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Assume we have a society that has sophisticated enough engineering to build a SSTO chassis that can survive the rigors of repeated launch (MaxQ) and re-entry (thermal stress). What thrust to weight ratio would they have to achieve from their propulsion systems in order to get the craft into orbit around an Earth-clone planet without the need for external boosters?

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  • Are you aware of this? McDonnell Douglas DC-X - "https://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X" - Screw it, if you follow this you find NASA Kills it. Shortly there after SpaceX and Blue Origin have some new employee's and exciting Reusable Rockets. – Enigma Maitreya Mar 04 '17 at 22:20
  • I've migrated this from Worldbuilding to Space Exploration as per the asker's request. I think it's okay for here, but if it's not, we'll take it back. – HDE 226868 Mar 04 '17 at 22:31
  • This is an interesting question with no easy answer. The SSTO needs TWR lower than 1 for flight on wings, but to reach any considerable fraction of orbital speed it must leave the atmosphere, as Ram Rise heating would obliterate it. For that it needs TWR approaching 1 - but not quite, because a part of its gravity drag is gone. Regardless, unless you have some magical propulsion of impossible delta-V, you'll want to make it in a good time to orbit to reduce gravity losses, so TWR>1 will be definitely welcome... – SF. Mar 04 '17 at 23:06
  • Do you talk some specific type of SSTO rocket style, jet style or in a general one? – MolbOrg Mar 04 '17 at 23:28
  • https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation DeltaV = Ve * ln(Mi/Mf) where DeltaV is the change in velocity -- about 7 km/s for earth. Ve is the effective exhaust velocity ln is the natural logarithm Mi is the initial mass with fuel Mf is the final dry weight. So DeltaV/Ve = ln (Mi/Mf) e^(DeltaV/Ve) = Mi/Mf – Sherwood Botsford Mar 04 '17 at 22:11

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What thrust to weight ratio would they have to achieve from their propulsion systems in order to get the craft into orbit

This wording indicates that you think there is a threshold. All designs with a TWR lower than this will fail to reach orbit, and designs with a higher TWR will succeed. This premise is incorrect. As SF says in his comment, a winged SSTO can get away with a TWR lower than 1. So depending on body design (large wings, small wings, lifting body or cylindrical rocket) you get a large range of possible TWR figures.

Other numbers are more critical than TWR:

  • You need sufficient delta-V to reach orbit.
  • You need a mass fraction that's good enough to bring some payload to orbit.

Those two combined (via the Rocket Equation) give a minimum specific impulse required to get that payload to orbit. TWR is a consideration via gravity losses: the longer you take to get to orbital speed, the more gravity losses you'll have and the more delta-V you'll need to account for.

So in order to be able to calculate anything, we need 2 parameters:

  • required payload
  • available specific impulse

Those two are enough to calculate the starting mass of the SSTO. Economics then decides if such an SSTO is worth building.

