SSTO engine types

[Hummingbird Aerospace]

I'm just tweaking my brand-new SSTA which is about 50 tones and has a crew of 1 kerbalnaut and I'm not sure if I should keep my layout of 4 turbo ramjets, two NERVAs, and two LV-T45 "Swivel" engines.I am torn between that or 4 rapiers and 2 NERVAs. What would you choose?

[KerikBalm]

I would choose rapiers and nervs, the Isp difference on swivels and Rapiers is only 15, is that worth increasing your dry mass by 2 tons? Now if you had said aerospikes or a KR-2L or a poodle... I'd be more reluctant and I'd ask for more stats on the craft

[FinalFan]

I am struggling to imagine the situation where a Swivel is desired on your spaceplane.  Why not Aerospike or Terrier or NERV or Thud?  

[AVaughan]

The other argument in favour of nervs and rapiers, is that with 4 rapiers you can probably ditch the turbo ramjets, and save even more mass.

[ARS]

Go for Rapier and Nuke

You can give us the detailed information of the craft, such as pic, engineering readout or craft file, that should give us enough data to give you the best advice

[Hummingbird Aerospace]

2 hours ago, ARS said:

You can give us the detailed information of the craft, such as pic, engineering readout or craft file, that should give us enough data to give you the best advice

Ahhhh... (insert 4 letter profanity in all caps)! I made the SSTA on a different computer while I was on a trip that happen so to be 4,000 kilometres from where I am now, so I emailed the craft file to myself, opened it up on KSP version 1.4.2, and bullcrap happens and it proclaims I cannot open the craft since there are “unknown parts”, even though it is fully stock and stuff and nonsense happens and I scream and rage quit KSP. I spent like 45 minutes of my precious life designing that SSTA and now it is gone. 

Anyway, I will post the file here, gotta go somewhere for 2 days, can’t post right now, so I will put it up later.

I don’t wanna use rapiers because the NERVs will be doing most of the interplanetary travel and I only need an engine like the closed-cycle RAPIER mode for powering me through the upper atmosphere on a suborbital trajectory where airbreathing engines flame out and NERVs have a very low thrust and it would take too much time to circularize, so I don’t need four of them. I have heard rapiers are not very efficient, and doing the Jool-5, which I am doing with this ship, I need maximum efficiency. 

Although I guess landing on Tylo would me much easier with 4 rapiers in closed-cycle...

Maybe I just PermaPruned a part with Janitor’s Closet and I forgot about it.

[FinalFan]

4 hours ago, Hummingbird Aerospace said:

Ahhhh... (insert 4 letter profanity in all caps)! I made the SSTA on a different computer while I was on a trip that happen so to be 4,000 kilometres from where I am now, so I emailed the craft file to myself, opened it up on KSP version 1.4.2, and bullcrap happens and it proclaims I cannot open the craft since there are “unknown parts”, even though it is fully stock and stuff and nonsense happens and I scream and rage quit KSP. I spent like 45 minutes of my precious life designing that SSTA and now it is gone. 

Anyway, I will post the file here, gotta go somewhere for 2 days, can’t post right now, so I will put it up later.

I don’t wanna use rapiers because the NERVs will be doing most of the interplanetary travel and I only need an engine like the closed-cycle RAPIER mode for powering me through the upper atmosphere on a suborbital trajectory where airbreathing engines flame out and NERVs have a very low thrust and it would take too much time to circularize, so I don’t need four of them. I have heard rapiers are not very efficient, and doing the Jool-5, which I am doing with this ship, I need maximum efficiency. 

Although I guess landing on Tylo would me much easier with 4 rapiers in closed-cycle...

Maybe I just PermaPruned a part with Janitor’s Closet and I forgot about it.

Sorry to hear about that.  But hey, it's always a lot easier the second time around (unless you've completely forgotten what you did), and a lot of the time the rebuild is better than the original.  

Disclaimer!  I am not a serious spaceplane guy, just a dabbler.  Maybe you already know more than me and I'm just being dumb.  But having said that...

Regarding the Rapier being inefficient:  although that is true, I think you may have one of two misapprehensions.  Unless I do, in the case of #1.  

One:  It is true that the Rapier gets worse Isp than the other jet engines, but jets are all ridiculously better than rockets, including the Rapier.  The big complaint about the Rapier is that it can (depending on your craft) struggle to get you supersonic.  After going supersonic it's usually fine, but some planes have to waste a lot of time and fuel coaxing Rapiers up to the speeds where they really shine.  AFAIK craft with this problem are the only ones that need to worry about the Rapier being an inferior jet engine.  It barely weighs more than the Whiplash.  

Two:  It is true that the closed cycle rocket form of the Rapier gets bad ISP compared to most rockets, but the other jet engines have literally nothing!  So you aren't really losing out there by going with Rapier vs. another jet engine.  

Perhaps you could compromise by using a Whiplash/Rapier/NERV combo?

[sdj64]

On 6/22/2018 at 11:28 AM, Hummingbird Aerospace said:

I don’t wanna use rapiers because the NERVs will be doing most of the interplanetary travel and I only need an engine like the closed-cycle RAPIER mode for powering me through the upper atmosphere on a suborbital trajectory where airbreathing engines flame out and NERVs have a very low thrust and it would take too much time to circularize, so I don’t need four of them. I have heard rapiers are not very efficient, and doing the Jool-5, which I am doing with this ship, I need maximum efficiency. 

Although I guess landing on Tylo would me much easier with 4 rapiers in closed-cycle...

Try 4 Rapiers and 2 NERVs, you'll probably find that the weight savings from eliminating two engines are more than worth the slight hit to ISP.  If you really need the vacuum ISP for a Tylo landing, you can replace the Swivels with Darts, or a single Skiff, Poodle, or Wolfhound if your design allows it.

[Hummingbird Aerospace]

@sdj64 Im going to start from scratch and build a Mk3 SSTO, so that I can have a bigger Convert-O-Tron, a bigger drill, AND just if I add an extra one seat module with some science stuff I can compete at Jeb’s level for the Jool-5. Also, thanks for the advice. 

Edited June 23, 2018 by Hummingbird Aerospace

[beomagi]

I found the mk3 parts heavy and prone to breaking. You can still use the larger drill and convert-o-tron without them.

Did a video of my last trip to Laythe which includes building the spaceplane. For under 2.5 tons, you can have 3 crew, which means having an engineer.  Drilling to refuel happens in a local day or two on Minmus, Laythe or Pol with one. Plane here : https://kerbalx.com/Beomagi/sp-4200-Pheonix

 

Rapiers + nukes are highly effective. I've found a ratio of 2:1/3:1 rapiers:nukes an effective balance. Engines are dead weight when in space, but not having enough rapiers means you may waste a lot of time and fuel getting up to speed, or just not leave the atmosphere with much speed, meaning more DV spent in closedcycle+nukes mode to LKO.

 

Edited June 24, 2018 by beomagi

[Nich]

whiplashes are dead weight always add more rapiers.

If you are having trouble going supersonic you may need to clean up your craft (reduce drag) possibly less wing.  you can blip LFO mode but I find this is generally a waist.  I have not played stock in forever but generally 20 ton/ raipier is the max.