Stock SSTOs, Rescaled Kerbin

[KerikBalm]

In a Q&A about SSTOs, I started sharing my SSTOs which could make orbit with payloads for a 3x rescaled Kerbin (if it can SSTO there, it can send payload on a Jool flyby in stock).

@AeroGav had questions, and I have questions for him and larger scale kerbin SSTOs in general. Rather than going off topic in that thread, I thought we could discuss here.

11 hours ago, AeroGav said:

What's a realistic payload fraction for a RSS stock SSTO?  Not much I'm guessing.   If It can carry a science lab and hitchhiker module, i'm happy.

I'd like to point out that what you (and I) are playing with is not RSS. I don't think stock SSTOs are really possible in RSS, because stock parts are generally pretty heavy and underpowered relative to real life. That's why I find the 3x parameters that I play under to be fairly good.

For the parameters I play with -I think you used the same- I think over 10% payload fraction is good.

Now just before that post in a video, I showed a SSTO on a 3x Kerbin (root 3 rotation period, 1.25x atmosphere height) taking a 114.49 ton payload to orbit, starting from 897.325.46 tons. I forget how much the mas of the payload fairing was (for something like an Ore tank Payload, the fairing mass can be dramatically reduced, and added to payload).

That's a 12.75% payload fraction.... over 13% if you include excess fuel still in the SSTO as payload.

11 hours ago, AeroGav said:

@KerikBalm I  did not load any payload,  and also the oxidizer tanks are empty.    I  wasn't actually planning to go to orbit , rather just make sure it could break mach 1 and that i hadn't done anything particularly dumb with the staging.

2 Panthers. 3 RAPIERs, 8 NERVs.    With no cargo or oxidizer, but a full load of LF, it weighs 101 Tons.  Dry mass is 55 tons.  Its rather docile but the handles like a Donkey with OCD -  tell it to pitch up, pitch down or bank the wings it does its best to ignore you.

  Hide contents

Max speed on three RAPIER.    It might have gone faster lower down,  but i couldn't get it to go down. Might be less skittish with some payload and oxidizer aboard.

After lighting the nukes and removing nose down pitch trim, it does a gentle zoom climb to 40km,  then starts into a shallow dive.   I was mighty relieved when it started climbing again, for good this time, because stuff was getting hot.

And there we are, in orbit with a little bit of fuel left.   Of course, there is no way something this size could bring an orange tank.  What's a realistic payload fraction for a RSS stock SSTO?  Not much I'm guessing.   If It can carry a science lab and hitchhiker module, i'm happy.

 

I find that to be a very interesting design. I didn't think LF only was possible at those higher scales. The jet to LV-N ratio is completely reversed from my design. Mine had 40 rapiers and 8 nukes (5 jets/rapiers per nuke). You had 8 nukes and 5 jets, or 3 rapiers if you prefer... 2.67 nukes per rapier, 1.6 nukes per jet. I may want to consider putting in more nukes and less rapiers.

I also note that you seem to have 10 shuttle wings, and I guess enough of other wing types to be roughly equivalent to 12 shuttle wings for 101 tons starting mass. So 8 tons per shuttle wing, and 1.7 tons per "relative lift unit" or relative wing area, or whatever unit the lift rating is of wings.... Quite a bit more wing per unit mass than my design.

Also, its impressive in a way that your dry mass is so high (over 50% of the wet mass), whereas my ~13% payload fraction design was like 280-290 tons dry mass, with a takeoff weight of about 900 tons (about 33% of the wet mass with payload). I guess you have more engine mass and less fuel mass, whereas I used more fuel mass and had less engine mass (per ton of craft)... the difference being that I went heavy on rapiers, and you went heavy on nukes.

As for your payload, I'm willing to count unused fuel as payload. Taking your numbers at face value, 55 tons dry mass (exactly for the purposes of calculation). Then you must have had 62.7 tons in orbit to get 1028 m/s of dV as your screenshot shows. That's 7.7 tons of fuel in orbit, from 101 tons starting mass. I'm willing to assign that demo flight a payload fraction of 7.6%.

