Ideal SSTO's Versus Two Staging...Settled?

[Spacescifi]

 Picture an SSTO that can boost 100 tons to orbit Mon-Friday...several times a day. You may laugh but let us consider what that would actually require.

 

What you need for scif/ ideal SSTO:

1. High thrust with high efficiency, meaning virtually all your energy input is going directly into thrust, instead of just enough wasted energy contributing to melting your high thrust/high efficiency engine.

2. Drop in reentry rather than ballistic reentry. Ballistic reentry chars the hull and kills rapid reusuability. At the very least a very low ballistic trajectory could be tolerated. Ideally none,  just fall straight down.

Consequences: A high efficiency/high thrust rocket may well be radioactive in nature, since that requires higher energy than chemical reactions alone can generate.

But theoretically, what if you engineered the rocket engineto emit low to no cancer causing radiation?

Then you still have heat. If you have a true SSTO that is not melting itself and carrying 100 tons or greater regularly, then you are surely charring wherever you land or launch. Since the plume you set off will be a lot hotter than anything man has made to date.

Has to be to be a true and ideal SSTO.

The real irony is that even with the ability to be a true SSTO you may two stage it.

Why?

Unless the super SSTO can throttle it's heat waaay down to lose some of it's high thrust/efficiency in the process, it will nuke the launch/landing site every time.

So if it cannot throttle that far back, two staging could get the second stage up high enough where it won't nuke beam damage the land.

 

Question: Anyone want to calculate the energy cost of sending the equivalent of Spacex mass (super heavy and starship second stage) as a complete SSTO, by sending it to orbit, and then stopping it's orbit to hover over the landing site in space before dropping straight down (linear reentry rather than ballistic)?

 

Because the answer will reveal whether a true SSTO must be two stage or not. If it must it will be because it will land in lava otherwise....besides cancer rays.

Edited July 3, 2021 by Spacescifi

[RyanRising]

I think you might be thinking of the wrong concept. An SSTO is a single-stage-to-orbit. Nothing more, nothing less. It says nothing about reusability, nor efficiency, nor payload.

[tater]

For a thread on SSTO physics, it's odd that it contains no physics.

[Spacescifi]

3 minutes ago, RyanRising said:

I think you might be thinking of the wrong concept. An SSTO is a single-stage-to-orbit. Nothing more, nothing less. It says nothing about reusability, nor efficiency, nor payload.

 

It's a waste of resources if not resuasable. Which is why we do not do it.

SSTO's won"t be much viable until they meet OP...outside of emergencies or military use.

[RyanRising]

1 minute ago, Spacescifi said:

 

It's a waste of resources if not resuasable. Which is why we do not do it.

SSTO's won"t be much viable until they meet OP...outside of emergencies or military use.

Being impractical to use does not make something not a “true” SSTO. You’re thinking of something more like “rapidly reusable spacecraft.”  Plus, what tater said - your assertations don’t appear to have much foundation in real physics, only observations of what we’ve done or thought up as concepts for spacecraft.

[Spacescifi]

6 minutes ago, RyanRising said:

Being impractical to use does not make something not a “true” SSTO. You’re thinking of something more like “rapidly reusable spacecraft.”  Plus, what tater said - your assertations don’t appear to have much foundation in real physics, only observations of what we’ve done or thought up as concepts for spacecraft.

 

Little use arguing semantics when the meaning is clear.

[Rakaydos]

1 hour ago, Spacescifi said:

 

Little use arguing semantics when the meaning is clear.

So why are you arguing for any other definition than... single stage, into orbit?

[Spacescifi]

9 minutes ago, Rakaydos said:

So why are you arguing for any other definition than... single stage, into orbit?

 

This is not an anwer.

I made this OP because I read another thread not far away about SSTOs, and I thought it good to take the idea of an SSTO to it's maximum ideal potential and see whether or not it can compete with 2 stage to orbit.

The answer is yes..but only for niche circumstances that absolutely require it.

Which is only somewhat better than today where they clearly cannot even compete.

Edited July 3, 2021 by Spacescifi

[Terwin]

1 hour ago, Spacescifi said:

Little use arguing semantics when the meaning is clear.

I would suggest that your meaning is not nearly as clear as you seem to think it is, thus the questions/suggestions.

