TWR of midrange SSTOs?

[noname_hero]

I've always thought that I (kinda) suck at SSTOs. Okay, I *can* build them, but I kind of hate flying most of my SSTO designs, and the main reason is horrible TWR. So I keep hoping that maybe I don't suck so bad, maybe I have unrealistic wishes. I have no problem building small and swift SSTOs that can ferry say two kerbals to a station in a 100km orbit and back to Kerbin. But if I want something midrange, with say 1500+ m/s of dV from LKO? My designs feel so sluggish I only ever test the designs, never use them for regular mission. The latest example? A Mk2-based SSTO that can get six people into LKO with 2000+ m/s of dV left.

It has a probe core, a claw, an RTG, radiators, drogue chute, the works. I can take it into LKO and return it to the KSC runway. It can rescue several kerbals from LKO and toss their wrecks into the atmosphere in the process. But it has four Rapiers that run out of oxidizer on the way up and the two NERVs that remain give it an awful TWR. It is another design I'll probably hardly ever fly again. So...

Do I suck at SSTO design that bad? Do people actually design and use midrange SSTOs with a reasonable TWR? My own experience is that it is much more comfortable to use short-range SSTOs to get kerbals to/from an LKO and to use reusable specialist craft with better TWR from then on, and fuel is free in LKO once you have ISRU tankers on Minmus. But what do those better than I am think about this?

[Lt_Duckweed]

The answers you get are going to very much depend on what you feel a "high" twr is.

For me, a lko twr of .2 or higher is enough to count as "high".  With this twr you can get up to about 8000m/s in orbit with a well optimized craft.

If you double your nukes for a lko twr of about .4, you can still achieve something in the region of 6000m/s.  This is for me quite the generous twr, and I would feel comfortable landing on anything duna or lighter.

If you go up to .6 twr, you are edging close to the point at which nukes are no longer your best option, but you can still get something like 4500m/s

It is worthwhile to note that these numbers do not account for passengers or isru.  You would need to adjust the numbers down a bit in that case.

[noname_hero]

7 hours ago, Lt_Duckweed said:

The answers you get are going to very much depend on what you feel a "high" twr is.

You're hitting the nail on the head there, I'm afraid. The design I mentioned (probe core, 2-kerbal pilot cabin, 4 passengers) has TWR of 0.3 with 2500 m/s left in LKO and it does feel sluggish to me, as I like having a TWR of 0.5 or higher for regular flights inside Kerbin's SOI. That being said...

I hate to sound like an incompetent and jealous a*s, but how the eff you get 8000 m/s left in LKO on a non-ion SSTO? I can only do that if I use a reusable booster, separate the 8k+ stage, and land the booster.

[Lt_Duckweed]

8 hours ago, noname_hero said:

You're hitting the nail on the head there, I'm afraid. The design I mentioned (probe core, 2-kerbal pilot cabin, 4 passengers) has TWR of 0.3 with 2500 m/s left in LKO and it does feel sluggish to me, as I like having a TWR of 0.5 or higher for regular flights inside Kerbin's SOI. That being said...

I hate to sound like an incompetent and jealous a*s, but how the eff you get 8000 m/s left in LKO on a non-ion SSTO? I can only do that if I use a reusable booster, separate the 8k+ stage, and land the booster.

This album details some of the design and flight of an efficient cargo lifter: https://m.imgur.com/gallery/fTWS13F

Though this is a cargo lifter, if the carried tank + docking ports was instead replaced with liquid fuel, it would have a smidge over 8000 dv.

If I added a cockpit and mining equipment it would drop the overall delta v in orbit by about 1000, still leaving 7000 in orbit.  Adding 2 nukes would drop it again to 5000.

When it comes to the most extreme of craft, I have been able to get 11,000 m/s in orbit with pure liquid fuel by minmaxing to an extreme degree.  104 tons takeoff mass, 2 rapiers, 2 nukes. Big S strakes for wings (6:1 wet:dry vs big s wing 4:1) a kerbal in a payload bay, hundreds of the tiny mk0 tanks (they have a 11:1 wet:dry ratio vs the standard tanks 9:1) stuffed along with all the engines under a tiny fairing, etc.

[Gavin786]

An amazing tool is : Kerbal Wind Tunnel.  You can actually get away with a lot less TWR than you think you can by flying the flight envelope of the aircraft.

Often, especially for raper engines, there can be a "hump" that until one gets past the engine will work with low TWR.  Kerbal Wind Tunnel lets you see where these transition points are, and what can actually be achieved by your space plane in its atmospheric flight mode.  A lot of space planes need to be "flown right".  That is held at a certain altitude until they are at the other side of the hump, only then will the TWR pick up and they can be put into their to-orbit trajectory.  KWT lets you see where to transition, how hard it is going to be.  Proper pro tool for any aircraft design.  Use notes(another mod) to create a flight plan/manual for your space plane so you can remember what you want to do at what stage in flight from the KWT graph.

 

[Lt_Duckweed]

In addition to kerbal wind tunnel (which is absolutely fantastic btw), correctCoL (correct center of lift) and rcs build aid are both extremely useful for spaceplane builders.

CorrectCoL because it shows the true center of aerodynamic pressure on your ship, and has handy stability charts as well.

RCS build aid because it can show wet and dry CoM at the same time, and show you how each fuel type on the craft contributes to CoM shift.

[TheFlyingKerman]

No I don't build 'midrange' SSTO's. Instead I add a transfer module to my payload. I reckon that is cheaper (even if the engine is single use) than the fuel hauling the wings and engines and cargo bay around.

[Xyphos]

since you're concerned about TWR, I assume you're speaking about rockets and not spaceplanes?
'SSTO' by itself is a very indescriptive generic term.

if SSTO spaceplanes, the wings do all the lifting so a TWR of >0.6 is needed to stay aloft. RAPIER engines should be used since they're good upto 22~24km then switch to Closed cycle until >35km and from there, you should be able to reach LKO and beyond with LV-N's so do try to limit the amount of oxidizer on-board.

if SSTO rockets, you will need a TWR of >1.0 to get off the ground, 1.2 is usually the bare minimum, I tend to aim for 2.0 minimum so I can yeet it off the pad without wasting fuel.

in either case, TWR will greatly vary depending on the payload, fuel mass and engine(s) used. there used to be an SSTO payload fraction contest somewhere?

Edited August 25, 2020 by Xyphos