I think I recall one flight that was a 17% payload fraction (I may be mistaken, my memory is fuzzy, and I had a computer crash a while back and lost most of my files), and had a veeeeerrrryyyyy long burn on the LV-Ns that went to space, back into the atmo, and back into space.

Maybe more nukes is the way to go...

So you had, all totalled, 8*3+3*2+2*1 = 32 tons of engines for 100 tons of craft, and 8 tons of payload (rounding here). So 3 tons of craft per engine, and 4 tons of engine per ton of payload.

My craft had 40 rapiers and 8 nukes, so 40*2+8*3 = 104 tons of engine for 900 tons of craft, and 115 tons of payload. so 8.5 tons of craft per engine, and about a 1:1 payload:engine mass ratio.

Our craft have very different ratios in multiple parameters, but the performance isn't all that dissimilar... interesting.

I look forward to what you come up with.

If possible, try to accommodate large and heavy payloads, and keep part count manageable.

 

And if interested, some old videos I put on youtube of my 3x system and some of my designs:

Spoiler

https://www.youtube.com/watch?v=1O5SqRqygHo

https://www.youtube.com/watch?v=T6bRj0SuEe8

 

Note that I'm not always trying to push the payload fraction to the max... I just wanted a practical surface to orbit shuttle to let me get a wide range of payloads to orbit (check out the payload in the 2nd video)

[KerikBalm]

Just did a new SSTO design in 3x: 764.126 tons on the runway, 160 parts

I decoupled the payload at 119.32 tons (I had to use a little LF from the payload).

After the deorbit burn, my craft had 1.1 tons of fuel left (therefore I could have dumped it into the payload, which had some empty tankage... it was launched with some empty space to account for this). Also, the payload fairing was 3.95 tons

Anyway... 119.32/764.126 = 15.6% payload fraction.

This one took a lot longer to get to orbit though. I dropped my rapier count from 40 to 32, and increased the LV-N count from 8 to 9.

I turned on Aero UI, and found that I'm losing 1/2 to 1/4 of my LV-N thrust to high altitude drag though... Maybe more LV-Ns is the way to go...

Still no panthers though

*edtit* increased LV-N count to 17, added a bit more LF and wings (15% more wing area)... came up with a lower payload fraction, and actually a slightly lower total payload... I'm not sure more LV-Ns are the way to go.

It may have been the flight profile though...

Edited February 1, 2019 by KerikBalm

[Mephisto81]

Do I understand the 3x correctly, in that you have 9.81 m/s² gravity, just like in stock, but the sizes of planet and atmosphere are bigger, resulting in a higher dV needed?
And if so, how much dV do you need to make orbit in your game?

[KerikBalm]

Yes, you seem to understand correctly.

Rockets seem to need about 5500 m/s with these settings...

Orbital velocity is about 4,100 m/ in low orbit, observe this pic, which still has the Pe well be low the surface:

So I tried again a slightly modified version: 32 rapiers, 48 big S wings, 17 LV-Ns.

The payload (encased in a fairing of just under 4 tons) was 2 Kerbodyne S3-14400 Tanks with the 2nd one emptied of oxidizer.. which is 122.4 tons. I had some left over oxidzer in the craft (except for a small amount for the SSTO), so I dumped that in the tank, increasing its mass to 128.34 tons. I then released the payload, redocked the rear section, and did the deorbit burn. The result was I still had 3.5 tons of leftover LF (which I could have removed and added oxidizer to the tank instead).

Therefore, its payload capacity to orbit was 131.84 tons, from a starting mass of 822.623 tons.. just barely breaking the 16% payload fraction- 16.027%

Anyone else want to try to do better?

