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Getting Interstellar

sevenperforce
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sevenperforce

sevenperforce

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We spend too much time close to Kerbin -- it's time to think beyond!

I was building a Sepratron-powered orbiter for this excellent challenge:

When I used my crewed rocket with just a probe core instead, I discovered that I was very nearly interstellar. Adding two extra stages and a few more tweaks got me out of Kerbol's SOI on a Sepratron-only rocket massing just over 16.2 tonnes.

It made me think...what are the limits of going interstellar? 

Who can go interstellar with the lowest liftoff mass? Who can get interstellar the fastest? Who can go interstellar with the fewest parts? Who can get onto an interstellar trajectory as low as possible within Kerbin's atmosphere? Is it possible to take an SSTO to Kerbin's atmosphere without staging? What is the lowest mass-to-interstellar without using nukes or ions?

Rather than doing a leaderboard with this challenge, I was thinking of doing a challenge-within-a-challenge. Each entrant should post an interstellar vehicle optimized for whatever they are good at and challenge other entrants to beat them. I'll kick it off -- go interstellar in under 16 tonnes!

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mystifeid

mystifeid

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This is the creatively named 'Interstellar', a 7.885 ton probe capable of Kerbol escape. I seem to remember a few people being very good with these sorts of things so doubtless they will not find it hard to get under that weight - I know I was still getting the weight down but sooner or later, enough is enough...

 

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sevenperforce

sevenperforce

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12 minutes ago, mystifeid said:

Not really - and after that sepratron effort I've got a feeling you're holding on to something about a ton lighter.

Anyway ... this is 2.65 tons.

 

Good idea using the Twitches -- they have better SL isp.

On my end, I just managed to go interstellar on 1.812 tonnes using a Dawn on top of a Spark on top of a Juno. It took a little cleverness with a drag-killing intake, though.

T5rvsyL.png

It has 800 m/s to spare past interstellar velocity but other than just reducing some of the initial propellant load I'm not sure what else I could do.

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sevenperforce

sevenperforce

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1 minute ago, mystifeid said:

Called it. Awesome again!

Hat trick: if you flip a 1.25-m circular intake and slap it on the back of a fairing, then slide it up almost all the way into the fairing, an engine mounted inside the fairing can fire through it without injury. It also completely kills the tail drag of the fairing, which is the important thing. I hit 680 m/s on the Juno at 8 km before I fired up the Spark, blew off the Juno, and pulled up under power using props in the fairing to get my apoapse to 50+ km. Then I used the decoupler to blow off the fairing and burned what remained of the Spark to get just shy of a circular orbit.

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mystifeid

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9 hours ago, sevenperforce said:

Hat trick: if you flip a 1.25-m circular intake and slap it on the back of a fairing, then slide it up almost all the way into the fairing, an engine mounted inside the fairing can fire through it without injury. It also completely kills the tail drag of the fairing, which is the important thing. I hit 680 m/s on the Juno at 8 km before I fired up the Spark, blew off the Juno, and pulled up under power using props in the fairing to get my apoapse to 50+ km. Then I used the decoupler to blow off the fairing and burned what remained of the Spark to get just shy of a circular orbit.

Very good to know.

I think I've given up trying to go straight up - just can't seem to quite do it at that mass. So, I hope you get a kick out of this one.

1.727 tons.

(It has a battery clipped into the top fuel tank which I'm not proud of but this was more of a test than anything else.)

 

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sevenperforce

sevenperforce

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4 hours ago, mystifeid said:

I think I've given up trying to go straight up - just can't seem to quite do it at that mass. So, I hope you get a kick out of this one.

1.727 tons.

(It has a battery clipped into the top fuel tank which I'm not proud of but this was more of a test than anything else.)

Mother of god, that was impressive. I've never used anything from Breaking Ground but maybe it is time.

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mystifeid

mystifeid

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18 minutes ago, sevenperforce said:

Mother of god, that was impressive. I've never used anything from Breaking Ground but maybe it is time.

You can put both props on the bottom but the only way I know to make a bearing between them is to use another engine. It looks prettier but it's marginally heavier and a lot harder to steer. Anyway, I'm guessing you still have another quarter of a ton to lose.