 

Earth orbital velocity: just under 30km/s

Time to orbit: 8.5 min(for the shuttle)

8.5*60*9.8=~5000m/s  (used as gravity loss estimate)

means we need at least 70km/s without aero-breaking (or accounting for air friction during launch)

So just calculate the fuel fraction for the isp of the engine you want to use and see if it is feasible(and no, anti-mater + pusher-plate is never feasible under the laws of physics as we currently understand them, so use something else)

[Shpaget]

6 minutes ago, Spacescifi said:

I read another thread not far away about SSTOs, and I thought it good to take the idea of an SSTO to it's maximum ideal potential and see whether or not it can compete with 2 stage to orbit.

So why not keep the discussion in that thread?

Also, in that thread it has been said multiple times that, given the same tech for SSTO and TSTO, the two stage configuration will always be more capable.

If you want to to talk fiction, then just give you ship the capability to launch into orbit and land back down multiple times without the need for refueling and be done with it. In case such tech is available, it may be worthwhile considering SSTO as a better configuration than TSTO, but until such time TSTO is the way to go.

In any case, what's this talk about vertical dropping into atmosphere? Putting back breaking braking aside, even the most optimistic hypothetical SSTO designs can just barely reach orbit. You now want to double the dv requirements? No.

[tater]

21 minutes ago, Terwin said:

Earth orbital velocity: just under 30km/s

Orbital velocity is closer to 9-10 km/s (with gravity/drag losses). It's not 70 km/s.

You are mistaking the tangential velocity of the Earth around the sun for the LEO velocity.

 

2 hours ago, Spacescifi said:

Little use arguing semantics when the meaning is clear.

The meaning is not clear at all.

Your subject is: "The Physics Of Ideal SSTO's Versus Two Staging...Settled?"

The OP then proceeds to have nothing to do with SSTOs (Single Stage To Orbit—the only meaning of SSTO), while also containing no physics to demonstrate anything at all, much less the specific subject.

(and that subject is a change in title from the original)

When I clicked on this thread, I expected to see mostly math.

Edited July 3, 2021 by tater

[Spacescifi]

35 minutes ago, Shpaget said:

So why not keep the discussion in that thread?

Also, in that thread it has been said multiple times that, given the same tech for SSTO and TSTO, the two stage configuration will always be more capable.

If you want to to talk fiction, then just give you ship the capability to launch into orbit and land back down multiple times without the need for refueling and be done with it. In case such tech is available, it may be worthwhile considering SSTO as a better configuration than TSTO, but until such time TSTO is the way to go.

In any case, what's this talk about vertical dropping into atmosphere? Putting back breaking braking aside, even the most optimistic hypothetical SSTO designs can just barely reach orbit. You now want to double the dv requirements? No.

 

LOL.

 

All I am saying is to compete with 2 stages the tech advancement required goes way up. That's all. I see things as what is needed if we could, not limits of today.

 

22 minutes ago, tater said:

Orbital velocity is closer to 9-10 km/s (with gravity/drag losses). It's not 70 km/s.

You are mistaking the tangential velocity of the Earth around the sun for the LEO velocity.

 

The meaning is not clear at all.

Your subject is: "The Physics Of Ideal SSTO's Versus Two Staging...Settled?"

The OP then proceeds to have nothing to do with SSTOs (Single Stage To Orbit—the only meaning of SSTO), while also containing no physics to demonstrate anything at all, much less the specific subject.

(and that subject is a change in title from the original)

When I clicked on this thread, I expected to see mostly math.

 

Sorry I did not indulge your math fix LOL.

 

You still can actually!

 

Just take Spacex. super heavy included, COMBINED as an ideal fictional SSTO and boost it to orbit, then flip over and slow it to a stop in space and drop straight down and boost to slow descent for landing.

 

Math will tell you if it lands in lava or not.

 

Either way it is obvious the plume will be a LOT hotter and more energetic than what Spacex is using.

[tater]

That's nonsense, why would anyone do 100% propulsive EDL on a world with a useful atmosphere?

The rocket equation is your friend, you should noodle around with it sometime.

