Jump to content

Recommended Posts

I've been building a lot of SSTOs lately. I'm fairly far into a career-mode game and it was annoying me that I didn't have a cheap way to get Kerbals into orbit and back down again, but while I have a good portion of the tech tree unlocked it wasn't complete yet and I didn't have access to the R.A.P.I.E.R. engines. Not to be deterred, I built the Grasshopper, a spaceplane based around a pair of Whiplash engines for atmospheric flight and a Toroidal Aerospike for space. The thing turned out fairly well, able to easily transport 4 Kerbals up to 250km orbits (which is where I keep my Kerbin space station) and back down again and, when pushed to its limits, can get up to a 500km circular orbit (although once up there I had trouble getting back down again, needing to use my RCS thrusters for that last bit of delta-V that I needed to get back into the atmosphere.)

Today, after completing a successful mission with the Grasshopper (several, actually; I had four different Kerbals from four different rescue contracts on my station and brought them all down at once) I took a look at my new contracts and discovered that I'd been offered one to test the R.A.P.I.E.R. engine, effectively unlocking it for the duration of the contract without needing to invest the science points to get it normally. I immediately upgraded the Grasshopper with the new technology, replacing its Whiplash engines with R.A.P.I.E.R.s, ditching the Aerospike (putting a second shielded docking port in its place) and rebalancing my fuel tanks to better suit the new configuration. The new craft turned out to be slightly lighter than the old one (but also more expensive) and I immediately took it to the runway, got it into orbit and discovered that it performed... well... pretty much exactly as well as the old one. It could get to a 500km circular orbit but didn't have enough oxidizer to come back down again. The new design did have some advantages, with that second docking port meaning that I could attach a NERV tug to the back for long-range journeys and still have it dock with a station (where before I could only have it docked with one thing at a time) and I managed to fit a science container in where I used to have a small LiquidFuel tank, but it was also substantially harder to fly due to the R.A.P.I.E.R.'s poor low-speed performance and, as I said, more expensive.

That got me thinking about the various SSTO designs I'd seen for this game. Nearly all of them are R.A.P.I.E.R.-based (with the occasional NERV engine for when you absolutely positively have to take a pair of wings and a set of air-breathing engines with you to Eeloo,) to the point where I don't think that a lot of builders are even considering the possibility of alternate designs. The R.A.P.I.E.R. is certainly convenient, being a jet engine and a rocket in a single package, but it has significant disadvantages in both modes, with its odd thrust curve requiring an exacting (and often tediously long) ascent profile in atmosphere and just being generally inefficient in vacuum. On the other hand, the Whiplash (and even the Panther) are still able to get a craft into the upper atmosphere with a significant fraction of its orbital velocity, are cheaper, get unlocked earlier and offer some significant advantages for in-atmosphere flying (with the Whiplash's smoother performance curve and the Panther's dry mode offering a very long cruise-time for when your re-entry falls wide of the KSC.)

So what do you think? Is it time to re-examine the R.A.P.I.E.R.'s status as the be-all and end-all of SSTO engines?

Link to post
Share on other sites

They're still better. If you build one space plane and use whips you will use them on Kerbin, but from there on there is no place to use them anymore and they just become weight that you carry around in space with no use. Good if you keep it local, but the moment you want to land on another planet they become an issue. At which point you wish you had Rapiers because you can use them to take off and land on other planets. Plus, if you're going too fast you can really slow down almost instantly. You can use the nukes, but they are so heavy and very bad at landing on anything with significant gravity, which is where it becomes obvious that you really need more power.

Link to post
Share on other sites

As I said, my original SSTO's space performance was about the same as my R.A.P.I.E.R.-based one, with the extra efficiency of the Aerospike making up for the dead weight of the Whiplashes. That said, saying that an SSTO spaceplane isn't very good at landing on airless worlds is like saying that a Volkswagen Beetle isn't very good for transporting an entire marching band. Of course it isn't, but it isn't supposed to be. The right tool for the right job. I called my ship the Grasshopper for a reason; it goes up and then comes back down. That's all it does, and all it's meant to do. I have other craft that I use for long-range work, but those ones stay in orbit, lacking the heat shields and parachutes necessary to land. All I need an SSTO for is to get my Kerbals to orbit so that they can transfer to one of those other craft to carry out the rest of the mission. Even without a set of purely air-breathing engines, going on a long-range mission with all the stuff needed to take off from and land at Kerbin means that you're going to have a lot of useless dead-weight slowing you down. Hence my joke about people strapping NERVs to their SSTOs.

