R.A.P.I.E.R. versus Turboramjet? Surprise, surprise

[SemiaCloud]

Nice SSTO!

[waterlubber]

I use SSTO's to bring crew or small probes to orbit. I use almost exclusively rapiers, but the turbos are a good idea. What is the best wing, though?

[georgTF]

So I figured out how to mount both turbo and nuke engines in the middle by canting them outwards, and this is the SSTO I came up with:

...snip

I haven't successfully landed it on the Mun yet, but i should have enough deltaV to make it and back. Based on its performance, I think 4 RAPIERs, 1 turbojet, and 1 nuke is indeed the magic ratio for optimal SSTO performance. I'd love to see all-stock designs that can do better than this one with a different configuration....

Well i worked on a spaceplane that had enough fuel to orbit Mun or Minmus and it could even land on Minmus. It was a twin turbojet + aerospike design that could seat 5 passengers (+1 pilot), BUT it required refuelling in Kerbin orbit in order to go anywhere. Going straight to the Mun and back would be considerably more difficult to pull off. BTW if you're having trouble with Munar landings, try Minmus since the cost in fuel of both landing and return is cheaper than the Mun. Dv required to get there is not much more than the Mun.

Maybe once i get a refinery on Minmus, i could take tourists there, refuel, THEN land on the Mun and return to Kerbin. Quite a dull mission, but i'd do it once or twice. At any rate, not using Kerbin orbit to refuel SSTO handicaps them a lot, and the price of replenishing a fuel depot in LKO can be offset by a contract or two.

That said, kudos on getting a non-refuelling SSTO design!

...What is the best wing, though?

For medium (15-30t) or Mk2 SSTO craft it's probably swept wings. You don't want a lot of them esp since Mk2 fuselage provides lift. It mostly depends on where you need to place the Center of Lift, so it's not 100% certain that delta or flat wings are superior. It will allways depend on the design.

Wings tend to be OP and it's a pretty good idea to try to use them to position the CoL roughly where it needs to be then use additional winglets or elevons to make the craft controlable.

Edited June 3, 2015 by georgTF

[herbal space program]

I use SSTO's to bring crew or small probes to orbit. I use almost exclusively rapiers, but the turbos are a good idea. What is the best wing, though?

The 42 ton design I posted above has 2 small winglets as vertical stabilizers, 4 wing strakes as main lifting surfaces, and 2 Elevon 4's. It has a takeoff speed of around 170 m/s. In this new aero system, I would never use a wing that stuck out too far. It's just too much drag when you're up there trying to rip a hole in the stratosphere. My fastest planes all look like flying Mrs. Butterworth's bottles.

- - - Updated - - -

Going straight to the Mun and back would be considerably more difficult to pull off. BTW if you're having trouble with Munar landings, try Minmus since the cost in fuel of both landing and return is cheaper than the Mun. Dv required to get there is not much more than the Mun.

That said, kudos on getting a non-refuelling SSTO design!

Setting that thing down on Minmus would be too easy. Landing it on the Mun is way harder because of the much deeper gravity well and the barely adequate thrust of the single nuke. If I can manage the Mun, I think I might have a decent shot at Duna.

[GoSlash27]

I've been spending a lot of time fooling around with this in the last few days, and for me it seems that 3-4 Rapiers, 1 turbojet, and 1 nuke is the combo makes for the best deltaV on orbit. It seems like with the current atmosphere model, breaking the sound barrier is the biggest hurdle in any ascent profile that's trying to get as much mass/deltaV per ton as possible on orbit. In a 3-hull Mk2 design with 4 RAPIERs plus a nuke, it's always been a long process for me to break the sound barrier and go into the silly thrust zone. I would have to climb to 14km on the RAPIERs at like 290 m/s,then nose down and engage the nuke, hoping that I could hit like 420 m/s and pull back up before getting below 10km. Very tedious and fuel-consuming. Adding one turbojet to the mix dramatically improved the efficiency with which I could get to that point by providing that extra little push at the transition. With 4 RAPIERS and 1 turbo, I was easily able to get a 42 ton craft to the 400+ m/s range on a constantly ascending profile. Keeping the RAPIER/Turbo ratio to 3 or 4 to 1 left enough thrust at the high end for me to get up to 12-1300 m/s surface at 24km with enough climb rate that I didn't burn off all my rocket deltaV getting up out of the soup. Unfortunately, there's no symmetrical way to place 3+1+1 or 4+1+1 engines, so I had to mount the turbo to an empty tank behind the nuke on a decoupler, then jettison it for the circularization burn. Obviously this is no longer technically a SSTO design, although dragging the turbo the rest of the way to orbit, if it were somehow possible, would still have left this ship with close to 4000 dV on orbit. I haven't taken this ship to the Mun and back yet, but I'm pretty sure I'll make it when I try. Landing a big ship like that on just the nuke is pretty costly though. A previous version without the tacked-on Turbo could make Munar orbit and back, but just didn't have enough deltaV to land and get back off.

