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Engine reusability - possible implications for the SSTO economy


eempc

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I've been designing a lot of SSTOs, both aerospace plane and rocket based, in anticipation of the contracts-based economy. I have found that the rocket-SSTOs are far easier to build, fly and can lift a bigger mass. As an example of a simple SSTO-rocket: command pod on top of fuel tank on top of a big engine with wings. Launch vertically, land horizontally. I could not descibe the construction and piloting of an aerospace plane in such simple words.

Which made me wonder what drawbacks the rocket-SSTO was supposed to have. The issue I see is that I am using a rocket engine that was designed for a single launch. I know that the Ariane 5's main rocket engine can be reignited, it was such a big deal that even Wikipedia mentions it. Even if an engineer could somehow recover the Ariane 5's rocket, can it be reused?

By contrast, nobody raves on about the reusability of a jet engine, since it is obviously supposed to be reusable.

If you had to assign a number of reusable flights to various engines, what could they be?

Example numbers (comparative):

RAPIER: 200 flights, based on the Skylon's airframe's reusability of 200 flights.

S1 SRB-KD25k: 100 flights, analogous to the space shuttle's SRBs.

Mainsail: 5 flights, by analogy to the Ariane 5's engine's tentative reignitability.

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SSTO and engines reusability has pros and and cons.

First, let's talk about SSTO. In real life there isn't any, because:

1. It's very very hard to fit 8 Km/s of Dv into a single stage

2. They're inefficient to bring payload to orbite (low payload mass percentage over total mass)

3. Reusing them might be dangerous

The third point came up when the Challenger exploded 73 seconds after launch, due to an heavy hot exaust leakage in the right booster. The reason has been attribuited to the low temperature, but since that tragic day the SRB have not been reused any more.

Reusing it's a big word for space ingeneers: it means saving loads of money. However it's not so easy. Re-igniting doesn't mean reusing: engines (and pumps and takns and heating coils and auxiliary power units...) are designed to work for a certain number of seconds or minutes, either for a determined amount of times, so yeah, reusing and Arian V engine mean likely creating an huge hole in the ground (and in your capital:)).

The worse thing is that usually the more is powerful, the less is resistant. Your RAPIER could hardly go past the third use without heavy manteinance, not to talk about your booster. Reusing is hard, requires high expenses in manteinance and needs advanced ways to check damage on the components without being invasive or distructive.

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This idea is discussed in every field where performance is an issue. Durability and performance don't generally run alongside. Suprisingly, some things are cheaper to be replaced than to be fixed.

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KSP allows you to do things not normally in real life (such as recovering a Mainsail engine), so try to think in terms of a video game. I think it has been stated that random critical failures are not going to be part of the game (correct me if I am wrong). If maintenance were to be required when reusing an engine, it would most likely just be a deduction in money. I can't see how a mini-game where you take apart an engine to inspect for damage would be fun in any way.

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Which made me wonder what drawbacks the rocket-SSTO was supposed to have.

That's exactly my point for quite a while. In KSP, SSTOs have just one disadvantage compared to conventional rockets and that's limited payload capacity. Besides that there are only advantages as KSP does not implement any kind of part wear or random failures so from the financial point of view, running a fully reusable SSTO is going to cost way less than running a space program based on conventional rockets that lose stages in process.

In my opinion, players should be rewarded for changing and improving their designs over time rather than for using the same ship over and over again. Reusable technology should not be favored over classical rockets.

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As an example of a simple SSTO-rocket: command pod on top of fuel tank on top of a big engine with wings. Launch vertically, land horizontally. I could not descibe the construction and piloting of an aerospace plane in such simple words.

Simple SSTO plane: Cockpit in front of fuel tank, in front of engine, with wings. Take off horizontally, land horizontally.

Not so difficult.

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I've been designing a lot of SSTOs, both aerospace plane and rocket based, in anticipation of the contracts-based economy. I have found that the rocket-SSTOs are far easier to build, fly and can lift a bigger mass.

First let me compliment you on the exactitude of your terminology :-)

I think one of the problems of vertical-launch rocket SSTOs is that the wings can make them difficult to control during atmospheric flight. Have you had that problem and have you had to adopt a 'non-conventional' ascent path?

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Being familiar with game design, you will have to do very poorly to be worried about money in this game. I'm betting they will make it very forgiving, and nearly impossible to actually "lose" the game, even if a lot of launches fail. Given revert and quicksave, most players will be "kertrillionairs" by the end of the tech tree. Reusable crafts won't be necessary to save money, and will remain a just for fun thing. I don't have a point really, just saying there will be plenty of extra kerbucks to build whatever you want.

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Being familiar with game design, you will have to do very poorly to be worried about money in this game. I'm betting they will make it very forgiving, and nearly impossible to actually "lose" the game, even if a lot of launches fail. Given revert and quicksave, most players will be "kertrillionairs" by the end of the tech tree. Reusable crafts won't be necessary to save money, and will remain a just for fun thing. I don't have a point really, just saying there will be plenty of extra kerbucks to build whatever you want.

I think you might be right, we'll have to wait and see. It might have a far larger impact on the ironman crowd where launch or mission failures might have an impact. Who knows, maybe they'll implement a dollar cost to using the revert key. I'm as curious as anyone.

My suspicion is that they'll simply return a fixed percentage of the dollar value of the recovered equipment back to your bank account rather than model actual reuse of the parts.

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Being familiar with game design, you will have to do very poorly to be worried about money in this game. I'm betting they will make it very forgiving, and nearly impossible to actually "lose" the game, even if a lot of launches fail. Given revert and quicksave, most players will be "kertrillionairs" by the end of the tech tree. Reusable crafts won't be necessary to save money, and will remain a just for fun thing. I don't have a point really, just saying there will be plenty of extra kerbucks to build whatever you want.

Yeah, this is an issue of game balance. It can be difficult to walk the line between 'challenge' and 'forgiving of failure'.

In a sim like KSP, it would be interesting to see "ongoing mission" costs in addition to the upfront vehicle purchase costs. The purpose of this mechanic would be allow players in a "failing" game state to free up cash by ending some missions. Ideally, this could accommodate both the "challenge" and "forgiving" goals. I would say you should only be able to terminate probes midflight, but some people are heartless, sooooo... rep penalty?

Early game you would be awash in cash (Kash? Kredits?), and failing would actually take some effort. Later game you could choose to ditch some old missions to free up cash, or take on some easy contracts (which shouldn't require maintence fees after the contract is filled), or switch to a more SSTO-based system to save dollars.

Anyway, it's just a thought to avoid the budgets become irrelevant if the "kertrillionaire" out is a practically guaranteed late game situation.

Edit: Yikes, I just realized this could have huge implications on maintaining an Remotech comm network... But maybe that would become part of the challenge of that mod. It certainly isn't there to make the game easier.

Edited by LethalDose
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