Hobbes
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  • I would respectfully point out your answer is Off Topic because it does not answer the question the OP asked. While I agree this will always be about delta-v that was not what was asked for. – Enigma Maitreya Mar 05 '17 at 17:03
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    Pointing out that a question has an incorrect premise is on topic. – Hobbes Mar 05 '17 at 18:13
  • With all due respect it would seem that is what messages are for else why this "This does not provide an answer to the question" as a reason for deleting an answer? – Enigma Maitreya Mar 05 '17 at 18:20
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    "The premise is incorrect but here is an answer to a closely related question that the correction of premise might reasonably lead to" is helpful enough to be worth making an exception. – Russell Borogove Mar 05 '17 at 18:29
  • @RussellBorogove - Thanks for that reply and regardless of how people may take things ... I prefer "helpful enough ..." but as I have read through a lot of questions and answers I get the impression that the value judgment is ... extremely broad indicating a possible preferential implementation. Regardless I accept your answer and thank you for confirming the Rules are not Literally the rules, rather guidelines to be used by people to do what they (subjectively) think is right. – Enigma Maitreya Mar 05 '17 at 19:01
  • @EnigmaMaitreya: Well, in case of this question - if we want any realistic answer, we need the extra data. Otherwise the answer is trivial: anything above 1.0 for non-lifting-body. For lifting body it will be inverse of the lift to drag ratio; for typical good lift to drag of 25, that's 0.04, less for better LtD. Of course these are completely unrealistic due to other factors, like ram rise, realistic specific impulse and so on. – SF. Mar 06 '17 at 10:30
  • @SF. :) RussellBorogove answered my question and confirms my concern. Either the Rules apply to every one equally or they do not. On this forum they do not, anytime subjectivity is allowed in a "rules based system" then division occurs and the rules get weaponized. One only need to understand the true nature of an environment to be able to survive in it. Do you see the applicability of this? "The Oracle: [referring to the Architect] Oh please! You and I may not be able to see past our own choices, but that man can't see past any choice!". Does one value Dissent or Kill it. Google it :) – Enigma Maitreya Mar 06 '17 at 18:18
  • @EnigmaMaitreya: you're advocating for a very narrow definition of on-topic because your first answer on this site attracted some downvotes and close votes. Going back through its edit history, it seems those votes were caused by the first version of your answer, which was incomplete. The way you worded your first sentence might also have thrown some people off. Some users of this site are quick to downvote new users because we get a lot of low-quality answers (and spam) from new users. See http://meta.space.stackexchange.com/questions/645/how-to-handle-first-questions-answers-by-new-users – Hobbes Mar 06 '17 at 18:39
  • The "This does not provide an answer to the question" reason to close exists because we sometimes get off-topic answers like the recently-deleted one here: http://space.stackexchange.com/questions/1382/how-frequently-do-asteroids-collide-with-each-other – Hobbes Mar 06 '17 at 18:40
  • @Hobbes - It is probably my ignorance by I can not identify that post as being deleted as it apparently would let me post an answer. (I HATE ENTER MEANS DONE). I have done my best to indicate that "judgement" is better than "Literal". I accept the consequence that goes with that. The weaponization of the system. I have never personally since GEinie / Prodigy MUDS, Ultima Online, Everquest etc. seen a community be judgmental AND not divided, as we are all humans. Meaning do not read things into what I say and attribute them to me. There are consequences to being divided Dissent is vulnerable. – Enigma Maitreya Mar 06 '17 at 18:56
  • That question has two answers, one from 2013, and one from today which starts with "i like pizza" - this is the deleted answer. – Hobbes Mar 06 '17 at 18:56
  • @Hobbes - Shrug I take your word for it as when I go there, there is only 1 answer and I do not see any references to a 2nd answer. Trust me I do not dispute the need to moderate that has, to me NEVER been an issue. For what it is worth I deleted then added the previous comment. because I prematurely hit enter and got caught on the 5 minute rule – Enigma Maitreya Mar 06 '17 at 19:01
  • @EnigmaMaitreya: it's there; you need quite a bit of karma to see deleted answers. or "answers". If the answer is even remotely on-topic and non-harmful, it's to stay. Only classes of answers deleted are either utter junk or a really, really bad advice (one directly leading to harm/injury/damage). Example non-spam answer deleted, on Sports.SE, to my question about choice of goggles for ASG was one that could easily lead to eye injury. The standard required from questions is considerably higher. – SF. Mar 06 '17 at 22:49
  • I disagree with your two parameters that decide if a SSTO can be built. First, you omitted the utterly vital propellant fraction. Second, if you're flying a lifting body you have lower gravity losses and thus can do it with less delta-v. – Loren Pechtel Jun 14 '19 at 05:33
  • propellant fraction is covered by "a mass fraction that's good enough", and I mention lifting bodies too. – Hobbes Jun 14 '19 at 06:55
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There is a chart and the explanation at wikipedia

Being limited by the performance of the existing chemical fuels and being limited by the sane take-off mass of less than 3K metric tonns (the weight of a Saturn V, the biggest rocket ever) then dry weight (everything but the fuel itself - the body, the payload itself, engines, re-entry fuel, safety margin fuel, landing gears, wings etc - everything) must fit into less than ~10% of the total mass, close to 5%, which is very challenging task to achieve.

Usage of better fuels like hydrogen improves the thrust to weight ratio but affects the dry weight because of the decreased density and insulating material for the cryogenic fuel tanks, so overall it does not make a big difference.

In order to SSTO to be technically possible we need the next generation of fuels / engines. Something like metallic hydrogen, thermonuclear fusion engine, photon engine etc.

ilyakharlamov
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The question asked I am answering

What thrust to weight ratio would they have to achieve from their propulsion systems in order to get the craft into orbit around an Earth-clone planet without the need for external boosters?

