Larger Jet Engines

[Gargamel]

A few posts and comments have been removed.  Please don't get personal, and leave the moderation to us, thanks. 

[Pds314]

On 5/13/2020 at 6:31 PM, Northstar1989 said:

The numbers I quoted are for rocket ascents.

It takes 4 - 3.5 km/s vacuum Delta-V to reach orbit in stock-sized Kerbin.  Most players underestimate this because they forget most engines get less Thrust/ISP in atmosphere.  I also use SpaceX-style launch stage recovery (including in RSS 64k), which is more comparable to spaceplane ascents in terms of cost, but requires more Delra-V...

It definitely takes 6.5 - 7 km/s in RSS 64k: because aero-drag doesn't decrease (in fact increases, like you said), but goes from maybe 1-2 km/s of the ascent costs (with 2 - 2.5 km/s for grwvity-losses) to 1.5 - 2.5 km/s with gravity-losses being 2.5x as much (sqrt 6.4 is 2.5) around 5 - 6 km/s.  The numbers I gave before add up...

 

Stock Kerbin NEVER should take 4 km/s unless you mean to Duna intercept. A sleek design will do 2.9 of vacuum DV. A less sleek one or one with marginal TWR might take 3.2.

Edited May 27, 2020 by Pds314

[Klapaucius]

I realize this is not exactly what you are asking, and there are some in-built compromises, but might I suggest strapping on some solid rockets?  This challenge proved the viability to solid only SSTOs, so a hybrid would be totally doable.  One Clydesdale packs a lot of punch.

 

 

[Lanley Kerman]

On 5/14/2020 at 8:38 AM, Pds314 said:

Yeah.... 8-12? more like 50... this design for a space bus is from awhile ago but it's very representative of the issue. I believe it was made in oldFAR, but that's actually a lot more similar to current stock than it is to oldstock.

Holy Kraken!!! Howm many kerbals could fit in that Hitchhiker mess?!

[KerikBalm]

Its not that hard...

I count 26 hitchhikers (13 on one side) , 2 cockpits, and 1 crew cabin. 26x4+2x1+1x3= 109 Kerbals

[Lanley Kerman]

On 6/8/2020 at 10:17 PM, KerikBalm said:

Its not that hard...

I count 26 hitchhikers (13 on one side) , 2 cockpits, and 1 crew cabin. 26x4+2x1+1x3= 109 Kerbals

Lanley Kerman poofed out of existence.

[Pds314]

On 5/26/2020 at 12:47 PM, KerikBalm said:

Yes, I was referring to incidence, I don't know why you want to call it built in angle of attack, when you can have positive incidence and negative angle of attack, and your statement about getting incidence by pitching up is just rubbish.

Dude, there is no inherent difference in trim drag from elevators vs canards. if you trim an elevator down, it makes more lift behind the CoM, if you trim a canard up, it makes more lift in front of the CoM. If a plane is balanced so that you need to pitch down as much as you need to pitch up, then it doesn't matter.

And the payload in those designs is at the CoM, and thus if the CoM is at the approximate center, then the lever arms are approximately equal.

They reduce lift when you need to pitch down.

If an elevator is constantly making lift because of incidence then it doesn't reduce lift of the whole plane for small deflections.

And canards in neutral or very low defelction generate little/no lift but still generate drag. That canard can generate extra lift to push the nose up is exactly equal to an elevator that is generating extra lift to lift the tail up... 

This is not complicated.

You have to remember that Canards generating no lift are basically deadweight you have to push to orbit.

And the same holds true for elevators with incidence, so the amount of deflection isn't that great (since to avoid stall, you also need deflection that isn't that great.

If you use incidence, your elevators are generating positive lift. The key is simply not to place too much burden on the elevators relative to their area . they should be able to maintain control with maybe 6 or degrees MAX deflection (so most of the time they only deflect 2-3 degrees)

Seriously, your arguments are so easy to defeat by simple word substitution... [snip]

It most definitely lies outside the nozzle

[snip]

 

If your craft is dynamically unstable, your elevators can be at higher incidence than your wing.

