IV first stage with the Centaur upper stage could get ca. 20 tons to LEO, comparable to the Falcon 9 Full Thrust expendable.

 So you use the SSTO only for smaller payloads and the newly optimized TSTO for larger payloads or to GEO.

 By the way, I used the phrase “high performance engines” in describing this. But the point of the matter is even mid level performance engines such as the RS-68 and Merlín can be made into high performance engines engines by addition of altitude compensating nozzles in a relatively low cost fashion.

 

  Robert Clark

 

[Shpaget]

But why?

Using the same tech for TSTO that you use for STSO will always lead to TSTO having an advantage, so it always makes sense to use TSTO.

[Exoscientist]

14 minutes ago, Shpaget said:

But why?

Using the same tech for TSTO that you use for STSO will always lead to TSTO having an advantage, so it always makes sense to use TSTO.

 Not every launch will need the full size rocket. If you’re going to launch a small payload, why use the added expense of the upper stage when it is not needed? There are also operational simplifications when it’s only the one stage that needs to be prepared for another launch.

  Robert Clark

[Exoscientist]

On 6/24/2021 at 9:44 AM, tater said:

You seem to sometimes refer to an SSTO that must be expendable (RS-68s, for example), and sometimes to recovering it from orbit. Which is it? If the vehicle actually reaches orbit (vs disposal), it requires some orbital engines—as RS-25s cannot restart. It requires wings/TPS/etc. We're back to some sort of space shuttle, only less capable.

 A TSTO can be expendable or reusable. By the same token an SSTO can be expendable or reusable. You’re right about using the SSME for the deorbit burn or landing, because of the relighting issue and also they have limited throttling so couldn’t be used for landing. The throttling issue on landing also applies to the RS-68 if were to use an alt.comp. version of that for our SSTO. However, the amount of delta-v needed for deorbit or cancelling out terminal velocity on landing is so small, < 100 m/s, we might as well use small auxiliary engines for this purpose.

 If the modified RS-68 were used for our SSTO, it might be even possible to use the RS-68 itself in just a pressure-fed mode for the purpose, i.e., bypassing the turbopumps and just running off the pressurized fuel tanks. I’m fairly sure this would work for the RS-68 since this has worked for other engines that use the “gas generator cycle” mode of combustion. I’m not sure it would work for the “staged combustion cycle” for the SSME’s. See this discussed in regards to the Merlin’s here:

Hovering capability for the reusable Falcon 9, page 2: Merlin engines in a pressure-fed mode?https://exoscientist.blogspot.com/2015/07/hovering-capability-for-reusable-falcon.html

  Here are the points I’m making in general for the usefulness of the SSTO:

1.)Contrary to the often reported statement that a rocket reaching orbit in a single stage is not currently possible, it has been possible since the 70’s by using high performance engines and lightweight tanks, with existing and in use tech.

2.)Instead of it being said technically unfeasible, it has sometimes been said to be uneconomical because the payload that can reach orbit would be so small. This is also incorrect. The payload of an expendable SSTO using both high performance engines and lightweight tanks can reach and exceed the 3% payload common with current expendable multi-stage rockets.

3.)By using high performance engines and lightweight tanks on the first stage, this also radically improves the payload for a TSTO as well. For example for the RS-68 on the Delta IV it’s vacuum Isp is only, 412 s. With a higher performance engine this can be raised to 450+ s. This can as much as double the payload for the TSTO.

4.)Surprisingly, a midlevel performance engine like the RS-68 or Merlin can be converted to a high performance engine by using known and low cost altitude compensation methods on just the nozzle, no expensive and complicated engine redesigns like converting to a toroidal combustion chamber for an  aerospike nozzle required.

 5.)In short, SSTO’s using currently existing tech can radically improve space access and reduce costs.

 

     Robert Clark

[RCgothic]

If it were economical someone would be doing it.

[DDE]

3 minutes ago, RCgothic said:

If it were economical someone would be doing it.

No-one's actually gone ahead and replicated the Falcon 9 architecture either. Shall we draw conclusions from that?

Heck, if we use the inverse of that logic, we end up with the Buran, built despite all evidence being contrary to the viability of the Shuttle.

Edited June 26, 2021 by DDE

[tater]

1 hour ago, DDE said:

No-one's actually gone ahead and replicated the Falcon 9 architecture either. Shall we draw conclusions from that?

Not yet, multiple outfits are working on it.

Blue Origin, Relativity, Rocket Lab, Linkspace (? Chinese company, I want to say there are 2 doing it), Arianespace now talking about it.

5 hours ago, Exoscientist said:

 5.)In short, SSTO’s using currently existing tech can radically improve space access and reduce costs.

Don't think so. Unless an SSTO is operationally reusable, it's a dead end.

I'd like to see someone try a novel engine layout (like some of the Phil Bono plug engine designs from the 60s).