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sevenperforce

sevenperforce

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16 minutes ago, mystifeid said:

You can put both props on the bottom but the only way I know to make a bearing between them is to use another engine. It looks prettier but it's marginally heavier and a lot harder to steer. Anyway, I'm guessing you still have another quarter of a ton to lose.

The Spark has more thrust than I need, but I've been using it to burn off the Juno. Maybe it's worth the mass loss to add a decoupler.

**looks**

Yep, the mass of a decoupler is less than the mass difference between a Spark and an Ant. So I may swap them out and see if I can ditch one of those donut tanks entirely.

Also you can save more mass on your current vehicle by attaching the cubic strut to the lower stage, attaching the prop decoupler to it, and then rotating/translating it up to the top.

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mystifeid

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13 minutes ago, sevenperforce said:

Yep, the mass of a decoupler is less than the mass difference between a Spark and an Ant. So I may swap them out and see if I can ditch one of those donut tanks entirely

Why don't you ditch the Dawn and just use an Ant?

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mystifeid

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9 minutes ago, sevenperforce said:

No way an Ant can sport the 4700 m/s that a minimal Dawn stage has.

Okto2 + OscarB + Ant in LKO = game over.

Just have to make sure the second last stage points the rocket pro-grade. No EC needed after staging and the Ant starts to burn. No reaction wheel needed. Launch at the right time and your circularisation burn also becomes your ejection burn.

Look at that last video again.

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sevenperforce

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20 minutes ago, mystifeid said:

Okto2 + OscarB + Ant in LKO = game over.

Just have to make sure the second last stage points the rocket pro-grade. No EC needed after staging and the Ant starts to burn. No reaction wheel needed. Launch at the right time and your circularisation burn also becomes your ejection burn.

Look at that last video again.

Doing the math...that's 3612 m/s.

Dawn + OKTO2 + single 1x6 solar panel + small radial xenon tank = 5164 m/s. No contest.

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sevenperforce

sevenperforce

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1 hour ago, RoninFrog said:

Oscar B + Ant is actually significantly lighter.

Hmmmm, good point. Let's see here.

The ion setup is 424 kg. The Ant+OKTO2+Oscar setup has a dry mass of 90 kg and a wet mass of 290 kg for 3612 m/s. 

Add another Oscar and you're up to a dry mass of 120 kg with a wet mass of 520 kg, for 4527 m/s. So more mass, less dV.

You could try to add the Oscar (or a pair of Dumplings) as a drop tank but that adds complexity and dry mass. And you'd need guidance on the drop tank stage which hurts more.

Plus, the Dawn can do useful things like firing at full thrust while occluded, which allows you to use it as a parallel stage while stacked serially.

The disadvantage of the ion setup is that you either need to do a retrograde orbit or you need to do a zillion periapsis kicks. 

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mystifeid

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8 hours ago, sevenperforce said:

Doing the math...that's 3612 m/s.

Dawn + OKTO2 + single 1x6 solar panel + small radial xenon tank = 5164 m/s. No contest.

You might have to explain exactly why you need that 5164m/s

Looking at the closest frames that I can to the relevant events in my last video:

Initial orbital velocity - 2282.9m/s
Escape orbital velocity -  4954.5m/s
dV - 2671.6m/s

Rocket dV remaining at initial orbit - 3079m/s
Rocket dV remaining at Kerbol escape -  136m/s
Rocket dV used - 2943m/s

That's a 271.4m/s loss, presumably simply because we're not burning in a straight line. (I wasn't locked on prograde either.) Maybe some gravity loss too.

Even if you bounce a lot with a Dawn, I wonder what sort of difference you're seeing. Must be pretty substantial.

Anyway, the point is that the goal is to make Kerbol escape. Why do you need something with a reported 5164m/s dV when I've used less than 3000? The Dawn stage is heavy. And it requires much more energy - read mass - to put into orbit.

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sevenperforce

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27 minutes ago, mystifeid said:

You might have to explain exactly why you need that 5164m/s

<snip>

Anyway, the point is that the goal is to make Kerbol escape. Why do you need something with a reported 5164m/s dV when I've used less than 3000? The Dawn stage is heavy. And it requires much more energy - read mass - to put into orbit.

Oh, I use it to complete circularization. Which means you don't need as much mass below.

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