 

Edited July 4, 2021 by tater

[GoSlash27]

 To be clear, The entire point of SSTO is that it's inexpensive to operate and rapidly reusable. If it doesn't fulfill those criteria, it's got no practical purpose for existing.

 Tsiolkovsky would point out that given identical tech, the 2 stage design will always use less fuel than the one stage design for an equal payload to orbit. Furthermore, we have figured out how to recycle the booster and are rapidly working out how to recycle the sustainer as well, it would seem that SSTO just plain doesn't offer enough fiscal advantage to be worth pursuing anymore. The turnaround time might have some niche applications, but most of today's launch providers and customers don't see that as a priority.

Best,

-Slashy

[Shpaget]

7 hours ago, Spacescifi said:

All I am saying is to compete with 2 stages the tech advancement required goes way up. That's all. I see things as what is needed if we could, not limits of today.

And as soon as that tech advancement is made, TSTO designs implement it, continue to outperform and stay out of reach of SSTO.

There is no competition. TSTO is more capable configuration. Always.

[Scotius]

Unless you have magical fusion drives, pulling extra power out of subspace via handwavey sorcery

https://www.sarna.net/wiki/DropShip

Then... you can have everything you want and be happy forever.

[jimmymcgoochie]

Two stage to orbit spaceplanes are feasible- use air-breathing rockets (like Skylon’s SABRE) to reach a ballistic suborbital trajectory, then throw the payload out to boost itself to orbit and the spaceplane can glide back down to land on a runway. Depending on the target orbit, they could operate trans-Atlantic from the US to the UK or possibly the other way around for a sun-synchronous orbit. Flying significantly slower than orbital velocity means there’s a lot less heat, so no need for fancy heat shielding when using naturally heat-resistant materials will do. There are plenty of reserved Space Shuttle runways on both sides of the Atlantic for something like Skylon to operate from, with a few additional pieces of infrastructure required (servicing, hydrogen refuelling and payload installation). Making the second stage recoverable is its own challenge though, but it’s feasible to do so with what would be a relatively small rocket- it’s already being done now.

But that’s just an alternative TSTO setup; making the spaceplane go all the way to orbit and back would make things even harder, probably requiring a dedicated vacuum engine for orbital insertion and proper heat shielding, both of which are heavy and on top of the plane-related gubbins will really cut down the payload fraction.

 

Re. the idea of braking in orbit to then drop vertically to the landing site- that’s a horrible idea, throwing away over 7km/s worth of fuel (that you had to haul up there with you, wrecking the rocket equation and your payload fraction) when there’s an atmosphere to do it for free. If you’re at the point where you can afford to drag that much fuel into space when you don’t need to, you’re probably at the stage where a space elevator is becoming feasible to get rid of rocket launches entirely, or if not that then some kind of orbital wheel to grab stuff in the atmosphere and fling it into space.

[Spacescifi]

5 hours ago, jimmymcgoochie said:

Two stage to orbit spaceplanes are feasible- use air-breathing rockets (like Skylon’s SABRE) to reach a ballistic suborbital trajectory, then throw the payload out to boost itself to orbit and the spaceplane can glide back down to land on a runway. Depending on the target orbit, they could operate trans-Atlantic from the US to the UK or possibly the other way around for a sun-synchronous orbit. Flying significantly slower than orbital velocity means there’s a lot less heat, so no need for fancy heat shielding when using naturally heat-resistant materials will do. There are plenty of reserved Space Shuttle runways on both sides of the Atlantic for something like Skylon to operate from, with a few additional pieces of infrastructure required (servicing, hydrogen refuelling and payload installation). Making the second stage recoverable is its own challenge though, but it’s feasible to do so with what would be a relatively small rocket- it’s already being done now.

But that’s just an alternative TSTO setup; making the spaceplane go all the way to orbit and back would make things even harder, probably requiring a dedicated vacuum engine for orbital insertion and proper heat shielding, both of which are heavy and on top of the plane-related gubbins will really cut down the payload fraction.

 

Re. the idea of braking in orbit to then drop vertically to the landing site- that’s a horrible idea, throwing away over 7km/s worth of fuel (that you had to haul up there with you, wrecking the rocket equation and your payload fraction) when there’s an atmosphere to do it for free. If you’re at the point where you can afford to drag that much fuel into space when you don’t need to, you’re probably at the stage where a space elevator is becoming feasible to get rid of rocket launches entirely, or if not that then some kind of orbital wheel to grab stuff in the atmosphere and fling it into space.