[AeroGav]

On 1/31/2019 at 8:47 AM, KerikBalm said:

 

I'd like to point out that what you (and I) are playing with is not RSS. I don't think stock SSTOs are really possible in RSS, because stock parts are generally pretty heavy and underpowered relative to real life. That's why I find the 3x parameters that I play under to be fairly good.

For the parameters I play with -I think you used the same- I think over 10% payload fraction is good.

 

That's a 12.75% payload fraction.... over 13% if you include excess fuel still in the SSTO as payload.

 

I find that to be a very interesting design. I didn't think LF only was possible at those higher scales. The jet to LV-N ratio is completely reversed from my design. Mine had 40 rapiers and 8 nukes (5 jets/rapiers per nuke). You had 8 nukes and 5 jets, or 3 rapiers if you prefer... 2.67 nukes per rapier, 1.6 nukes per jet. I may want to consider putting in more nukes and less rapiers.

I also note that you seem to have 10 shuttle wings, and I guess enough of other wing types to be roughly equivalent to 12 shuttle wings for 101 tons starting mass. So 8 tons per shuttle wing, and 1.7 tons per "relative lift unit" or relative wing area, or whatever unit the lift rating is of wings.... Quite a bit more wing per unit mass than my design.

Also, its impressive in a way that your dry mass is so high (over 50% of the wet mass), whereas my ~13% payload fraction design was like 280-290 tons dry mass, with a takeoff weight of about 900 tons (about 33% of the wet mass with payload). I guess you have more engine mass and less fuel mass, whereas I used more fuel mass and had less engine mass (per ton of craft)... the difference being that I went heavy on rapiers, and you went heavy on nukes.

As for your payload, I'm willing to count unused fuel as payload. Taking your numbers at face value, 55 tons dry mass (exactly for the purposes of calculation). Then you must have had 62.7 tons in orbit to get 1028 m/s of dV as your screenshot shows. That's 7.7 tons of fuel in orbit, from 101 tons starting mass. I'm willing to assign that demo flight a payload fraction of 7.6%.

I think I recall one flight that was a 17% payload fraction (I may be mistaken, my memory is fuzzy, and I had a computer crash a while back and lost most of my files), and had a veeeeerrrryyyyy long burn on the LV-Ns that went to space, back into the atmo, and back into space.

Maybe more nukes is the way to go...

So you had, all totalled, 8*3+3*2+2*1 = 32 tons of engines for 100 tons of craft, and 8 tons of payload (rounding here). So 3 tons of craft per engine, and 4 tons of engine per ton of payload.

My craft had 40 rapiers and 8 nukes, so 40*2+8*3 = 104 tons of engine for 900 tons of craft, and 115 tons of payload. so 8.5 tons of craft per engine, and about a 1:1 payload:engine mass ratio.

Our craft have very different ratios in multiple parameters, but the performance isn't all that dissimilar... interesting.

I look forward to what you come up with.

If possible, try to accommodate large and heavy payloads, and keep part count manageable.

 

And if interested, some old videos I put on youtube of my 3x system and some of my designs:

  Reveal hidden contents

https://www.youtube.com/watch?v=1O5SqRqygHo

https://www.youtube.com/watch?v=T6bRj0SuEe8

 

Note that I'm not always trying to push the payload fraction to the max... I just wanted a practical surface to orbit shuttle to let me get a wide range of payloads to orbit (check out the payload in the 2nd video)

RE: My airplane has a lot of LV-N's  + wings

Indeed - i don't know if changing the engine ratio on your SSTO will  work though,   since it was designed that way from the outset.   It might be a case of you have to go all in on something like that ,  or not at all.    Alternatively,  it's about aerodynamics.   For an spaceplane to be able to gain energy during the start of the non airbreathing part of the ascent ,  TWR  x  (L/D Ratio)  must be greater than 1.    If your engines are RAPIERs that weigh 2T and kick out 160kn, it's easier to just add more thrust and concentrate on reducing mass of things that are not engines, fuel or payload.    If your engines are NERVs that weigh 3T and only have 60kn,  it's easier to add more wings to improve L/D,  and reducing mass of other stuff won't have much impact.