Link to post
Share on other sites

Rapiers go faster than whiplashes in the atmosphere.  This decreases the amount of fuel you need to burn with rockets, which is much more expensive.  Rapiers also do better when used in conjunction with high efficiency engines (like the Nerv or Ion) because they can accelerate to a large fraction of orbital velocity, then kick the apoapsis out of the atmosphere using a small amount of on-board oxidizer to allow the lower t/w engines to circularize and go to your destination.  You get superior performance to whiplashes with bonus mediocre rocket performance for only .2t.

Link to post
Share on other sites

I think it just comes down to KSP is a game of trade-offs.  Usually every significant changelog mentions something about balancing this and re-balancing that.  Take the "Dawn" ion engines for example: an ion powered craft has ridiculous amounts of delta-V, blowing even the LV-N's out of the water on that account.  Yet, they are extremely expensive (as is the xenon fuel), require a lot of electricity, are low thrust, and cannot be supported with ISRU equipment.

I believe the main deciding factor on which technique to use comes down to your own proficiencies and preferences in KSP craft design.  Some players prefer to make SSTU (Single-Stage-To-Universe) spaceplanes that can cover the entire Kerbol system, I personally don't take spaceplanes past Kerbin orbit.  So to answer you question, IMO I don't think RAPIER's are overrated, I think it's simply a choice of how you want to design your spaceplane to function and/or look.

EDIT: Oh, IIRC one additional consideration is RAPIER's can run in much lower air densities than the Whiplashes, so you can push higher into the atmosphere before switching to pure rocket power.  Again, a given spaceplane design may make this irrelevant depending on how the player engineers it.

Edited by Raptor9
Link to post
Share on other sites

I'm not saying that R.A.P.I.E.R.s are never useful, but that their omnipresence in SSTO designs is undeserved. It's quite possible to make a practical SSTO without them, and there are some advantages to doing so, but nobody ever does this. Every SSTO spaceplane I see runs almost exclusively on R.A.P.I.E.R.s.

Link to post
Share on other sites

 

1 hour ago, Whisky Tango Foxtrot said:

That got me thinking about the various SSTO designs I'd seen for this game. Nearly all of them are R.A.P.I.E.R.-based (with the occasional NERV engine for when you absolutely positively have to take a pair of wings and a set of air-breathing engines with you to Eeloo,) to the point where I don't think that a lot of builders are even considering the possibility of alternate designs.

 

33 minutes ago, Whisky Tango Foxtrot said:

 Hence my joke about people strapping NERVs to their SSTOs.

I always had the impression that people are free to do what they want to do in KSP without being laughed at or having to justify their design descisions. Looks like I've been wrong.

Edited by lodger
Link to post
Share on other sites
42 minutes ago, Whisky Tango Foxtrot said:

As I said, my original SSTO's space performance was about the same as my R.A.P.I.E.R.-based one, with the extra efficiency of the Aerospike making up for the dead weight of the Whiplashes. That said, saying that an SSTO spaceplane isn't very good at landing on airless worlds is like saying that a Volkswagen Beetle isn't very good for transporting an entire marching band. Of course it isn't, but it isn't supposed to be. The right tool for the right job. I called my ship the Grasshopper for a reason; it goes up and then comes back down. That's all it does, and all it's meant to do. I have other craft that I use for long-range work, but those ones stay in orbit, lacking the heat shields and parachutes necessary to land. All I need an SSTO for is to get my Kerbals to orbit so that they can transfer to one of those other craft to carry out the rest of the mission. Even without a set of purely air-breathing engines, going on a long-range mission with all the stuff needed to take off from and land at Kerbin means that you're going to have a lot of useless dead-weight slowing you down. Hence my joke about people strapping NERVs to their SSTOs.

 

16 minutes ago, Whisky Tango Foxtrot said:

I'm not saying that R.A.P.I.E.R.s are never useful, but that their omnipresence in SSTO designs is undeserved. It's quite possible to make a practical SSTO without them, and there are some advantages to doing so, but nobody ever does this. Every SSTO spaceplane I see runs almost exclusively on R.A.P.I.E.R.s.

 

 

Bolded some of your keypoints for making my point You made an SSTO for a very specific purpose, take low quantity of Kerbals into orbit & bring them back down. As a mission for an SSTO, very acceptable, and as you observed you didn't really gain much in performance.