[swjr-swis]

On 5/13/2020 at 8:55 AM, Northstar1989 said:

A simple request- but the game could really use larger (1.875 meter or 2.5 meter) jet engines

Agreed. We have precisely one jet engine of that size, the Goliath, and it's near to useless for any practical spaceplane design as it's not even meant to go supersonic (somewhat relevant: look at this challenge and see what compromises are being made just to get something -anything- delivered to orbit with a Goliath-powered spaceplane). We are definitely missing a supersonic jet in that size.

Looking at stock only though, we don't have much practical use for them: there are no cargo bays beyond the Mk3 ones, and any reasonable (*) Mk3 cargo can be lifted pretty comfortably with the engines we already have. So if we go this route, please also include some bigger cross-section cargo bays. We need to lift Mk3 and size 3 cargo!

(*: yes, I do realize this sounds ridiculous coming from me... I don't build all that much 'reasonable' stuff anymore -- did I ever? Not the point.)

[Northstar1989]

On 6/21/2020 at 2:36 PM, swjr-swis said:

there are no cargo bays beyond the Mk3 ones, and any reasonable (*) Mk3 cargo can be lifted pretty comfortably with the engines we already have.

Comfortable?

For optimum efficiency a spaceplane with a Mk3 fuselage requires at least 8-10 jet engines.  Any less than that and you're wasting fuel on an excessively-slow ascent and run for speed at altitude.

With 1.875 meter supersonic jets, we could do this with just 4 jet engines (each 1.875 meter jet could have 2.25 times the Thrust of a 1.25 meter jet, to get the same Thrust per unit of cross-sectional area: so 4 of these would be equivalent to NINE 1.25 meter jets).  This would also generate less Drag- for the same reason four 1.875 meter stacks generate less Drag than nine 1.25 meter stacks: less wetted surface area due to the Square-Cube Law...

Edited June 26, 2020 by Northstar1989

[swjr-swis]

6 hours ago, Northstar1989 said:

For optimum efficiency a spaceplane with a Mk3 fuselage requires at least 8-10 jet engines.  Any less than that and you're wasting fuel on an excessively-slow ascent and run for speed at altitude.

With 1.875 meter supersonic jets, we could do this with just 4 jet engines

You're doing something wrong if you need 8-10 engines for an Mk3 cargo spaceplane. 3 jet engines is plenty, and that's without optimizations.

 

Spoiler

Let's deliver a full Mk3 cargo bay to LKO. 3 RAPIERs should do the trick.

No problem getting supersonic. Climb profile is as simple as holding orbital prograde all the way up, and switching to closed cycle around 23km / 1800+ m/s.

LKO with fuel to spare even when departing with partially filled tanks.

There and back again - a spaceplane's tail.

Full album: https://imgur.com/a/38iytZO

Craft file: https://kerbalx.com/swjr-swis/Mk3-Spaceplane1

Edited June 27, 2020 by swjr-swis

[Northstar1989]

On 6/26/2020 at 11:33 PM, swjr-swis said:

You're doing something wrong if you need 8-10 engines for an Mk3 cargo spaceplane. 3 jet engines is plenty, and that's without optimizations.

Don't just automatically assume you know better than another player more experienced than you and post a Craft File: that's just rude.

Yours is a grossly suboptimal design: if you're going to use a Mk3 fuselage, it should only be for much larger payloads than that (or MULTIPLE payloads in that weight range).

If I were going to lift a payload of that size to orbit, I'd use a long Mk2 fuselage, with 1.875 meter fuel tanks on the payload.  This will get you to orbit for much less fuel, as the Mk2 is a Lifting Body and more aerodynamically efficient (in FAR and especially in Stock- which lacks the FAR code that creates Body Lift from ALL fuselages to some degree) than the Mk3 for payloads in that size range.

