Spaceplanes with other engines..

[gc1ceo]

I've been spending the last month or so learning how to build airplanes and finally space planes and information from AeroGav (and others) has been invaluable to working with the rapier, whiplash, nerva, panther and aerospike engines.

I'm looking to expand my design possibilities and try and incorporate other rocket engines such as the LV-T30/T45 so are there any good guidelines and suggestions (aside from don't do it) to using these on a space plane?

I should clarify this means a single stage space plane which takes off from a runway with additional atmospheric engines and not something launched vertically from a separate rocket.

Edited February 20, 2019 by gc1ceo

[Aegolius13]

I guess it depends in part on what other engines you're using.  I.e., if you're using Rapiers, my personal opinion is that it's never going to be worth it to put a Swivel or Reliant onboard.  Yeah, the vacuum ISP is a tiny bit higher, but it comes at a very significant mass and drag penalty.   Whereas if you have a Rapier anyway for jet mode, its rocket function is effectively free.  As a general rule, the further your mission is going, the more important vacuum delta-v will be.  If you need so much vacuum delta-v that you need an upgrade over the Rapier in rocket mode, the best answer  is probably a nuke.  

If you're using a lower-ceiling jet engine like the Whiplash or Panther, then yeah, rockets will have more of a use-case.  But even then, keep in mind that you're likely not lighting up the rockets until you're well over 10km in altitude.  At that point, you can essentially disregard the sea-level performance figures and just look at vacuum.  Given that, the Reliant is a pretty bad fit, since it has poor vacuum ISP and no gimbal to help with attitude control.  The Swivel is a little better, but still not ideal for this role.

You might also take a look at more vacuum-oriented engines: the Terrier, Poodle and (if you have Making History) the Cheetah, Wolfhound or Skiff.  As mentioned above, by the time you turn them on, these "vacuum" engines should already beat the sea-level engines on performance.  The Spark can work nicely on small planes, and I guess the Skipper is a possibility as well, though I would think the Poodle would have adequate thrust most of the time if you're going 2.5m.   The Vector could work if you need a ton of thrust on a 1.25m stack, but its vacuum ISP is not ideal.

I'd suggest using fewer, but bigger engines over a lot of small engines.  E.g., a Poodle mounted to the back of a 2.5m fuselage might work better than 4 Terriers on 1.25m nacelles.  The jet engines might have to go on nacelles, but you don't really have an option with those since they're all 1.25m anyway.  

[bewing]

Aegolius' suggestion about vacuum engines is exactly on point. Either you want an all-LF design (with nukes for your rocket engines), or you want to use a low-mass, vacuum rated LFO engine. So reliants and swivels are out.

I'd disagree with his point about fewer+larger rocket engines, though. Sticking with 1.25m engines gives you the opportunity to terminate each nacelle/stack with a rocket engine plus a displaced jet engine. The reduction in drag will pay for the extra jet engine. Even spark/juno combos on .625m stacks give you good enough performance for spaceplane work.

 

[Aeroboi]

2 hours ago, gc1ceo said:

I've been spending the last month or so learning how to build airplanes and finally space planes and information from AeroGav (and others) has been invaluable to working with the rapier, whiplash, nerva, panther and aerospike engines.

I'm looking to expand my design possibilities and try and incorporate other rocket engines such as the LV-T30/T45 so are there any good guidelines and suggestions (aside from don't do it) to using these on a space plane?

I should clarify this means a single stage space plane which takes off from a runway with additional atmospheric engines and not something launched vertically from a separate rocket.

If you play sandbox the Rapier, Nerva, Whiplash and Aerospike are the only real candidates you want to go for, the Panther being very much optional as a VTOL assist (only if your building VTOL) or using the panther as a transonic assist (when using draggier payloads) to cross the transonic region. IMO you just have to beat the drag using pointier fairings and less stacks and more aerodynamic end cap attachments and never use Mk2.

On the other hand if your playing career then Panthers early on can really pack a punch. Sidenote: I can even make small cargo SSTO's using Wheesley engines (don't expect to much of it)

When using panthers a Reliant can really be useful. While the swivel has better vacuum ISP the reliant is less heavy 1.25T vs 1.50T.
Often I find the Reliant does better then a swivel due to having less dry mass.

Adding to that the reliant doesn't swivel sp it suffers no cosine losses.

The only trick is to have the Center of thrust aligned right through the center and then make sure the CoM doesn't move up or down. If your fuel tanks drain evenly and the CoT of the Reliant engines is through the CoM the thrust should be balanced normally across fuel expenditure.

The big winner in SSTO designs is drag. I have a few SSTO's that use a whiplash and a vector. With proper drag reduction a whiplash can reach ~1600ms at ~22km altitude.

The Vector has the lowest thrust to cross section, meaning it produces most thrust for a 1.25m form factor. If your SSTO is very streamlined it can reach orbital velocity ~22/2300m/s at 25-30km altitude without losing much speed when coasting to apoapsis.
I make these kind of SSTO's primarily to get crew to LKO for the least in game and real life time expenditure.

In general the combinations of engines on SSTO's can be very broad oriented. Some designs can be very unique and use odd engine combinations beyond those I mentioned that can perform better.
Simply put, some engines work better across multiple flight conditions. It isn't always easy to tell which combination works best. Some engines may have better ISP, but if they are required for very limited applications you should scrap them.

 

[gc1ceo]

I started out playing with the rapiers which can do everything in the world (and more) but then I read your guide about using nervas and panthers and been building some space planes based on your excellent guide. It only got me more interested in what you can do with engines that aren't frequently utilized for space planes. I've read a few posts where people reportedly have made space planes work with other engines like the reliants but there never seems to be enough information on how they did it.

Your use of the Wheesleys with SSTOs sounds really interesting -- do you use the fanned out series of wings approach to make it work or something else?

 

[FleshJeb]

@gc1ceo Just FYI, if you want someone to see your message, you have to quote or @ them like I just did.

Good luck with your planes. Gav is definitely someone to listen to, although he has a very particular style, and there is more than one good way to build a spaceplane.

[TheTripleAce3]

I made ONE spaceplane that had a Wolfhoud on it that made orbit with a satellite about the size (and close to the mass) of a Kerbal, I wouldn't really go with big engines like that when you have NERVs and rapiers (or aerospikes, people should use them more).

If you want a plane to use a Reliant or Swivel, best prepare it to stage at some time in flight or have the center of lift quite far back, it is doable but not very easy to fly most of the time. Terriers also work if you perhaps stage Panthers off the rear of the aircraft.