Skylon is having issues with heat exchangers I have read, otherwise they would be out testing like Musk.

Why drop straight down?  Lack of infrastructure on a new colony. Reduce wear and tear on a do-it-all colony LOL (launch, orbit, land) ship.

Right now the only SSTO that has enough power to stop an orbit with thrust to spare is a nuclear pusher plate/rocket hybrid (opposite engines at opposite ends).

Who is to say if smaller, less radioactive bombs can be made one day that are just as powerful? Besides known theories on this (often involving AM or MH), the possibility of unforseen breakthroughs are another possibility.

I will agree with you that so long propellant is a concern that a skylon variant that actually works would be fine, but it limits payload severely because of drag issues with flight.

A more powerful engine setup could get more payload to orbit than Skylon ever could.

Edited July 4, 2021 by Spacescifi

[GoSlash27]

48 minutes ago, Spacescifi said:

1) Right now the only SSTO that has enough power to stop an orbit with thrust to spare is a nuclear pusher plate/rocket hybrid (opposite engines at opposite ends).

2) I will agree with you that so long propellant is a concern that a skylon variant that actually works would be fine, but it limits payload severely because of drag issues with flight.

3) A more powerful engine setup could get more payload to orbit than Skylon ever could.

1) I'm not sure what you mean "stop an orbit". Are you proposing to nullify orbital velocity instantaneously while still at orbital altitude? If so, I can point out a half a dozen reasons why that's a bad idea.

2) The payload issue with Skylon has more to do with the rocket equation than drag.

3) Current engine setups can already get more payload to orbit than Skylon ever could. Again, reference the rocket equation (in my sig below).

[Terwin]

15 hours ago, tater said:

Orbital velocity is closer to 9-10 km/s (with gravity/drag losses). It's not 70 km/s.

You are mistaking the tangential velocity of the Earth around the sun for the LEO velocity.

Sorry, my google-fu failed me.  (if only I had some common-sense to back it up)

ISS orbital velocity: 27,600 kilometers per hour  -> 7 2/3 km/s  (ref: https://planetseducation.com/international-space-station-orbit/ )

So it only takes ~24 2/3 km/s of delta-v to follow the specified trajectory(assuming little or no atmospheric drag for take-off)

As we have seen in Star Trek, Star Wars, Battlestar Galactica and other science-fantasy; this is clearly not an issue, and vehicles the size of a large van or small bus can repeatedly make such trips with a cargo fraction of 80%+

 

In the real world, Falcon 9FT is 549t and can loft 15.6t  (22.8t expended) for a cargo fraction of ~2.8%(~4.2% expended)

Stage 1 dry-mass: 22.2t - 1 falcon9 engine no longer needed on 2nd stage: 0.47t  =21.7t

cargo capacity without staging:  -6.1t  (1.1t expended or 0.2% )

Giving the STSO ~4.8% of the efficiency of the tsto using this rocket(the most updated version of one of the most recently developed rockets that currently exists)

Also note that this is fully expended, so actually being able to land again would greatly reduce this.

 

Even if we assumed that the first-stage tanks are massless after their normal separation point and we only need to carry 8 additional engines to orbit(engines are only ~19% of the first stage dry-mass btw), that puts us at 11.8t  or ~75% of the tsto performance in this scenario(and this requires either staging 21.7t of empty tanks or a  ~80% dry-mass reduction only applied to the SSTO)

 

So yes, a fantasy-tech ssto can indeed beat out a real-tech tsto, but if they both have access to the same technology, then the TSTO will trounce the SSTO every time.(Martians and Lunites can have useful SSTO with current tech, but even there TSTO will be more efficient)

[Vanamonde]

Some comments have been removed. Please don't get carried away with the joke answers. 

[sevenperforce]

On 7/3/2021 at 4:10 PM, Spacescifi said:

Unless the super SSTO can throttle it's heat waaay down to lose some of it's high thrust/efficiency in the process, it will nuke the launch/landing site every time.

So if it cannot throttle that far back, two staging could get the second stage up high enough where it won't nuke beam damage the land.