To use a Mk3 (which is really meant for use as a Shuttle fuselage, and is terribly-shaped for horozontal takeoff spaceplanes- meaning I'd probably use a Stail fuselage from OPT instead of not playing Stock-only...) I'd have to be lifting a payload MUCH bigger than that- at which point I would need the extra engines I just talked about.

Edited June 28, 2020 by Northstar1989

[Incarnation of Chaos]

21 minutes ago, Northstar1989 said:

Don't just automatically assume you know better than another player more experienced than you and post a Craft File: that's just rude.

Yours is a grossly suboptimal design: if you're going to use a Mk3 fuselage, it should only be for much larger payloads than that (or MULTIPLE payloads in that weight range).

If I were going to lift a payload of that size to orbit, I'd use a long Mk2 fuselage, with 1.875 meter fuel tanks on the payload.  This will get you to orbit for much less fuel, as the Mk2 is a Lifting Body and more aerodynamically efficient (in FAR and especially in Stock- which lacks the FAR code that creates Body Lift from ALL fuselages to some degree) than the Mk3 for payloads in that size range.

To use a Mk3 (which is really meant for use as a Shuttle fuselage, and is terribly-shaped for horozontal takeoff spaceplanes- meaning I'd probably use a Stail fuselage from OPT instead of not playing Stock-only...) I'd have to be lifting a payload MUCH bigger than that- at which point I would need the extra engines I just talked about.

Why are you so confrontational? He saw that you mentioned needing 8-10 engines for a MK3 and demonstrated that you can easily get a MK3 sized payload and craft into orbit with 3 engines.

And using less fuel isn't really that good of a point; Fuel is the cheapest part of the whole craft while the engines (Especially RAPIER) are the most expensive. Also why are you talking about MK2 parts? He was directly responding to something about MK3 parts.

 

[KerikBalm]

Plus, in stock, mk2 fusalages are pretty much the worst for drag.

Their L/D is terrible, you get less drag with a cylindrical fusalage and wings.

Cylinders are more mass efficient too. In stock (dunno about FAR), mk2 is for looks, or surviving re-entry.

Mk3 isn't great either, but its the only reasonable option for a payload bay that you can reuse (unlike fairings).

It will be interesting to see if the fairing changes in 1.10 make mk3 cargo bays a thing of the past

[swjr-swis]

11 hours ago, Northstar1989 said:

Don't just automatically assume you know better than another player more experienced than you

Someone is making assumptions here alright, but it might not be who you think it is.

 

11 hours ago, Northstar1989 said:

Yours is a grossly suboptimal design

It might indeed be. I never claimed it to be an optimal design - in fact I specifically mentioned it not being optimized. Just a craft lifting a full CRG-100 bay of payload to LKO, which is the biggest Mk3 cargo bay we have in stock, without having to spam engines.

Let's recall how this exchange went: I said in current stock, an Mk3 payload could be comfortably lifted with the engines available to us. You seemed to take exception with that particular assessment. So I provided proof for my statement by slapping a simple, low part-count plane together that lifts such a payload on just 3 engines, with about the easiest ascent profile I can imagine. In my opinion, that is pretty 'comfortable'.

 

In any case, I'm always up for learning what a really optimized design looks like for such a payload. I promise I won't call you rude if you post a craft file that blows mine out of the water in terms of optimization (for any definition of 'optimal' of your choice). A big part of what I enjoy out of participating in this forum is learning new design tricks and optimizations. Information exchange is awesome.

[Jesusthebird]

not sure if anyone has mentioned this yet, but take a look at the Sabre mod, they have lots of beefy jet engines and even rapier like engines(airbreathing and closed cycle) Now obviously I think your idea would be very nice as I agree the jet engine dept in stock is lacking...and console players wont benefit from a mod like sabre, but as far an an immediate solution...if you run...1.7 at least I think?? Sabre mod can be a nice addition to your game