hsp,

3 RAPIERs 1 TJ and 1 nuke seems a bit much considering I can orbit a 54 tonne spaceplane with just 4 RAPIERs.

This "Texaco" design is 107 tonnes and makes orbit with 6 RAPIERs.

They represent 22% and 26% payload fraction, respectively.

Best,

-Slashy

[georgTF]

Setting that thing down on Minmus would be too easy. Landing it on the Mun is way harder because of the much deeper gravity well and the barely adequate thrust of the single nuke. If I can manage the Mun, I think I might have a decent shot at Duna.

Oh, i didn't realise you were aiming for Duna. I thought it was going to operate in the Kerbin system. Well since Duna has some athmosphere, you can use parachutes to help land it there. I still question the wisdom of lugging 5 useless and quite heavy engines all the way. Not to mention the fuel tanks needed to achieve a stable Kerbin orbit. All part of the challenge i suppose.

[herbal space program]

hsp,

3 RAPIERs 1 TJ and 1 nuke seems a bit much considering I can orbit a 54 tonne spaceplane with just 4 RAPIERs.

This "Texaco" design is 107 tonnes and makes orbit with 6 RAPIERs

Best,

-Slashy

I'd say it depends what you're trying to do. With due respect to those truly lovely designs, my ship is not intended just to tag LKO and go home. It's intended to land a Kerbal on the Mun and come back to the runway at the KSC. That's why it has that heavy nuke, which is indispensable AFAICT for what I'm trying to do. I was also incorrect FWIW about how much it weighs. That version is actually 45t on the runway, which can be seen in the album I posted.

Anyway, I'm not sure how you calculated your payload fraction, but I'm arriving at a 100km orbit with 9.5 tons of fuel , a Mk1 cockpit (1t), docking port (0.1t) , 4 LT-2 landing struts (0.4t), and 8 RCS thruster blocks (0.4t). That would represent a payload fraction of 25%, which is comparable to what you posted. If you also count the nuke as payload, we're talking 34.2%. I've really got that nuke on board just to land on the Mun. It's totally dispensable to get to orbit. If I replaced it with an LV-909, I would probably end up with just about the same payload fraction at LKO. Another thing this design has going for it is that you don't need to do any fancy bobbing and weaving to get this through the sound barrier. It flies straight to orbit, and although I totally understand the mass-based argument for just being able to get over that hump, planes I've flown that work that way have somehow always seemed to end up in orbit with less fuel than ones with enough power to break the SB on a continually ascending profile. So I guess what I should have said to communicate my design goals properly is that I believe this is the magic formula for a well-performing, fully functional interplanetary SSTO.

One important thing however that I do take away from looking at your designs is that I definitely seem to have too many intakes! I should try losing about a third of the ones I have on my plane and see if it performs significantly better. I also haven't explored the MK3 parts yet to see if they have a better lift/drag ratio than the Mk2. Mk1 AFAICT is just an all-around loser.

...So after flying the thing for a couple of hours last night with fewer intakes, which definitely improved performance, I was able to get to LKO with 12 tons of fuel left, for a 26% payload fraction in just propellants. Unfortunately, the tilted engine performed great until the actual landing, by which time the CoM had moved so far back that it no longer wanted to push the ship in a straight line at all. I'm pretty sure I could land it eventually, but that's no way to fly. I guess tonight I will see if I can lose the turbojet by cutting down the intake drag to a minimum, but I feel like I've been down that road already with this design.

Edited June 5, 2015 by herbal space program