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AeroGav

30Ton SSTO, oxidizer for max range or not?

Question

So , my most recent creation was based off @lodger 's Xkos.

20161121161935_1_zpsubvnyqzj.jpg

https://kerbalx.com/AeroGav/ASES-SP2-Xkos-Dual-Nukes

It weighs about 30 tons,  and it's a 3 holer.

My rule of thumb is that you want one Rapier or Whiplash for every 30 tons  and 60kn Closed cycle power for every 15 tons mass (if nuclear) or every 10 tons mass (if chemical).

So, it's got about the right amount of jet engine power by my book.  In rocket mode, we have an embarrassment of riches.   With 120kn nuke power in a 30 ton ship, that is sufficient by itself.   But it also has a Rapier producing 180kn in closed cycle mode.  On it's own , that would also be sufficient. With both, it's already in LKO before I manage to burn all the Ox off.   If I continue to run the nukes till our LF is gone, our PE ends up halfway out to Dres.

It's carrying 2 tons of Rapier, 6 tons of NERV, 11.4 tons of LF and 2 tons of Oxidizer.

I'm wondering what's going to work best here -

1. Fly it with full tanks as i have done so far.

2. Leave the oxidizer tanks empty, because we're wasting LF burning it at 305ISP in a Rapier when we could be feeding it into a nuke for 800ISP

3. Use configurable containers mod to swap the adapters to hold all LF and no ox, and don't use the Rapier's closed cycle mode

4. Drop down to a single nuke, saving 3 tons, and bring enough oxidizer to get us through the roughest part of the ascent  (1 ton may be enough?).   Now, how do you fit one rapier and one nuke to a ship that size and not have horrendous off-axis thrust probs?

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14 minutes ago, AeroGav said:

2. Leave the oxidizer tanks empty, because we're wasting LF burning it at 305ISP in a Rapier when we could be feeding it into a nuke for 800ISP

Ding ding ding!  We have a winner!

The trick is that you may have to adjust the ascent profile somewhat.


Happy landings!

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50 minutes ago, AeroGav said:

Now, how do you fit one rapier and one nuke to a ship that size and not have horrendous off-axis thrust probs?

One way of doing it.

uURJ5yK.png

Notice the rear body is bent in a slight S-shape, aka the nuke translated down, to have it thrusting through CoM and pointing in the same direction as the cockpit. The RAPIER is just rotated to thrust through CoM.

My experience is that as long as the nuke is thrusting through CoM and inline with the cockpit, you're fine to mount the RAPIER any way you like as long as it doesn't produce too much torque when the atmosphere becomes thin, above 22 km. Below that control surfaces can easily compensate for any up or down pitch created by the engines.

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1 hour ago, Starhawk said:

 

The trick is that you may have to adjust the ascent profile somewhat.


 

Actually, it appears not !

 

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1 hour ago, Val said:

One way of doing it.

uURJ5yK.png

Notice the rear body is bent in a slight S-shape, aka the nuke translated down, to have it thrusting through CoM and pointing in the same direction as the cockpit. The RAPIER is just rotated to thrust through CoM.

My experience is that as long as the nuke is thrusting through CoM and inline with the cockpit, you're fine to mount the RAPIER any way you like as long as it doesn't produce too much torque when the atmosphere becomes thin, above 22 km. Below that control surfaces can easily compensate for any up or down pitch created by the engines.

Does it matter that the RAPIER's output varies rather a lot during the flight?  Or if you're pointing at CoM, and have adjusted with rotate tool to get almost 0 torque on RCS build aid, then it doesn't matter how much power it has?

Could you do this with a rapier and nuke on a bicoupler, or would you have to  angle them so sharply to thrust through CoM that you'd suffer cosine losses?

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I happened to have one of my SSTO of a similar mass (~32t). That one uses 2 rapiers and 1 nuke. I carry oxidizer only enough for rapiers to work in closed cycle until I reach ~1.8km/s. I find this combination fairly comfortable for me: 2 rapiers give me a comfortable ascent TWR; 1+2 engine setup enables me to do a symmetric design - nuke in the central 1.25m stack and rapiers on side 1.25m stacks; more usage of 1.25m stack further utilizes tank mass more efficiently.

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40 minutes ago, AeroGav said:

Does it matter that the RAPIER's output varies rather a lot during the flight? Or if you're pointing at CoM, and have adjusted with rotate tool to get almost 0 torque on RCS build aid, then it doesn't matter how much power it has?

No. Below 22 km you need very little aerodynamic control surfaces to compensate, even if you don't thrust exactly through CoM. If using a Whiplash you could probably get away without rotating for most designs.

Thrusting through CoM only becomes important once you get above 22 km and engage Closed Cycle.

 

40 minutes ago, AeroGav said:

Could you do this with a rapier and nuke on a bicoupler, or would you have to  angle them so sharply to thrust through CoM that you'd suffer cosine losses?

I don't recommend rotating the nuke. At least I find it really annoying to fly in vacuum with a craft that doesn't have engines and cockpit aligned.

You could rotate the cockpit to compensate, but that just makes the navball horizon be offset while flying atmospheric. AoA readout would be offset, too.

With my trials (designing single stage to Laythe and back) I didn't notice cosine losses in atmospheric or the Closed Cycle part. I was more focused on maintaining control of the craft. :D

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This is why I want little .625m nukes. So I can have 2 of them for balance. 1 rapier and 2 mini-nukes.