When you're going NERV heavy,  lift/drag ratio  and drag / fuel capacity ratio trumps all.    That's the main reason i had so many wings,   the Big S wing part has about the same drag as a mk1 liquid fuel fuselage,  I was worried about  adding mk3 tanks as that adds a lot of extra drag without any lift.      

The design could use more fuel.   I normally reckon on one NERV per 15 - 20 tons on stock scale for a comfortable and easy flight.  On 3x,  i still think 1 per 15 T would be ok because i've designed low tech Panther SSTOs on that ratio whose air breathers only get a small fraction of orbital velocity.   However, this design has 8 LV-N at 100T.   As soon as you stop forcing  the nose down during the air breathing speed run and go to prograde hold, starting the nukes,  it balloons up to 40km (still accelerating)  then dips back to 30km  before going up again for good,  by that point,  it has a TWR of 0.78 to 1,  with thrust nearly 10x my drag value.   I should probably try putting on some mk3 fuselage,  right now it's hard to find room for more wings without giving an unrealistic and ugly appearance.

Also,   I found it really problematic finding room for engines on this design.    Panthers, NERVS,  Rapiers...   i'm effectively using 3 engines to do the job of 1,  but saving on fuel burn as a result.   However,  this makes it hard to find a place where your exhaust wont be blocked by a wing , and i have lot of wings.

Just to re-iterate,   this airplane has a lift to drag ratio in the hypersonic climb of > 3.5 to 1 when you go on prograde hold mode.    That is what i think enables it to do well with nerv-heavy design.

[AeroGav]

Takeoff is not as exciting as yours.    Hmm,  i need to fix the lack of pitch authority.  Not being able to command more than 2 deg above prograde is ok on ascent to orbit,  but might make for some rather firm landings..

 

[AeroGav]

Here's a video of the climb under nerv power.    At no point does drag exceed 200kn (most of the time it is much less) and we have 480kn from 8 engines,  so it's fairly comfortable.   Max thermal percentage is 78% , reached on the faring that  covers the probe core, batteries and stuff at the front.

 

[AeroGav]

OK,  version 2.0 of this craft has now been test flown.

https://www.dropbox.com/s/tb0x9qdylyp59il/Bright Hawk2.craft?dl=0

I got rid of that fustercuck of strakes at the front end and put a short mk3 tank at the front of the cargo bay.

For front end lift (and control) ,   a pair of shuttle tail fins acting as canards.   Much more efficient in terms of part count.

I've saved three parts by simplifying the landing gear too.

The dry centre of mass is now too far forward,  what a nice problem to have.   The panthers now get moved alongside the rapiers (which looks better and reduces tail strike risk).   We need more LF aft of CoM - so, i add a bunch of mk1 tanks in front of the panthers all the way to the CoM,  with a shock cone fronting the stacks.

Finally, the oxidizer tanks are filled.

I was leery of doing this,  but i decided to imitate SpaceX and iterate, iterate.    I can see the logic in swapping wings for mk3 in the name of part count efficiency,  but was it right to replace with mk1s ?

So, v1 of this craft is 82 parts,  102t wet .   V2 is 145 tons, but only 77 parts.   Oxidizer is much of this extra mass.   If you're trying to win payload mass fraction,  i can see why going liquid fuel only can help you win,  because your takeoff weight will be significantly less.  To make the 15% payload fraction ,  this v2 craft is going to have to lift much more because it is heavier itself.      Not feeling terribly optimistic, I loaded a rockomax 32 tank and crossed my fingers...