 

However, the vast majority of SSTO's other people show off, that are Rapier based, are the observed SSTU's, designed to take off from Kerbin and go anywhere and everywhere. Role determines design, their role as an SSTU/SSTE (Single-Stage-To-Everywhere) requires the wider performance range of the Rapiers.

 

And the very purpose of an SSTO, cost means nothing because you should be landing at, or very near the KSC, for near 100% cost return; so using a more costly engine means little if you only gain a few drawbacks like your observed low-thrust at low altitudes (it's a spaceplane, after you take off staying low altitude is like taking a Formula 1 car and staying in gear 1), but you gain many benefits like considerably wider performance range and functioning at lower air pressure, higher thrust values and I believe even a higher ISP in vaccum.

Link to post
Share on other sites
1 minute ago, Somtaaw said:

 

 

 

Bolded some of your keypoints for making my point You made an SSTO for a very specific purpose, take low quantity of Kerbals into orbit & bring them back down. As a mission for an SSTO, very acceptable, and as you observed you didn't really gain much in performance.

 

However, the vast majority of SSTO's other people show off, that are Rapier based, are the observed SSTU's, designed to take off from Kerbin and go anywhere and everywhere. Role determines design, their role as an SSTU/SSTE (Single-Stage-To-Everywhere) requires the wider performance range of the Rapiers.

 

And the very purpose of an SSTO, cost means nothing because you should be landing at, or very near the KSC, for near 100% cost return; so using a more costly engine means little if you only gain a few drawbacks like your observed low-thrust at low altitudes (it's a spaceplane, after you take off staying low altitude is like taking a Formula 1 car and staying in gear 1), but you gain many benefits like considerably wider performance range and functioning at lower air pressure, higher thrust values and I believe even a higher ISP in vaccum.

Couldn't have said it better myself. To shorten this for time-conserving purposes; use whatever you need to get to your destination and more relatedly (no typo here); use whatever ENGINE you need.

Link to post
Share on other sites
1 hour ago, Whisky Tango Foxtrot said:

That said, saying that an SSTO spaceplane isn't very good at landing on airless worlds is like saying that a Volkswagen Beetle isn't very good for transporting an entire marching band. Of course it isn't, but it isn't supposed to be. The right tool for the right job.

I am building an SSTO that can fly to other planets. The right tool for the right job is the Rapier in this case. Because they're not dead weight that I just carry around so I can use on one planet out of all. People use Rapiers because you can create more compact ships also, by not wasting two slots for doing the job that one engine can do while occupying less space, which creates utility. I created SSTO using a few rapiers that can go to orbit and back, but creating one that can go places won't work with your design very well because you have weight that has no reaction.

Link to post
Share on other sites

WhiskeyTangoFoxtrot,

 You are seeing the same results because you're plugging RAPIERs into a hybrid design. The advantage of RAPIERs isn't the weight savings, it's the parallel node savings. Using RAPIERs allows for a more efficient layout, which means both less drag and more payload per engine, but you have to actually design the ship to take advantage of this.

Best,
-Slashy

 

Enceos,

Spoiler

 

GiantSSTO0_zps42wvtpwy.jpg

GiantSSTO4_zpssl5ar7us.jpg

 

They're also good for really freakin' huge space planes.

Best,
-Slashy

 

Edited by GoSlash27
Link to post
Share on other sites

I just ran a couple of test-flights and recorded the numbers. Both were to a 500km orbit and back again (I seem to have gotten my ascent profiles a bit more efficient since I was able to get back into the atmosphere with both craft this time, although my periapsis was a bit lower with the Whiplash version.)

The Whiplash + Aerospike SSTO started with 1530 units of LiquidFuel and 1320 units of oxidizer. By the time it reached a 70km orbit it had 363 units of LiquidFuel and 340 units of oxidizer.

The R.A.P.I.E.R. version started with 1700 units of LiquidFuel and 1133 units of oxidizer. After reaching orbit it had 492 units of LiquidFuel and 370 units of oxidizer.

The first thing that stands out here is that the R.A.P.I.E.R. has significantly more fuel left upon reaching orbit, however it still has the same performance in space as the W+A version. This means that, even carrying the dead weight of the Whiplash engines the higher efficiency of the Toroidal Aerospike rocket means that that version of the craft performs better in space. For a mission profile that spent a longer portion of its time in space vs. ascending through an atmosphere, a W+A craft would be able to outdo a R.A.P.I.E.R. craft.