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28 minutes ago, bewing said:

This is why I want little .625m nukes. So I can have 2 of them for balance. 1 rapier and 2 mini-nukes.

Tweakscale?  Interstellar?

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2 hours ago, bewing said:

This is why I want little .625m nukes. So I can have 2 of them for balance. 1 rapier and 2 mini-nukes

I do want the 0.625m mini-nukes, if only for superefficient 0.625m probes and size0 spaceplanes. Yes please.

But it is possible to use 1 RAPIER and 1 Nerv in a completely balanced way with all the thrust through the CoM: node-attach the RAPIER on the bottom node of the Nerv, then offset the RAPIER 5.5 clicks (or 11 in fine mode) into the Nerv. The exhaust of both engines will work perfectly without overheating either one.

With a shock cone on the RAPIER end, rotated to face front and offset upward, it even forms a nice smooth transition between the Nerv chamber and the RAPIER front edge. Or placed behind a pre-cooler, the Nerv can be offset fully into the cooler to neatly align the RAPIER to it.

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7 hours ago, swjr-swis said:

With a shock cone on the RAPIER end, rotated to face front and offset upward

This is a bad idea from a drag perspective, though. If you rotate a tailcone, KSP starts treating it as a nosecone. Which means you now have a single stack with the drag of 2 stacks, minus the tailcone drag.

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4 hours ago, Val said:

This is a bad idea from a drag perspective, though.

I was merely making an aesthetics suggestion with this: it visually smoothes the transition between the two engines, both in shape and texture. It's about the smoothest fit I've found.

But you are correct about the drag penalty. For a craft design that easily breaks Mach 1 at sea level it'll hardly make a difference (top speed affected by perhaps 1-3%), but if it is struggling to break Mach 1, every bit of drag can make the difference. So one should weigh aesthetics against performance.

Relevant tidbit: in 1.2.1, shock cones are again the least draggy nose and tail cones, by a noticeable margin. Very worth swapping a midge of fuel for the extra weight compared to other cones (and it also allows more compact stack designs).

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My solution to 2-engine SSTO:

2015-07-07_00014.jpg

LV-N attached to COStrut, then rotated 180 degrees, clipped into LF tank.

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Regardless, if you arrive into orbit with spare oxidizer, my calculations say "BURN".

The delta-V of fuel lost to worse ISp of RAPIER will be more than recuperated by reducing dry mass of the craft for LV-N.

Say, your craft is 30 tons, arriving into orbit with 5 tons of OX and 15 tons of LF. (assuming 1:1 usage for simplicity, although with current ratio the result is worse.)

Scenario 1: LF only, ferry that useless oxidizer around. Wet mass 50 tons, dry mass 35 tons, ISp = 800

dV=2800m/s.

Scenario 2: First burn away the 5 tons of oxidizer. a) Wet mass 50 tons, dry mass 40 tons, ISp = 305. b) Wet mass 40 tons, dry mass 30 tons, ISp = 800.

dV = 667m/s + 2260m/s = 2860m/s.

Never mind any transfer/departure burn will cost less delta-V both due to Oberth effect (burn ending in LKO with highest speed, instead of somewhere far out) and due to better precision (closer to impulse burn).

 

 

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7 hours ago, Sharpy said:

Regardless, if you arrive into orbit with spare oxidizer, my calculations say "BURN".

The delta-V of fuel lost to worse ISp of RAPIER will be more than recuperated by reducing dry mass of the craft for LV-N.

Say, your craft is 30 tons, arriving into orbit with 5 tons of OX and 15 tons of LF. (assuming 1:1 usage for simplicity, although with current ratio the result is worse.)

Scenario 1: LF only, ferry that useless oxidizer around. Wet mass 50 tons, dry mass 35 tons, ISp = 800

dV=2800m/s.

Scenario 2: First burn away the 5 tons of oxidizer. a) Wet mass 50 tons, dry mass 40 tons, ISp = 305. b) Wet mass 40 tons, dry mass 30 tons, ISp = 800.

dV = 667m/s + 2260m/s = 2860m/s.

Never mind any transfer/departure burn will cost less delta-V both due to Oberth effect (burn ending in LKO with highest speed, instead of somewhere far out) and due to better precision (closer to impulse burn).

 

 

Ah yes, i forgot you should burn your lowest ISP propellant first.

My old launch technique was to run the rapier till thrust drops below 80kn,  bring in the 2 nukes (+120kn), then when the rapier flames out completely, switch to close cycle (+180kn)

So i should not start the nukes till i've used up all my oxidizer?  The single rapier has 180kn in close cycle which is more than enough thrust, given how easily it goes to space with just the 2 nukes.

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2 hours ago, AeroGav said:

So i should not start the nukes till i've used up all my oxidizer?  The single rapier has 180kn in close cycle which is more than enough thrust, given how easily it goes to space with just the 2 nukes.

Good TWR reduces gravity losses and gets you to orbit faster, so use all the engines you can during non-airbreathing ascent. But once your apoapsis is above atmosphere you can stop the nukes. Although the savings by doing that are minuscule. If your LV-N has ten minutes worth of fuel and your Rapier has 15 seconds left, then the overlap is really meaningless.

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My vote goes to LF only - And it seems to me I follow a very similar pattern - This one weights 31t at take off and I've got 1 rap and 2 nukes, so it fits. 

KQBKZPq.png

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