It now needs about three quarters of the runway to get airborne, and climbs out at 5-10 degrees.   But,  i think it flies better with extra wing loading.    I'm not fighting the tendency to climb too high for the airbreathers to work.   And that huge ballooning effect you got when going prograde at the end of the speedrun, is gone.   The climb is steadier.   Right about the point where the v1 airplane starts to fall down again,  v2 has just burned off all its LFO courtesy of the rapiers, and thus lightened, it just stagnates for a bit then starts to go up some more.

As you can see this is  very much a prototype.  Payload deployment doesn't exactly go to plan,   but the contract only said deliver 18 tons of rocket fuel to orbit,  at least the airplane survived.

 

[KerikBalm]

Interesting... I note that it took you one hour to get to your payload deployment video (the payload seems to clip with the wings that are clipping in the cargobay). From your other video, it seems to be about a 30 minute flight to space... this is perhaps too long for me... When part count goes up, as when deploying large payloads, the real time to game time ratio gets pretty bad.

My 40 rapier 8 LV-N designs were getting to orbit in about 11-12 minutes of game time, so I got deploy a payload in about 30 minutes of real time. Going from that which got 13% payload fraction to my higher LV-N% design that got 16% resulted in a significant extension of the time to orbit.

For RP reasons, I also like to make nuke-less designs (or limit the use of nukes, 17 nukes as on my 16% design, I consider to be a lot), and those I again make rapier only, but I would consider using a LFO vacuum engine instead.

11 hours ago, AeroGav said:

 V2 is 145 tons, but only 77 parts.   Oxidizer is much of this extra mass.    Oxidizer is much of this extra mass.   If you're trying to win payload mass fraction,  i can see why going liquid fuel only can help you win,  because your takeoff weight will be significantly less.  To make the 15% payload fraction ,  this v2 craft is going to have to lift much more because it is heavier itself.      Not feeling terribly optimistic, I loaded a rockomax 32 tank and crossed my fingers.

...

the contract only said deliver 18 tons of rocket fuel to orbit,  at least the airplane survived.

Contract? you had a contract (career mode?)? If you meant the specification... 145*0.15 is 21.75. 18 tons would be a 12.4% payload fraction

However, in your display I see 1760 Ox, and 3554 LF... that has a mass of 26.57 tons. Plus the dry mass of the tank (2 tons)

Then lets subtract about 0.5 tons of fuel for the deorbit burn (we'll assume that you time it well and can glide in), you carried up 28 tons of excess weight from a 145 ton craft... impressive, 19.3% payload fraction. 2.75 parts per ton to orbit is still rather high. I got my design down to about 1.34 parts per ton to orbit, but the higher payload fraction one was at 1.5 parts per ton to orbit...

I'm still not sold on the Panther thing, but it does seem that pumping up the LV-N and wing number is the key to higher payload fractions... I'm fairly happy with what I've got for part count, flight time, general utility, and RP reasons though.

17 hours ago, AeroGav said:

I found it really problematic finding room for engines on this design.    Panthers, NERVS,  Rapiers...   i'm effectively using 3 engines to do the job of 1,  but saving on fuel burn as a result.   However,  this makes it hard to find a place where your exhaust wont be blocked by a wing , and i have lot of wings.

Tell me about it... I don't want to abuse part clipping - otherwise I could stack rapiers end to end, and clip them into each other, and clip wings into each other, and finding places for engines and wings (I also don't want to make a biplane) on a design that is meant to carry large and bulky payloads to orbit is a real challenge.

It would be interesting to change the specs to have no nukes, to settle if a mixed rapier-panther design really is better than all-rapier.

I know that all rapier designs work at this scale, thus they work with a large margin in stock. All rapier designs are conceptually simpler (only 1 airbreather's flight curve to factor in, no need to find places for airbreathers and rockets), and since they still have large margins in stock, that's why I recommend them for newbies.

16 hours ago, AeroGav said:

Takeoff is not as exciting as yours. 

My last 2 designs have much better  takeoff characteristics, and don't flop on the ground at the end of the runway before lifting off again, but actually just become airborne at the end of the runway.