No, that's not how rockets work. You can always use engine clusters or a low-thrust mode. Besides, a reusable two-stage solution requires that the upper stage also come back down and land, so you have the same problem. A two-stage solution also requires a first stage with more powerful engines than the second stage, unless you are merely looking to do a chemically-launched hop to a few km.

If you want a vehicle that can deliver a payload to low Earth orbit, loop once around the planet, and then propulsively lower its velocity to the point that it can make re-entry without a heat shield (kind of a weird waste, but whatever), then you need about 15.8 km/s. You'll need a little more for landing which is kind of slop at this point but let's kick it up to 16 km/s just to give us margin. 

Note that I didn't say anything about "100 tonnes" at all, because the payload amount is immaterial. Above a certain size, the specific impulse requirements involved are going to be on the same order regardless of the size of the payload. 

Starship+Superheavy has a dry mass of around 400 tonnes and carries 4600 tonnes of propellant. With a 100 tonne payload, that's a m₀/m_f of 10.2, so to squeeze 16 km/s out of that, you'd need an engine with a specific impulse of about 700 seconds. You can multiply or divide your payload, dry mass, and propellant mass by whatever number you want, and the specific impulse you need remains at 700 seconds. Probably well within the reach of a good near-future-scifi LANTR design, perhaps with methalox landing engines which would also provide added liftoff thrust.

[Spacescifi]

28 minutes ago, sevenperforce said:

No, that's not how rockets work. You can always use engine clusters or a low-thrust mode. Besides, a reusable two-stage solution requires that the upper stage also come back down and land, so you have the same problem. A two-stage solution also requires a first stage with more powerful engines than the second stage, unless you are merely looking to do a chemically-launched hop to a few km.

If you want a vehicle that can deliver a payload to low Earth orbit, loop once around the planet, and then propulsively lower its velocity to the point that it can make re-entry without a heat shield (kind of a weird waste, but whatever), then you need about 15.8 km/s. You'll need a little more for landing which is kind of slop at this point but let's kick it up to 16 km/s just to give us margin. 

Note that I didn't say anything about "100 tonnes" at all, because the payload amount is immaterial. Above a certain size, the specific impulse requirements involved are going to be on the same order regardless of the size of the payload. 

Starship+Superheavy has a dry mass of around 400 tonnes and carries 4600 tonnes of propellant. With a 100 tonne payload, that's a m₀/m_f of 10.2, so to squeeze 16 km/s out of that, you'd need an engine with a specific impulse of about 700 seconds. You can multiply or divide your payload, dry mass, and propellant mass by whatever number you want, and the specific impulse you need remains at 700 seconds. Probably well within the reach of a good near-future-scifi LANTR design, perhaps with methalox landing engines which would also provide added liftoff thrust.

 

Yes....but will we be landing in lava? A crater? Can the landing legs deal with that or will we need spares?

That is the point. The crazy thing about physics is EVERYTHING has a limit. That limit is often what we can survive or our materials can....otherwise what will work and what will not.

You can only build project orion so big until even the nukes used to propel it become inefficient.

[sevenperforce]

Just now, Spacescifi said:

Yes....but will we be landing in lava? A crater? Can the landing legs deal with that or will we need spares?

That is the point. The crazy thing about physics is EVERYTHING has a limit. That limit is often what we can survive or our materials can....otherwise what will work and what will not.

You can only build project orion so big until even the nukes used to propel it become inefficient.

Like I said, this set of requirements is completely within the range of near-future scifi. 700 seconds of isp is not that much. It can land on Raptors on any prepared concrete surface, and with these kinds of energy budgets you can have as beefy of landing legs as you want. It's all very simple and straightforward if you just get a nice powerful LANTR bundled in between your Raptors.

You're starting with the wrong problem. If you have a world where you're flying an LANTR-based SSTO multiple times per day, material engineering is not going to be a problem for you. Materials can survive all this easily.

[GoSlash27]

Just now, sevenperforce said:

700 seconds of isp is not that much.

I'm not aware of any chemical bipropellant that can achieve 700s specific impulse even theoretically, let alone with enough thrust to lift anything. I'd say that unless you invoke some kind of unobtainium propulsion system, 700s *is* a bit much.

Best,

-Slashy