The R.A.P.I.E.R. craft was able to perform better on ascent, however, burning only 1971 units of fuel in total vs. 2147 for the W+A. However I'd say that the superior ease-of-use that the Whiplash's smoother performance curve offers offsets that. The R.A.P.I.E.R. may perform better at high altitudes and speeds but getting it to those altitudes and speeds is an absolute pain.

So yes, I did manage to save a node by switching to R.A.P.I.E.R.s and the extra docking port that that gave me would certainly have its uses, but it doesn't justify the R.A.P.I.E.R. being treated as the absolute only choice for SSTO spaceplanes like it so often is, which has been my point from the beginning. The R.A.P.I.E.R. engine is a 1000-science part which is in many ways just a step sideways from earlier, 550-science parts.

Edited by Whisky Tango Foxtrot
Link to post
Share on other sites
15 minutes ago, Whisky Tango Foxtrot said:

So yes, I did manage to save a node by switching to R.A.P.I.E.R.s and the extra docking port that that gave me would certainly have its uses, but it doesn't justify the R.A.P.I.E.R. being treated as the absolute only choice for SSTO spaceplanes like it so often is,

You didn't eliminate the unnecessary node, you just tacked something else onto it. Had you designed the craft from the ground up to use 2 RAPIERs, you'd see a much larger improvement. But having said that, the RAPIER is not the absolute only choice for SSTO spaceplanes. There is a situation where you would want to employ a hybrid design instead: You haven't unlocked the RAPIER yet. Other than that, the RAPIER is the way to go.

Best,
-Slashy

Edited by GoSlash27
Link to post
Share on other sites

I use RAPIERS even on designs where I never light them in vacuum (e.g. RAPIER/LV-N no-oxidant SSTO). I can't quote performance specs like Slashy can, but my understanding is that I can get higher velocity at higher altitude in jet mode with RAPIER than I can with any other airbreathing engine. I've never managed to make a no-oxidant SSTO with anything but a RAPIER, but then again I haven't tried to since before the Panther came out...I'm sure someone else has a design somewhere. In any case, I'll agree that the RAPIER isn't the only possible SSTO engine, but it makes SSTO so much easier that it does tend to dominate the field :) 

Link to post
Share on other sites

Kuzzter,

7 minutes ago, Kuzzter said:

I can't quote performance specs like Slashy can, but my understanding is that I can get higher velocity at higher altitude in jet mode with RAPIER than I can with any other airbreathing engine.

 This is correct.

Best,
-Slashy

Link to post
Share on other sites

I concur. The RAPIER is not the end all be all. But you can achieve significant velocities in the atmosphere up to 1600-1800 ms with it up to a ceiling of 24-26km maybe higher, although I have not achieved this. Which makes getting to space that much easier and 3500 ISP over 310-350 to reach these velocities? yes please. I will say though that using the RAPIER in conjunction with a higher efficiency closed cycle engine is advantageous and can increase dv. But not always. If the craft is light and the additional weight of the closed cycle engine is a significant % of total vehicle mass then you wont see dv increasing from higher ISP. However on large crafts where the additional weight is 1 ton on a 140 ton craft, then yes, ISP will over come the difference in mass.  

Link to post
Share on other sites

As atmospheric pressure decreases, and speed increase to past Mach 3 the whiplash engines start to decrease in performance seeing as mach 3 is about 1029 m/s at 1500 ms they would suffer considerable performance loss. They are also the most air hungry. 

Link to post
Share on other sites
12 minutes ago, Leafbaron said:

As atmospheric pressure decreases, and speed increase to past Mach 3 the whiplash engines start to decrease in performance seeing as mach 3 is about 1029 m/s at 1500 ms they would suffer considerable performance loss. They are also the most air hungry. 

And here I thought the reason was nose drag was too high, causing me to flip. 

Link to post
Share on other sites

Depends. If your spaceplane is just a bus to orbit then RAPIERs are nearly always better. If it's designed to actually do something in space then it's the opposite. My Minmus/Mun/Laythe spaceplanes don't use RAPIERs as they're too heavy and useless in space.

Link to post
Share on other sites
5 hours ago, theend3r said:

Depends. If your spaceplane is just a bus to orbit then RAPIERs are nearly always better. If it's designed to actually do something in space then it's the opposite. My Minmus/Mun/Laythe spaceplanes don't use RAPIERs as they're too heavy and useless in space.

If you had a pair of docking ports, you could decouple your engines, and be 1-2 tons lighter.I've had more success with KIS/KAS and removing the unnecessary parts once I'm docked in space to the space station. I store the excess parts there.

Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...