The rapiers get it up to a sufficient speed, its just a matter of such a long craft with its weight spread out on all that heavy duty gear can't really pitch up.

Also I noticed one other difference in the parameters that we are using... it doesn't change much: I also have my terrain height multiplier set to 1.5 instead of 3x. Many terrain features are exaggerated on Kerbin relative to the body size (if its meant to be an earth analogue). At 3x, mountains are way too high. Its similar to why I only scaled the atmo up to 1.25x.

Scaling the atmo up 1.25x, and the ground up by 3x means the air is very thin on mountain tops. A 1.25:1.5 atmo:terrain height ratio keeps things more similar, and I think it looks better from up high (it does make slopes less steep though).

This has a very minor affect on the altitude of KSC, but I doubt it matters, and aside from takeoff, we can ignore the terrain height for this.

[AeroGav]

4 hours ago, KerikBalm said:

Interesting... I note that it took you one hour to get to your payload deployment video (the payload seems to clip with the wings that are clipping in the cargobay). From your other video, it seems to be about a 30 minute flight to space... this is perhaps too long for me... When part count goes up, as when deploying large payloads, the real time to game time ratio gets pretty bad.

The V1 prototype (no oxidizer)  took 13 minutes to get from launch to starting the rocket engines.      The V2 with 50% greater takeoff weight,  needed 18 minutes to reach that same point.    Main engine cutoff (MECO)  is about 20 minutes after launch on V1,  about 25 minutes on V2.   You've then got another 5 minutes coasting to get out of the atmosphere, after which you can do non-physics time warp.   The orbit ends up getting circularised on the dark side of the planet,   i had to warp to the PE (another half orbit) to get enough light to show payload deployment.    

If this is a criticism,  I have to say ,  "what do you expect?".   Payload fraction , Delta V,   and TWR  are the three basic parameters of a vehicle - one can only be improved by reducing the others.       I  am raising delta V  and  payload fraction,  so TWR decreases.       Using lots of wing parts to improve lift/drag ratio is a way of making the lower TWR not cause excess gravity losses.  Swapping some RAPIERs for Panthers is about sacrificing some performance above mach 2.5 to get better performance below mach 1 - enabling you to break the sound barrier with less tons invested in jet engines.    If you don't want to lower TWR from your original design,  then this whole exercise has been a waste of time.

Getting higher lift drag ratio implies raising the part count doesn't it ?  Your original design is efficient in terms of parts per tons of payload,  but negatively affects l/d.  

All rapier designs are conceptually simpler (only 1 airbreather's flight curve to factor in, no need to find places for airbreathers and rockets), and since they still have large margins in stock, that's why I recommend them for newbies.

I still must disagree here.     All rapier means you have the problem, which a lot of newbies fail at,  getting past mach 1.   A mix of rapier and panther gives a flatter overall power curve and means you don't have to worry about the engine curve so much.    Adding nervs does make things complicated, because that triples the number of engines  you have to find a place for.   

I suppose some newbies will break mach 1 when expect to climb at 20 degrees and still pass 20km at mach 4.  With a rapier / panther design you need to actually level off at 18km to hit mach 4,  and some people aren't capable of doing that,  or will lose control or break their wings off trying. 

to quote a clever person 

Everything Should Be Made as Simple as Possible, But Not Simpler

I

5 hours ago, KerikBalm said:

For RP reasons, I also like to make nuke-less designs (or limit the use of nukes, 17 nukes as on my 16% design, I consider to be a lot), and those I again make rapier only, but I would consider using a LFO vacuum engine instead.

As you probably know,  the exhaust of a nerv does not contain any radioactive materials,   but the reactor makes X rays, gamma rays and neutrons when operating.   For weight reasons,  only the side of the reactor facing the crew is shielded (via a shadow shield) so you don't want to overfly a city at low altitude with nervs running.      A reactor that has never operated does not emit radiation,   but once you start the nervs , nuclear reactions don't neatly stop  on engine shutdown.     