I had wanted to use the Delta Clipper X but had a Eureka Moment that it was a scale testing unit and switched to this instead. To the best of my knowledge, there has not been a "Public" SSTO made or launched. Some may consider Lockheed a credible source.

https://en.wikipedia.org/wiki/VentureStar - VentureStar

While never built the work presented here seems to be consistent with a Full Scale Ship (SSTO)

Function    Manned Re-usable Spaceplane
Manufacturer    Lockheed Martin
Country of origin   United States
Size
Height  38.7 m[1] (127 ft)
Diameter    N/A
Mass    1,000,000 kg[1] (2,200,000 lb)
Stages  1
Capacity
Payload to LEO  20,412 kg[1] (45,000 lb)
Launch history
Status  Cancelled
Launch sites    Unknown
Total launches  0
First stage - VentureStar
Engines 7 RS2200 Linear Aerospikes[1]
Thrust  3,010,000 lb[1] (13.39 MN)
Fuel    LOX/LH2[1]

It gives both weight and thrust.

The following is based on the MASS because I have no clue if the Payload is included or not.

0.1643017177 lbs per Newton would be your answer in Newtons.

Or for my bud SF that likes to groan a lot :) 164.3017177 lbs per kN

Or 164,301.717699776 lbs per MN

Treat lbs of mass as lbf of weight, divide thrust in lbf by that and you have the clean dimensionless TWR

3,010,000 / 2,200,000 gives 1.36818118118182 - Just for you

For those that want a good and interesting read on Real Life SSTO's go here

https://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X

The Delta Clipper, VentureStar, SpaceX, Blue Origin Are all linked together. The Delta Clipper was taken over by NASA killed in favor of their choice the VenturStar because of the landing being horizontal rather than vertical then canceled it for the Space Shuttle and guess were everyone went to work. Let us .... say the obvious, The Shuttle, Delta Clipper XA, VentureStar all ran separately and parallel to each other.

Enigma Maitreya
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    "lbs per kN" groan – SF. Mar 04 '17 at 22:59
  • The groaning presumably comes from mixing imperial (lbs) and SI (N) units, not imperial and kilo- or mega- units. – Nathan Tuggy Mar 04 '17 at 23:30
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    I like SF his taunts at me are cool and make me laugh (with both respect and all other good ways). Hopefully no one takes my response to him wrong. As in I would buy him a Beer any day of the week and have a good time. – Enigma Maitreya Mar 04 '17 at 23:32
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    @EnigmaMaitreya: Exactly. You've got both lbs and lbf, and kg and kN. What possessed you to mix the units? Treat lbs of mass as lbf of weight, divide thrust in lbf by that and you have the clean dimensionless TWR. – SF. Mar 05 '17 at 00:38
  • @SF - I would have thought that answer was obvious, being American and all and lbf vs kN = hum most scientist prefer kN in this scenario and lbs .... what can I say? Oh wait :) I am working a compromise here :) I note your not saying the mixing is wrong as in giving an error ... are you? To be clear as it seems people like to obfuscate things, I have dealt with the UK + Europe for longer than ... some posters and editors here have been alive. I knew exactly what you meant as it would not have been the first discussion of that nature .... not by a long shot. :) So are we good? – Enigma Maitreya Mar 05 '17 at 01:41
  • @SF. Not going to have this go to chat so :) Besides isn't the real issue here there is no Chicken so there can not be an Egg and isn't that a No No on this forum? The talking about the Egg that is. – Enigma Maitreya Mar 05 '17 at 01:49
  • You're still not providing TWR, just TMR. Get that TWR in whatever system you like. (although if you provide it in lbf per newton, I swear I'll sock you.) – SF. Mar 05 '17 at 03:52
  • @SF. You have the ability to edit go for it I don't mind :) I give you kart blanch permission to edit any of my post Shrug I trust you :) – Enigma Maitreya Mar 05 '17 at 05:09
  • Um, your last paragraph shows a great misunderstanding of the history of those programs. Considering that Delta Clipper and Venturestar all were started long after the Space Shuttle program began. – Organic Marble Mar 05 '17 at 17:38
  • @OrganicMarble - Hum did I say the Space Shuttle was started after them or did I say the Space Shuttle survived the killing of the Delta Clipper XA and VentureStar I would have sworn I said "canceled it for the Space Shuttle" If you like I can easily remove any doubts you have. – Enigma Maitreya Mar 05 '17 at 17:57
  • They were supposed to lead to a shuttle replacement, so "canceling them for the shuttle" makes no sense. The shuttle was already there. – Organic Marble Mar 05 '17 at 18:03
  • @OrganicMarble - I am not about to participate in the moving this to Chat. Answer this, IF the Shuttle Program did not exist AND IF the powerful Congressional Members, representing their states that stood lose jobs and money did NOT exist would the VentureStar have been canceled and IF SO then what program would have been selected. You get the last word as these messages are off topic and I have no interest in getting involved in that. – Enigma Maitreya Mar 05 '17 at 18:14