So,  for RP reasons i think you have two assume one of two political realities for your game

1.  "No nukes (present day political attitudes)".  Never fired RTGs and LVNs can be launched from kerbin,   but may not be operated within its SOI.    Once an RTG or LVN has been activated for the first time, it may never enter Kerbin's SOI again,  so there is no possibility of it re-entering the atmosphere.

2.  "1950s style nuke power is great"  .   These are the rules i am operating under.    My use of LVN's is over the ocean,  or so high in the atmosphere that people on the surface would be protected.    After landing,  the SSTO would have residual radiation emission so you'd leave the vessel and walk back to the hangar via an approved route that keeps the shadow shield between you and the hot stuff, rather than hang around taking selfies under the nozzles.

A practical chem only  SSTO is hard however.    Furthermore, if you're not going to use LVN's,  there is no use for high L/D ratio and no use for my skillset.  I might as well quit the game.

For chem only,  I think TSTO is more practical.     With high thrust chem engines however,  there is no need for the orbiter stage to have wings at all.     Why not develop a pegasus style system,   where the giant launcher craft is RAPIER powered  (air breathing mode only),   that drops a Rhino rocket stage to do the rest ?

5 hours ago, KerikBalm said:

Contract? you had a contract (career mode?)? If you meant the specification

Yeah it was my attempt at a joke.  18 tons of fuel was in orbit,  just no longer contained within a tank.   Grab a net or some plastic bags and go EVA, maybe you can catch some of it.

 

5 hours ago, KerikBalm said:

Also I noticed one other difference in the parameters that we are using... it doesn't change much: I also have my terrain height multiplier set to 1.5 instead of 3x. Many terrain features are exaggerated on Kerbin relative to the body size (if its meant to be an earth analogue). At 3x, mountains are way too high. Its similar to why I only scaled the atmo up to 1.25x.

Scaling the atmo up 1.25x, and the ground up by 3x means the air is very thin on mountain tops. A 1.25:1.5 atmo:terrain height ratio keeps things more similar, and I think it looks better from up high (it does make slopes less steep though).

This has a very minor affect on the altitude of KSC, but I doubt it matters, and aside from takeoff, we can ignore the terrain height for this.

Ah good tip.   Actually,  i do find marginal takeoffs exciting and imagine having the space centre surrounded by high terrain and trying not to hit any of it on the way out..   something to play with

 

[KerikBalm]

1 hour ago, AeroGav said:

If this is a criticism, 

Not a criticism, just an observation

Quote

 If you don't want to lower TWR from your original design,  then this whole exercise has been a waste of time.

Well, I wasn't so much wanting a lower TWR, I just wanted to look at options and optimization for SSTOs that have to operate at larger scales.

Quote

I still must disagree here.     All rapier means you have the problem, which a lot of newbies fail at,  getting past mach 1.

Well, I never had much problem with that, and the threat that this spun off from had the guy getting to 1,000 m/s on rapiers... I'm still not sold on panthers. I'd still like to see if panthers over rapiers can be used to improve payload fraction. LV-Ns can, for sure, but I already knew that (although I wasn't quite sure about how many to use, I was sure that the designs with them were doing better than those without them)

Quote

As you probably know,  the exhaust of a nerv does not contain any radioactive materials,   but the reactor makes X rays, gamma rays and neutrons when operating.  

Well, yes, it makes a lot of radiation when operating, and its even worse in the atmosphere, as you get backscatter from the atmosphere that can bypass your "shadow shield". Shadow shields work fine in space, but they don't work nearly as well in the atmosphere where there is mass to redirect the radiation.

Also, they are of course more dangerous for something that is going to be re-entering. Then they'd pose additional problems when refurbishing the SSTO for the next flight.

The lack of a radioactive exhaust from a closed cycle nuke engine doesn't mean they are still super friendly to use.